Setting WPF commands from multiple view models

Setting WPF commands from multiple view models - wpf

Like any MVVM WPF app I have a handful of view models. Each has a few commands. My view implements a Fluent UI (Office ribbon) so there are some items that light up based on the context of the application. The ribbon is a child to the main application.
The basic structure of my app is that it manages a COURSE. A COURSE has multiple MODULES in it, so I have a VM for course & module... and each has commands.
When the app loads I set the data context of the main window to the course so binding the course commands to the ribbon is easy and works fine.
The challenge comes when the user starts to work with a module. When a module is selected from a list the details are shown in another user control. Now... my challenge is how to wire up the commands to the ribbon.
I assume I could have some event handler that programatically wires up the current module's commands to all the relevant controls in the ribbon and removes everything when the context goes away. But that seems like a lot of unnecessary work. Is there a cleaner way of doing this?
I thought about routed commands/events, but someone told me that this wouldn't work because they won't bubble all thew ay up to the Window and back down to the ribbon.
Looking for some guidance here... I'm a bit of a noob to MVVM (but loving it!).

Idea: Introduce a ShellCommands class which is exposed as a service.
public class ShellCommands : IShellCommands
{
public ICommand SaveCommand { get; set; }
...
}
Then the CourseViewModel and the ModuleViewModel can use the same service to register their commands.
public class CourseViewModel : ViewModel
{
public CourseViewModel(IShellCommands shellCommands, ...)
{
this.ShellCommands = shellCommands;
...
}
public IShellCommands ShellCommands { get; private set; }
}
In XAML you can access the service via the ShellCommands property.
<MenuItem Header="Save" Command="{Binding ShellCommands.SaveCommand}"/>
.
More Informations: WPF Application Framework (WAF)

Related

Main Window starts to depend on too many things, but I don't want a service locator

I am creating a WPF application. Naturally my entry point is MainWindow.xaml, which is opened up by App.xaml
var mainWindow = container.Resolve<MainWindow>();
Application.Current.MainWindow = mainWindow;
Application.Current.MainWindow.Show();
I am using Dependency Injection and so far all the dependencies are passed as parameters in the ctor of the MainWindow's View Model.
i.e. my Main Window is
public partial class MainWindow : MetroWindow
{
private readonly MainWindowModel mainViewModel;
public MainWindow(MainWindowModel mainViewModel)
{
and its View model is:
public MainWindowModel(IDataRepository dataRepo, ICommand command1, ICommand command2, etc ...)
{
However, I am now starting to realize this might be a problem. Given that the MainWindow is the entry point to the entire app, it seems like any dependency, anywhere in the application will have to first pass through the MainWindow View Model constructor. This seems crazy.
I am coming from the background of ASP.NET MVC and there we have Controllers, which receive only the dependencies that they need. i.e. the concept of a main entry point there is missing and this makes things easier and more manageable.
Here is an example in my WPF app. A control, on the Main View needs to open up a dialog. This dialog is another Window and of course that window receives its ViewModel in its ctor. To me, it seems like to be able to resolve the dialog properly, I need to pass it through the Main Window View Model ctor first, keep it as private readonly field of the Main Window View Model and launch it when necessary. Ok, but what if I have 100 dialogs. That's just one of the examples. I have such issue with the ICommand implementations too.
To sum up my question:
How do I manage the dependencies in WPF properly, without using the Service Locator anti-pattern and without passing every single abstraction through the ctor of the main window view model? I could very easily pass a Container around and let, e.g., the create ABC command solve the ABCDialog before opening it, but I feel this will cause more issues than it would solve.
I am probably doing something wrong. Please advise me what is the best practice.

Caliburn Micro - ActivateItem using Container

I was going through the Caliburn Micro documenation here. Simultaneously, I was trying to put up some rough code for experiment. I am a little confused about how to activate item using a container and how to pass an object to the ViewModel that we are activating.
Lets consider a master/detail scenario. The master contains a list (say datagrid) and the details contain specific row from the master for update(say tab item inside tab control). In the documentation (for ease of understanding), I believe the detail ViewModel was directly instantiated using code like this
public class ShellViewModel : Conductor<IScreen>.Collection.OneActive {
int count = 1;
public void OpenTab() {
ActivateItem(new TabViewModel {
DisplayName = "Tab " + count++
});
}
}
So, to apply the above fundamental concept in real world app, we need to instantiate the DetailViewModel (TabViewModel above) using container(say MEF). The challenge then is to know whether the particular DetailViewModel is already opened in the TAB Control. The immediate crude thing that came to my mind was maintaining a List of the Opened Tabs (DetailViewModels). But then we are again referencing DetailViewModel in the MasterViewModel defeating the purpose. Is there any options available to solve this issue.
The second thing that is troubling me is how to pass the Objects from MasterViewModel (Selected Detail Item) to the DetailViewModel. If we use the EventAggregator here then each of the opened DetailViewModels will receive the event which I am not sure how to handle.
If anyone can throw some light on the above two issues, I would be grateful
Update:
The Master is Conductor like this
public class MainViewModel : Conductor<IScreen>.Collection.OneActive, IShell {
....
}
And the detail is defined like this
public class TabViewModel : Screen {
....
}
Both are in the same Window.

