Setting UserControl ViewModel Property - wpf

Question

All -
I am using Unity in my WPF application for DI (without prism). I have my MainWindow.xaml and MainWindowViewModel.cs. I have a usercontrol in my Mainwindow.xaml. The user control has its own uc1.xaml and uc1viewmodel.cs. The UC1 ViewModel is currently exposed as a property on MainWindowViewModel so I can set the datacontext on the usercontrol (as recommended by many ppl here).
The question I have is how/where can I set this property - will it be in app.xaml.cs or will it be in the constructor of mainwindowviewmodel. Code Snippets:
App.xaml.cs
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
//Step 1 - One Time - Creating an instance of the container
UnityContainer unity = new UnityContainer();
//Step 2 - Registering your MainWindowViewModel
unity.RegisterType<IViewModel, UserControl1ViewModel>();
//Step 3 - Creating an Instance
UserControl1ViewModel uc1_mwvm = unity.Resolve<UserControl1ViewModel>(); <-- doesnt help
MainWindowViewModel mwvm = unity.Resolve<MainWindowViewModel>();
MainWindow mw = unity.Resolve<MainWindow>();
mw.Show();
}
MainWindowViewModel.cs
public class MainWindowViewModel
{
public IViewModel IVM { get; set; }
public MainWindowViewModel()
{
//IVM = new UserControl1ViewModel(); <-- All I really want is an equivalent but letting Unity do the work.
}
}
MainWindow.xaml
<Window x:Class="_05_ViewFist_UC_Unity_Working.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:uc1="clr-namespace:_05_ViewFist_UC_Unity_Working"
xmlns:uc2="clr-namespace:_05_ViewFist_UC_Unity_Working"
Title="MainWindow" Height="350" Width="525">
<StackPanel Orientation="Vertical">
<TextBlock Text="{Binding NNN}" />
<uc1:UC1 DataContext="{Binding UC1VM}" />
<uc2:UC2 DataContext="{Binding UC2VM}" />
</StackPanel>
</Window>
UC1
<UserControl x:Class="_05_ViewFist_UC_Unity_Working.UC1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300" >
<StackPanel Orientation="Horizontal" Background="Red">
<TextBlock Text="UC1 " />
<TextBlock Text="{Binding FirstName}" />
</StackPanel>
</UserControl>
As you see from the code - Instance of UC1 is created in xaml (MainWindow.xaml) and hence when MainWindow instance is created in app.xaml.cs - it still doesnt create an instance of UserControl1ViewModel.
Question again is : Dont think its a good practice for me to call the Unity Resolve statement in the constructor of MainwindowViewModel. Is that correct??
Can somebody share a code snippet of how/where I can do this?
Thanks

Answer 1

I downloaded your solution from github and tried to solve your problem.
You did a great job just you forgot few details such as property attributes.
This is how your App.cs file shall look alike:
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
//Step 1 - One Time - Creating an instance of the container
UnityContainer unity = new UnityContainer();
//Step 2 - Registeration
unity.RegisterType<IMainWindowViewModel, MainWindowViewModel>();
unity.RegisterType<IUC1ViewModel, UC1ViewModel>();
unity.RegisterType<IUC2ViewModel, UC2ViewModel>();
//// Instance of MainWindowViewModel will be created once you call Resolve MainWindow.
MainWindow mw = unity.Resolve<MainWindow>();
mw.Show();
}
Here is what I changed:
public class MainWindowViewModel : IMainWindowViewModel
{
#region Public Properties
[Dependency]
public IUC1ViewModel UC1VM { get; set; }
[Dependency]
public IUC2ViewModel UC2VM { get; set; }
public string NNN { get; set; }
#endregion
public MainWindowViewModel()
{
NNN = "This value coming from MainWindowViewModel";
}
}
[Dependency] is a property attibute that tells Unity where to inject values.
I could merge my code to your repo in github if you wish so.
Let me know if this helped you any futher. Feel free to mark this as the answer.

Answer 2

You can use the service locator pattern. I use it with Unity as a DI.
internal class ServiceLocator
{
[...]
public MainViewModel Main { get { return container.Resolve<MainViewModel>(); } }
}
You can intantiate your class the way you want (DI or not, the class initializes the DI or receive it as a parameter, you can store the DI in a private static property, you can initialize your class if DI is null or when the application starts etc...).
In your App.xaml
<Application.Resources>
<vm:ServiceLocator x:Key="Locator"/>
</Application.Resources>
And now, you can set your datacontext
DataContext="{Binding Main, Source={StaticResource Locator}}"
Edit:
I found another way of doing it (among other):
Take a look at this article. In the command, you can resolve your viewmodel as you like.