MVVM : how to pass parameter to ViewModel's constructor - silverlight

Question

I'm using L. Bugnion's MVVM Light Framework.
What are some of the recommended approaches to pass parameters such as Customer's ID to ViewModel's constructor?
Edit:
The parameter I need for each ViewModel is not something that is shared across models. it is something unique to each viewmodel instance.

Answer 1

//Create a container class to pass via messenger service
public class CarSelectedArgs
{
#region Declarations
public Car Car { get; set; }
#endregion
#region Constructor
public CarSelectedArgs(Car car)
{
Car = car;
}
#endregion
}
//example of view model sending message.
public class SendingViewModel : ViewModelBase
{
private Car _car;
public Car SelectedCar
{
get { return _car; }
set
{
_car = value;
if (value != null)
{
//messenger will notify all classes that have registered for a message of this type
Messenger.Default.Send(new CarSelectedArgs(value));
}
}
}
}
//Example of ViewModel registering to recieve a message
public class SampleViewModel : ViewModelBase
{
#region Constructor
public SampleViewModel()
{
Messenger.Default.Register<CarSelectedArgs>(this, OnCarSelected);
}
#endregion
#region LocalMethods
void OnCarSelected(CarSelectedArgs e)
{
var NewCar = e.Car;
}
#endregion
}

Answer 2

For me the whole point of using MVVM Light is to avoid injecting anything into the constructor of a View Model. MVVM Light provides a Messaging facility that allows you to send your parameters to a listener registered inside of the View Model.
For example, this is my View Model from my WordWalkingStick project using VSTO and WPF:
using System;
using System.Xml.Linq;
using GalaSoft.MvvmLight.Messaging;
namespace Songhay.Wpf.WordWalkingStick.ViewModels
{
using Songhay.Office2010.Word;
using Songhay.OpenXml;
using Songhay.OpenXml.Models;
using Songhay.Wpf.Mvvm;
using Songhay.Wpf.Mvvm.ViewModels;
/// <summary>
/// View Model for the default Client
/// </summary>
public class ClientViewModel : ViewModelBase
{
/// <summary>
/// Initializes a new instance of the <see cref="ClientViewModel"/> class.
/// </summary>
public ClientViewModel()
{
if(base.IsInDesignMode)
{
#region
this._flatOpcSourceString = ApplicationUtility
.LoadResource(
new Uri("/Songhay.Wpf.WordWalkingStick;component/PackedFiles/FlatOpcToHtml.xml",
UriKind.Relative));
this._xhtmlSourceString = ApplicationUtility
.LoadResource(
new Uri("/Songhay.Wpf.WordWalkingStick;component/PackedFiles/FlatOpcToHtml.html",
UriKind.Relative));
#endregion
}
else
{
this._flatOpcSourceString = "Loading…";
this._xhtmlSourceString = "Loading…";
//Receive MvvmLight message:
Messenger.Default.Register(this,
new Action<GenericMessage<TransformationMessage>>(
message =>
{
var tempDocFolder =
Environment.ExpandEnvironmentVariables("%UserProfile%/Desktop/");
var inputPath = tempDocFolder + "temp.docx";
var outputPath = tempDocFolder + "temp.html";
var flatOpcDoc =
XDocument.Parse(message.Content.TransformationResult);
OpenXmlUtility.TransformFlatToOpc(flatOpcDoc, inputPath);
this.FlatOpcSourceString = flatOpcDoc.Root.ToString();
var settings = new SonghayHtmlConverterSettings()
{
PageTitle = "My Page Title " + DateTime.Now.ToString("U"),
UseEntityMap = false
};
OpenXmlUtility.WriteHtmlFile(inputPath, outputPath, settings);
var xhtmlDoc = XDocument.Load(outputPath);
this.XhtmlSourceString = xhtmlDoc.Root.ToString();
}));
}
}
/// <summary>
/// Gets or sets the flat opc source string.
/// </summary>
/// <value>The flat opc source string.</value>
public string FlatOpcSourceString
{
get
{
return _flatOpcSourceString;
}
set
{
_flatOpcSourceString = value;
base.RaisePropertyChanged("FlatOpcSourceString");
}
}
/// <summary>
/// Gets or sets the XHTML source string.
/// </summary>
/// <value>The XHTML source string.</value>
public string XhtmlSourceString
{
get
{
return _xhtmlSourceString;
}
set
{
_xhtmlSourceString = value;
base.RaisePropertyChanged("XhtmlSourceString");
}
}
string _flatOpcSourceString;
string _xhtmlSourceString;
}
}
You can see that MVVM Light is messaging (not injecting) values into the constructor (Messenger.Default.Register) with its Messenger.

Answer 3

Request anything you want, via injection, using interfaces.
If you have settings shared across models, instantiate a singleton containing the values and expose them via ISomethingProvider and ISomethingEditor interfaces.

Answer 4

Here is what I do:
ViewModel needs to show a car window with car id passed as parameter:
ViewModel -> message to codebehind for view to open window. Message sends id.
Essentially in code behind:
var vm = new viewmodel(id);
var view = new view();
view.datacontext = vm;
view.show();
my viewmodel has a constructor that takes in an id.

Answer 5

In the case of writing tests against the viewmodel I sometimes create an overload of the viewmodel constructor that takes an ISomething as a parameter. I have the default constructor call the second one with a default implementation of ISomething. In case of the test I call the constructor with a test implementation. I know it's not the best method, because it creates a dependency between the two classes... but sometimes you'll have to take the easy path...
public class SomeViewModel
{
private ISomething internalSomething;
public void SomeViewModel():this(new Something()){}
public void SomeViewModel(ISomething something)
{
this.internalSomething = something;
}
}
Update
Creating a view in xaml can be like this:
<UserControl xmlns="...."
xmlns:Example="SomeNamespace">
<UserControl.DataContext>
<Example:SomeViewModel />
</UserControl.DataContext>
<Grid>
...
</Grid>
</UserControl>