I'm not sure exactly what the issue is. In your conductor of many, you have an Items collection provided by Caliburn.Micro. When you come to display a detail view, you can check this collection for the existence of that detail view (using the primary key which you have from the master view).
If the item is already in the Items collection then just activate it (using the ActivateItem method). If the item isn't in the collection, then instantiate it (presumably using a factory if you're using MEF), and add it to the Items collection, and then activate it.

passing data to a mvvm usercontrol

I'm writting a form in WPF/c# with the MVVM pattern and trying to share data with a user control. (Well, the User Controls View Model)
I either need to:
Create a View model in the parents and bind it to the User Control
Bind certain classes with the View Model in the Xaml
Be told that User Controls arn't the way to go with MVVM and be pushed in the correct direction. (I've seen data templates but they didn't seem ideal)
The usercontrol is only being used to make large forms more manageable so I'm not sure if this is the way to go with MVVM, it's just how I would of done it in the past.
I would like to pass a class the VM contruct in the Xaml.
<TabItem Header="Applicants">
<Views:ApplicantTabView>
<UserControl.DataContext>
<ViewModels:ApplicantTabViewModel Client="{Binding Client} />
</UserControl.DataContext>
</Views:ApplicantTabView>
</TabItem>
public ClientComp Client
{
get { return (ClientComp)GetValue(ClientProperty); }
set { SetValue(ClientProperty, value); }
}
public static readonly DependencyProperty ClientProperty = DependencyProperty.Register("Client", typeof(ClientComp),
typeof(ApplicantTabViewModel),
new FrameworkPropertyMetadata
(null));
But I can't seem to get a dependancy property to accept non static content.
This has been an issue for me for a while but assumed I'd find out but have failed so here I am here.
Thanks in advance,
Oli

Oli - it is OK (actually - recommended) to split portions of the View into UserControl, if UI became too big - and independently you can split the view models to sub view models, if VM became too big.
It appears though that you are doing double-instantiations of your sub VM. There is also no need to create Dependency Property in your VM (actually, I think it is wrong).
In your outer VM, just have the ClientComp a regular property. If you don't intend to change it - the setter doesn't even have to fire a property changed event, although it is recommended.
public class OuterVm
{
public ClientComp Client { get; private set; }
// instantiate ClientComp in constructor:
public OuterVm( ) {
Client = new ClientComp( );
}
}
Then, in the XAML, put the ApplicantTabView, and bind its data context:
...
<TabItem Header="Applicants">
<Views:ApplicantTabView DataContext="{Binding Client}" />
</TabItem>

I answered a similar question as yours recently: passing a gridview selected item value to a different ViewModel of different Usercontrol
Essentially setting up a dependency property which allows data from your parent view to persist to your child user control. Abstracting your view into specific user controls and hooking them using dependency properties along with the MVVM pattern is actually quite powerful and recommended for Silverlight/WPF development, especially when unit testing comes into play. Let me know if you'd like any more clarification, hope this helps.

What approach should I use when creating a custom WPF control?

I'm about to redo an old WinForms application as a WPF application. The core of this applicaton is a custom "grid" component. I'd like some ideas on the best way to do this as a WPF component.
The application displays a grid of data for different countries/sectors. Each cell of the grid displays different information (e.g. graph, image) depending on the available data for that country/sector.
I have a domain model assembly that I want to keep clean - to maximize reuse. The structure is as follows:
Table
Continents
Countries
Sectors
Data[country, sector]
The grid displays countries down the left and sectors across the top.
In the current application, the grid component has a (POCO) Table property and an Refresh() methods to manually redraw it. So, if the Table is updated, the parent of the grid component refreshes it. The grid component also has a number of events that are fired if a continent, country or cell is clicked - so that the parent can response with pop-up menus, etc.
This all works fine.
However, I'm wondering whether this is the correct model to use for a WPF application. Looking at many of the WPF example, they support data-binding, etc. But, it's not clear, from the simple examples, how I might bind a complex object to my components - or whether it would even be worthwhile.
Also, the WinForms component is completely custom drawn - there are no sub-controls (e.g. Labels) in use. Would it be better to use a WPF user control and build the table from a GridLayout and lots of Label, Shape, etc controls? In practice, they are maybe 20 rows and 20 columns in the grid, and the user regular removes and adds countries/sectors (rows/columnms) while using the application.
My immediate goal is to make sure my design plays well in the WPF eco-system, but I have a secondary goal of learning how to do things in a WPFy way - given this is my first WPF app. I'm pretty on top of the use of building a general WPF app - it's just the custom control stuff that remains a little fuzzy (even after reading around it a little).
Any insights/guidance would be appreciated.

You definitely want to adapt the MVVM approach, as outlined by Josh Smith. Practically, this means that your custom grid component will be contained in it's own View. Backing the view will be your ViewModel, where you will define an ObservableCollection of objects containing your data. These objects will probably come from your Model. This interaction is shown below:
Models:
public class TableData
{
public string Country { get; set; }
public string Continent { get; set; }
public object Sector { get; set; }
}
public class TableManager : ITableManager
{
public Collection<TableData> Rows;
public void GetData()
{
this.Rows = new Collection<TableData>();
this.Rows.Add(...
}
}
ViewModel:
public class TableViewModel
{
private ITableManager _tableManager;
public TableViewModel() : base(new TableManager())
{
}
// for dependency injection (recommended)
public TableViewModel(ITableManager tableManager)
{
_tableManager = tableManager;
_tableManager.GetData();
}
public ObservableCollection<TableData> Rows
{
get { return _tableManager.Rows; }
}
}
View:
<ctrls:CustomDataGrid
ItemsSource={Binding Rows}
AutoGenerateColumns=True
>
<!-- Use AutoGenerateColumns if the # of sectors is dynamic -->
<!-- Otherwise, define columns manually, like so: -->
<DataGridTextColumn
Width="*"
Header="SectorA"
Binding="{Binding Country}
/>
</ctrls:CustomDataGrid>
I used CustomDataGrid in the view because I assume you're going to subclass your own DataGrid. This will allow you to override events to customize the DataGrid to your liking:
public class CustomDataGrid : DataGrid
{
public override Event...
}

Open File Dialog MVVM

Ok I really would like to know how expert MVVM developers handle an openfile dialog in WPF.
I don't really want to do this in my ViewModel(where 'Browse' is referenced via a DelegateCommand)
void Browse(object param)
{
//Add code here
OpenFileDialog d = new OpenFileDialog();
if (d.ShowDialog() == true)
{
//Do stuff
}
}
Because I believe that goes against MVVM methodology.
What do I do?

Long story short:
The solution is to show user interactions from a class, that is part of the view component.
This means, such a class must be a class that is unknown to the view model and therefore can't be invoked by the view model.
The solution of course can involve code-behind implementations as code-behind is not relevant when evaluating whether a solution complies with MVVM or not.
Beside answering the original question, this answer also tries to provide an alternative view on the general problem why controlling a UI component like a dialog from the view model violates the MVVM design pattern and why workarounds like a dialog service don't solve the problem.
1 MVVM and dialogs
1.1 Critique of common suggestions
Almost all answers are following the misconception that MVVM is a pattern, that targets class level dependencies and also requires empty code-behind files. But it's an architectural pattern, that tries to solve a different problem - on application/component level: keeping the business domain decoupled from the UI.
Most people (here on SO) agree that the view model should not handle dialogs, but then propose to move the UI related logic to a helper class (doesn't matter if it's called helper or service), which is still controlled by the view model.
This (especially the service version) is also known as dependency hiding. Many patterns do this. Such patterns are considered anti-patterns. Service Locator is the most famous dependency hiding anti-pattern.
That is why I would call any pattern that involves extraction of the UI logic from the view model class to a separate class an anti-pattern too. It does not solve the original problem: how to change the application structure or class design in order to remove the UI related responsibilities from a view model (or model) class and move it back to a view related class.
In other words: the critical logic remains being a part of the view model component.
For this reason, I do not recommend to implement solutions, like the accepted one, that involve a dialog service (whether it is hidden behind an interface or not). If you are concerned to write code that complies with the MVVM design pattern, then simply don't handle dialog views or messaging inside the view model.
Introducing an interface to decouple class level dependencies, for example an IFileDialogService interface, is called Dependency Inversion principle (the D in SOLID) and has nothing to do with MVVM. When it has no relevance in terms of MVVM, it can't solve an MVVM related problem. When room temperature does not have any relevance whether a structure is a four story building or a skyscraper, then changing the room temperature can never turn any building into a skyscraper. MVVM is not a synonym for Dependency Inversion.
MVVM is an architectural pattern while Dependency Inversion is an OO language principle that has nothing to do with structuring an application (aka software architecture). It's not the interface (or the abstract type) that structures an application, but abstract objects or entities like components or modules e.g. Model - View - View Model. An interface can only help to "physically" decouple the components or modules. It doesn't remove component associations.
1.2 Why dialogs or handling Window in general feels so odd?
We have to keep in mind that the dialog controls like Microsoft.Win32.OpenFileDialog are "low level" native Windows controls. They don't have the necessary API to smoothly integrate them into a MVVM environment. Because of their true nature, they have some limitations the way they can integrate into a high level framework like WPF. Dialogs or native window hosts in general, are a known "weakness" of all high level frameworks like WPF.
Dialogs are commonly based on the Window or the abstract CommonDialog class. The Window class is a ContentControl and therefore allows styles and templates to target the content.
One big limitation is, that a Window must always be the root element. You can't add it as a child to the visual tree and e.g. show/launch it using triggers or host it in a DataTemplate.
In case of the CommonDialog, it can't be added to the visual tree, because it doesn't extend UIElement.
Therefore, Window or CommonDialog based types must always be shown from code-behind, which I guess is the reason for the big confusion about handling this kind of controls properly.
In addition, many developers, especially beginners that are new to MVVM, have the perception that code-behind violates MVVM.
For some irrational reason, they find it less violating to handle the dialog views in the view model component.
Due to it's API, a Window looks like a simple control (in fact, it extends ContentControl). But underneath, it hooks into to the low level of the OS. There is a lot of unmanaged code necessary to achieve this. Developers that are coming from low level C++ frameworks like MFC know exactly what's going on under the hoods.
The Window and CommonDialog class are both true hybrids: they are part of the WPF framework, but in order to behave like or actually to be a native OS window, they must be also part of the low level OS infrastructure.
The WPF Window, as well as the CommonDialog class, is basically a wrapper around the complex low level OS API. That's why this controls have sometimes a strange feel (from the developer point of view), when compared to common and pure framework controls.
That Window is sold as a simple ContentControl is quite deceptive. But since WPF is a high level framework, all low level details are hidden from the API by design.
We have to accept that we have to handle controls based on Window and CommonDialog using C# only - and that code-behind does not violate any design pattern at all.
If you are willing to waive the native look and feel and the general OS integration to get the native features like theming and task bar, you can improve the handling by creating a custom dialog e.g., by extending Control or Popup, that exposes relevant properties as DependencyProperty. You can then set up data bindings and XAML triggers to control the visibility, like you usually would.
1.3 Why MVVM?
Without a sophisticated design pattern or application structure, developers would e.g., directly load database data to a table control and mix UI logic with business logic. In such a scenario, changing to a different database would break the UI. But even worse, changing the UI would require to change the logic that deals with the database. And when changing the logic, you would also need to change the related unit tests.
The real application is the business logic and not the fancy GUI.
You want to write unit tests for the business logic - without being forced to include any UI.
You want to modify the UI without modifying the business logic and unit tests.
MVVM is a pattern that solves this problems and allows to decouple the UI from the business logic i.e. data from views. It does this more efficiently than the related design patterns MVC and MVP.
We don't want to have the UI bleed into the lower levels of the application. We want to separate data from data presentation and especially their rendering (data views). For example, we want to handle database access without having to care which libraries or controls are used to view the data. That's why we choose MVVM. For this sake, we can't allow to implement UI logic in components other than the view.
1.4 Why moving UI logic from a class named ViewModel to a separate class still violates MVVM
By applying MVVM, you effectively structuring the application into three components: model, view and view model. It is very important to understand that this partitioning or structure is not about classes. It's about application components.
You may follow the widely spread pattern to name or suffix a class ViewModel, but you must know that the view model component usually contains many classes of which some are not named or suffixed with ViewModel - View Model is an abstract component.
Example:
when you extract functionality, like creating a data source collection, from a big class named MainViewModel and you move this functionality to a new class named ItemCreator, then this class ItemCreator is logically still part of the view model component.
On class level the functionality is now outside the MainViewModel class (while MainViewModel now has a strong reference to the new class, in order to invoke the code). On application level (architecture level), the functionality is still in the same component.
You can project this example onto the often proposed dialog service: extracting the dialog logic from the view model to a dedicated class named DialogService doesn't move the logic outside the view model component: the view model still depends on this extracted functionality.
The view model still participates in the UI logic e.g by explicitly invoking the "service" to control when dialog is shown and to control the dialog type itself (e.g., file open, folder select, color picker etc.).
This all requires knowledge of the UI's business details. Knowledge, that per definition does not belong into the view model component. Of course, such knowlegde introduces a coupling/dependency from the view model component to the view component.
Responsibilities simply don't change because you name a class DialogService instead of e.g. DialogViewModel.
The DialogService is therefore an anti-pattern, which hides the real problem: having implemented view model classes, that depend on UI and execute UI logic.
1.5 Does writing code-behind violates the MVVM design pattern?
MVVM is a design pattern and design patterns are per definition library independent, framework independent and language or compiler independent. Therefore, code-behind is not a topic when talking about MVVM.
The code-behind file is absolutely a valid context to write UI code. It's just another file that contains C# code. Code-behind means "a file with a .xaml.cs extension". It's also the only place for event handlers. And you don't want to stay away from events.
Why does the mantra "No code in code-behind" exist?
For people that are new to WPF, UWP or Xamarin, like those skilled and experienced developers coming from frameworks like WinForms, we have to stress that using XAML should be the preferred way to write UI code. Implementing a Style or DataTemplate using C# (e.g. in the code-behind file) is too complicated and produces code that is very difficult to read => difficult to understand => difficult to maintain.
XAML is just perfect for such tasks. The visually verbose markup style perfectly expresses the UI's structure. It does this far better, than for example C# could ever do. Despite markup languages like XAML may feel inferior to some or not worth learning it, it's definitely the first choice when implementing GUI. We should strive to write as much GUI code as possible using XAML.
But such considerations are absolutely irrelevant in terms of the MVVM design pattern.
Code-behind is simply a compiler concept, realized by the partial directive (in C#). That's why code-behind has nothing to do with any design pattern. That's why neither XAML nor C# can't have anything to do with any design pattern.
2 Solution
Like the OP correctly concludes:
"I don't really want to do this [open a file picker dialog] in my
ViewModel(where 'Browse' is referenced via a DelegateCommand).
Because I believe that goes against MVVM methodology.
2.1 Some fundamental considerations
A dialog is a UI control: a view.
The handling of a dialog control or a control in general e.g. showing/hiding is UI logic.
MVVM requirement: the view model does not know about the existence of an UI or users. Because of this, a control flow that requires the view model to actively wait or call for user input, really requires some re-design: it is a critical violation and breaks the architectural boundaries dictated by MVVM.
Showing a dialog requires knowledge about when to show it and when to close it.
Showing the dialog requires to know about the UI and user, because the only reason to show a dialog is to interact with the user.
Showing the dialog requires knowledge about the current UI context (in order to choose the appropriate dialog type).
It is not the dependency on assemblies or classes like OpenFileDialog or UIElement that breaks the MVVM pattern, but the implementation or reference of UI logic in the view model component or model component (although such a dependency can be a valuable hint).
For the same reasons, it would be wrong to show the dialog from the model component too.
The only component responsible for UI logic is the view component.
From an MVVM point of view, there is nothing like C#, XAML, C++ or VB.NET. Which means, there is nothing like partial or the related infamous code-behind file (*.xaml.cs). The concept of code-behind exists to allow the compiler to merge the XAML part of a class with its C# part. After that merge, both files are treated as a single class: it's a pure compiler concept. partial is the magic that enables to write class code using XAML (the true compiler language is still C# or any other IL compliant language).
ICommand is an interface of the .NET library and therefore not a topic when talking about MVVM. It's wrong to believe that every action has to be triggered by an ICommand implementation in the view model.
Events are still a very useful concept that conform with MVVM, as long as the unidirectional dependency between the components is maintained. Always forcing the use of ICommand instead of using events leads to unnatural and smelly code like the code presented by the OP.
There is no such "rule" that ICommand must only be implemented by a view model class. It can be implemented by a view class too.
In fact, views commonly implement RoutedCommand (or RoutedUICommand), which both are implementions of ICommand, and can also be used to trigger the display of a dialog from e.g., a Window or any other control.
We have data binding to allow the UI to exchange data with the view model (anonymously, from the data source point of view). But since data binding can't invoke operations (at least in WPF - e.g., UWP allows this), we have ICommand and ICommandSource to realize this.
Interfaces in general are not a relevant concept of MVVM. Therefore, introducing an interface (e.g., IFileDialogService) can never solve a MVVM related problem.
Services or helper classes are not a concept of MVVM. Therefore, introducing services or helper classes can never solve a MVVM related problem.
Classes an their names or type names in general are not relevant in terms of MVVM. Moving view model code to a separate class, even if that class is not named or suffixed with ViewModel, can't solve a MVVM related problem.
2.2 Conclusion
The solution is to show user interactions from a class, that is part of the view component.
This means, such a class must be a class that is unknown to the view model and therefore can't be invoked by the view model.
This logic could be implemented directly in the code-behind file or inside any other class (file). The implementation can be a simple helper class or a more sophisticated (attached) behavior.
The point is: the dialog i.e. the UI component must be handled by the view component alone, as this is the only component that contains UI related logic. Since the view model does not have any knowledge of a view, it can't act actively to communicate with the view. Only passive communication is allowed (data binding, events).
We can always implement a certain flow using events raised by the view model that can be observed by the view in order to take actions like interacting with the user using a dialog.
There exist solutions using the view-model-first approach, which is does not violate MVVM in the first place. But still, badly designed responsibilities can turn this solution into an anti-pattern too.
3 How to fix the need for certain dialog requests
Most of the times, we can eliminate the need to show dialogs from within the application by fixing the application's design.
Since dialogs are a UI concept to enable interaction with the user, we must evaluate dialogs using UI design rules.
Maybe the most famous design rules for UI design are the 10 rules postulated by Nielsen and Molich in the 90's.
One important rule is about error prevention: it states that
a) we must prevent any kind of errors, especially input related, because
b) the user does not like his productivity to be interrupted by error messages and dialogs.
a) means: input data validation. Don't allow invalid data to enter the business logic.
b) means: avoid showing dialogs to the user, whenever possible. Never show a dialog from within the application and let the user trigger dialogs explicitly e.g., on mouse click (no unexpected interruption).
Following this simple rule certainly always eliminates the need to show a dialog triggered by the view model.
From the user's perspective, an application is a black box: it outputs data, accepts data and processes the input data. If we control the data input to guard against invalid data, we eliminate undefined or illegal states and ensure data integrity. This would mean that there is no need to ever show a dialog to the user from inside the application. Only those explicitly triggered by the user.
For example, a common scenario is that our model needs to persist data in a file. If the destination file already exists, we want to ask the user to confirm to overwrite this file.
Following the rule of error prevention, we always let the user pick files in the first place: whether it is a source file or a destination file, it's always the user who specifies this file by explicitly picking it via a file dialog. This means, the user must also explicitly trigger the file operation, for example by clicking on a "Save As" button.
This way, we can use a file picker or file save dialog to ensure only existing files are selected. As a bonus, we additionally eliminate the need to warn the user about overwriting existing files.
Following this approach, we have satisfied a) "[...]prevent any kind of errors, especially input related" and b) "[...]the user does not like to be interrupted by error messages and dialogs".
Update
Since people are questioning the fact that you don't need a view model to handle the dialog views, by coming up with extra "complicated" requirements like data validation to proof their point, I am forced to provide more complex examples to address these more complex scenarios (that were not initially requested by the OP).
4 Examples
4.1 Overview
The scenario is a simple input form to collect a user input like an album name and then use the OpenFileDialog to pick a destination file where the album name is saved to.
Three simple solutions:
Solution 1: Very simple and basic scenario, that meets the exact requirements of the question.
Solution 2: Solution that enables to use data validation in the view model. To keep the example simple, the implementation of INotifyDataErrorInfo is omitted.
Solution 3: Another, more elegant solution that uses an ICommand and the ICommandSource.CommandParameter to send the dialog result to the view model and execute the persistence operation.
Solution 1
The following example provides a simple and intuitive solution to show the OpenFileDialog in a MVVM compliant way.
The solution allows the view model to remain unaware of any UI components or logic.
You can even consider to pass a FileStream to the view model instead of the file path. This way, you can handle any errors, while creating the stream, directly in the UI e.g., by showing a dialog if needed.
View
MainWindow.xaml
<Window>
<Window.DataContext>
<MainViewModel />
</Window.DataContext>
<StackPanel>
<!-- The data to persist -->
<TextBox Text="{Binding AlbumName}" />
<!-- Show the file dialog.
Let user explicitly trigger the file save operation.
This button will be disabled until the required input is valid -->
<Button Content="Save as"
Click="SaveAlbumNameToFile_OnClick" />
</StackPanel>
</Window>
MainWindow.xaml.cs
partial class MainWindow : Window
{
public MainWindow()
=> InitializeComponent();
private void SaveAlbumNameToFile_OnClick(object sender, EventArgs e)
{
var dialog = new OpenFileDialog();
if (dialog.ShowDialog() == true)
{
// Consider to create the FileStream here to handle errors
// related to the user's picked file in the view.
// If opening the FileStream succeeds, we can pass it over to the viewmodel.
string destinationFilePath = dialog.FileName;
(this.DataContext as MainViewModel)?.SaveAlbumName(destinationFilePath);
}
}
}
View Model
MainViewModel.cs
class MainViewModel : INotifyPropertyChanged
{
// Raises PropertyChanged
public string AlbumName { get; set; }
// A model class that is responsible to persist and load data
private DataRepository DataRepository { get; }
public MainViewModel() => this.DataRepository = new DataRepository();
// Since 'destinationFilePath' was picked using a file dialog,
// this method can't fail.
public void SaveAlbumName(string destinationFilePath)
=> this.DataRepository.SaveData(this.AlbumName, destinationFilePath);
}
Solution 2
A more realistic solution is to add a dedicated TextBox as input field to enable collection of the destination file path via copy&paste.
This TextBox is bound to the view model class, which ideally implements INotifyDataErrorInfo to validate the file path before it is used.
An additional button will open the optional file picker view to allow the user to alternatively browse the file system to pick a destination.
Finally, the persistence operation is triggered by a "Save As" button:
View
MainWindow.xaml
<Window>
<Window.DataContext>
<MainViewModel />
</Window.DataContext>
<StackPanel>
<!-- The data to persist -->
<TextBox Text="{Binding AlbumName}" />
<!-- Alternative file path input, validated using INotifyDataErrorInfo validation
e.g. using File.Exists to validate the file path -->
<TextBox x:Name="FilePathTextBox"
Text="{Binding DestinationPath, ValidatesOnNotifyDataErrors=True}" />
<!-- Option to search a file using the file picker dialog -->
<Button Content="Browse" Click="PickFile_OnClick" />
<!-- Let user explicitly trigger the file save operation.
This button will be disabled until the required input is valid -->
<Button Content="Save as"
Command="{Binding SaveAlbumNameCommand}" />
</StackPanel>
</Window>
MainWindow.xaml.cs
partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void PickFile_OnClick(object sender, EventArgs e)
{
var dialog = new OpenFileDialog();
if (dialog.ShowDialog() == true)
{
this.FilePathTextBox.Text = dialog.FileName;
// Since setting the property explicitly bypasses the data binding,
// we must explicitly update it by calling BindingExpression.UpdateSource()
this.FilePathTextBox
.GetBindingExpression(TextBox.TextProperty)
.UpdateSource();
}
}
}
View Model
MainViewModel.cs
class MainViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
{
private string albumName;
public string AlbumName
{
get => this.albumName;
set
{
this.albumName = value;
OnPropertyChanged();
}
}
private string destinationPath;
public string DestinationPath
{
get => this.destinationPath;
set
{
this.destinationPath = value;
OnPropertyChanged();
ValidateDestinationFilePath();
}
}
public ICommand SaveAlbumNameCommand => new RelayCommand(
commandParameter => ExecuteSaveAlbumName(this.TextValue),
commandParameter => true);
// A model class that is responsible to persist and load data
private DataRepository DataRepository { get; }
// Default constructor
public MainViewModel() => this.DataRepository = new DataRepository();
private void ExecuteSaveAlbumName(string destinationFilePath)
{
// Use a aggregated/composed model class to persist the data
this.DataRepository.SaveData(this.AlbumName, destinationFilePath);
}
}
Solution 3
The following solution is a more elegant version of the second scenario. It uses the ICommandSource.CommandParameter property to send the dialog result to the view model (instead of the data binding used in the previous example).
The validation of the optional user input (e.g. copy&paste) is validated using binding validation:
View
MainWindow.xaml
<Window x:Name="Window">
<Window.DataContext>
<MainViewModel />
</Window.DataContext>
<StackPanel>
<!-- The data to persist -->
<TextBox Text="{Binding AlbumName}" />
<!-- Alternative file path input, validated using binding validation
e.g. using File.Exists to validate the file path -->
<TextBox x:Name="FilePathTextBox">
<TextBox.Text>
<Binding ElementName="Window" Path="DestinationPath">
<Binding.ValidationRules>
<FilePathValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
<!-- Option to search a file using the file picker dialog -->
<Button Content="Browse" Click="PickFile_OnClick" />
<!-- Let user explicitly trigger the file save operation.
This button will be disabled until the required input is valid -->
<Button Content="Save as"
CommandParameter="{Binding ElementName=Window, Path=DestinationPath}"
Command="{Binding SaveAlbumNameCommand}" />
</StackPanel>
</Window>
FilePathValidationRule.cs
class FilePathValidationRule : ValidationRule
{
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
=> value is string filePath && File.Exists(filePath)
? ValidationResult.ValidResult
: new ValidationResult(false, "File path does not exist.");
}
MainWindow.xaml.cs
partial class MainWindow : Window
{
public static readonly DependencyProperty DestinationPathProperty = DependencyProperty.Register(
"DestinationPath",
typeof(string),
typeof(MainWindow),
new PropertyMetadata(default(string)));
public string DestinationPath
{
get => (string)GetValue(MainWindow.DestinationPathProperty);
set => SetValue(MainWindow.DestinationPathProperty, value);
}
public MainWindow()
{
InitializeComponent();
}
private void PickFile_OnClick(object sender, EventArgs e)
{
var dialog = new OpenFileDialog();
if (dialog.ShowDialog() == true)
{
this.DestinationPath = dialog.FileName;
}
}
}
View Model
MainViewModel.cs
class MainViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
{
private string albumName;
public string AlbumName
{
get => this.albumName;
set
{
this.albumName = value;
OnPropertyChanged();
}
}
public ICommand SaveAlbumNameCommand => new RelayCommand(
commandParameter => ExecuteSaveAlbumName(commandParameter as string),
commandParameter => true);
// A model class that is responsible to persist and load data
private DataRepository DataRepository { get; }
// Default constructor
public MainViewModel() => this.DataRepository = new DataRepository();
private void ExecuteSaveAlbumName(string destinationFilePath)
{
// Use a aggregated/composed model class to persist the data
this.DataRepository.SaveData(this.AlbumName, destinationFilePath);
}
}

The best thing to do here is use a service.
A service is just a class that you access from a central repository of services, often an IOC container. The service then implements what you need like the OpenFileDialog.
So, assuming you have an IFileDialogService in a Unity container, you could do...
void Browse(object param)
{
var fileDialogService = container.Resolve<IFileDialogService>();
string path = fileDialogService.OpenFileDialog();
if (!string.IsNullOrEmpty(path))
{
//Do stuff
}
}

I would have liked to comment on one of the answers, but alas, my reputation is not high enough to do so.
Having a call such as OpenFileDialog() violates the MVVM pattern because it implies a view (dialog) in the view model. The view model can call something like GetFileName() (that is, if simple binding is not sufficient), but it should not care how the file name is obtained.

The ViewModel should not open dialogs or even know of their existence. If the VM is housed in a separate DLL, the project should not have a reference to PresentationFramework.
I like to use a helper class in the view for common dialogs.
The helper class exposes a command (not an event) which the window binds to in XAML. This implies the use of RelayCommand within the view. The helper class is a DepencyObject so it can bind to the view model.
class DialogHelper : DependencyObject
{
public ViewModel ViewModel
{
get { return (ViewModel)GetValue(ViewModelProperty); }
set { SetValue(ViewModelProperty, value); }
}
public static readonly DependencyProperty ViewModelProperty =
DependencyProperty.Register("ViewModel", typeof(ViewModel), typeof(DialogHelper),
new UIPropertyMetadata(new PropertyChangedCallback(ViewModelProperty_Changed)));
private static void ViewModelProperty_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (ViewModelProperty != null)
{
Binding myBinding = new Binding("FileName");
myBinding.Source = e.NewValue;
myBinding.Mode = BindingMode.OneWayToSource;
BindingOperations.SetBinding(d, FileNameProperty, myBinding);
}
}
private string FileName
{
get { return (string)GetValue(FileNameProperty); }
set { SetValue(FileNameProperty, value); }
}
private static readonly DependencyProperty FileNameProperty =
DependencyProperty.Register("FileName", typeof(string), typeof(DialogHelper),
new UIPropertyMetadata(new PropertyChangedCallback(FileNameProperty_Changed)));
private static void FileNameProperty_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
Debug.WriteLine("DialogHelper.FileName = {0}", e.NewValue);
}
public ICommand OpenFile { get; private set; }
public DialogHelper()
{
OpenFile = new RelayCommand(OpenFileAction);
}
private void OpenFileAction(object obj)
{
OpenFileDialog dlg = new OpenFileDialog();
if (dlg.ShowDialog() == true)
{
FileName = dlg.FileName;
}
}
}
The helper class needs a reference to the ViewModel instance. See the resource dictionary. Just after construction, the ViewModel property is set (in the same line of XAML). This is when the FileName property on the helper class is bound to the FileName property on the view model.
<Window x:Class="DialogExperiment.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:DialogExperiment"
xmlns:vm="clr-namespace:DialogExperimentVM;assembly=DialogExperimentVM"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<vm:ViewModel x:Key="viewModel" />
<local:DialogHelper x:Key="helper" ViewModel="{StaticResource viewModel}"/>
</Window.Resources>
<DockPanel DataContext="{StaticResource viewModel}">
<Menu DockPanel.Dock="Top">
<MenuItem Header="File">
<MenuItem Header="Open" Command="{Binding Source={StaticResource helper}, Path=OpenFile}" />
</MenuItem>
</Menu>
</DockPanel>
</Window>

I use a service which i for example can pass into the constructor of my viewModel or resolve via dependency injection.
e.g.
public interface IOpenFileService
{
string FileName { get; }
bool OpenFileDialog()
}
and a class implementing it, using OpenFileDialog under the hood. In the viewModel, i only use the interface and thus can mock/replace it if needed.

I have solved it for me this way:
In ViewModel I have defined an interface and work with it in
ViewModel
In View I have implemented this interface.
CommandImpl is not implemented in code below.
ViewModel:
namespace ViewModels.Interfaces
{
using System.Collections.Generic;
public interface IDialogWindow
{
List<string> ExecuteFileDialog(object owner, string extFilter);
}
}
namespace ViewModels
{
using ViewModels.Interfaces;
public class MyViewModel
{
public ICommand DoSomeThingCmd { get; } = new CommandImpl((dialogType) =>
{
var dlgObj = Activator.CreateInstance(dialogType) as IDialogWindow;
var fileNames = dlgObj?.ExecuteFileDialog(null, "*.txt");
//Do something with fileNames..
});
}
}
View:
namespace Views
{
using ViewModels.Interfaces;
using Microsoft.Win32;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
public class OpenFilesDialog : IDialogWindow
{
public List<string> ExecuteFileDialog(object owner, string extFilter)
{
var fd = new OpenFileDialog();
fd.Multiselect = true;
if (!string.IsNullOrWhiteSpace(extFilter))
{
fd.Filter = extFilter;
}
fd.ShowDialog(owner as Window);
return fd.FileNames.ToList();
}
}
}
XAML:
<Window xmlns:views="clr-namespace:Views"
xmlns:viewModels="clr-namespace:ViewModels">
<Window.DataContext>
<viewModels:MyViewModel/>
</Window.DataContext>
<Grid>
<Button Content = "Open files.." Command="{Binding DoSomeThingCmd}"
CommandParameter="{x:Type views:OpenFilesDialog}"/>
</Grid>
</Window>

Having a service is like opening up a view from viewmodel.
I have a Dependency property in view, and on the chnage of the property, I open up FileDialog and read the path, update the property and consequently the bound property of the VM