Question

I wrote a really simple ViewModel Locator :

public static readonly DependencyProperty LocateForProperty =
    DependencyProperty.RegisterAttached("LocateFor", 
    typeof (Object), 
    typeof (ViewModelLocator), 
    new PropertyMetadata(null, OnLocateForChanged));

public static object GetLocateFor(DependencyObject dependencyObject)
{
    return dependencyObject.GetValue(LocateForProperty);
}

public static void SetLocateFor(DependencyObject dependencyObject, Object value)
{
    dependencyObject.SetValue(LocateForProperty, value);
}

private static void OnLocateForChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs eventArgs)
{
    if (dependencyObject != null)
    {
        var viewModelName = dependencyObject.GetType().AssemblyQualifiedName;
        if (! string.IsNullOrEmpty(viewModelName))
        {
            viewModelName = viewModelName.Replace("View", "ViewModel");
            var viewModel = DependencyLocator.GetInstance(Type.GetType(viewModelName));
            if (viewModel != null)
            {
                ((FrameworkElement) dependencyObject).DataContext = viewModel;
            }
        }
    }
}

And use it like so :

<vm:ViewModelLocator.LocateFor>
    <Binding RelativeSource="{RelativeSource Self}" />
</vm:ViewModelLocator.LocateFor>

It works perfect at runtime but I have lost design time location. I suspect it is because I am using the relative binding, I tried changing it to x:Static but that doesn’t work. Are there any suggested changes I could make that would give me back Blendability?

EDIT : About DependencyLocator, it is a static wrapper around Ninject:

using System;
using System.Collections.Generic;

/// <summary>
/// Defines an object that wraps around specific DI containers.
/// </summary>
public static class DependencyLocator
{
    /// <summary>
    /// Gets or Sets the function used for locating a single instance of a specifc type.
    /// </summary>
    public static Func<Type, object> GetInstance;

    /// <summary>
    /// Gets or Sets the function used for locating all instance of a specific type.
    /// </summary>
    public static Func<Type, IEnumerable<object>> GetAllInstances;

    /// <summary>
    /// Gets the implementation of the provided service.
    /// </summary>
    public static T Get<T>()
    {
        return (T)GetInstance(typeof(T));
    }

    /// <summary>
    /// Gets all implementations of the provided service.
    /// </summary>
    public static IEnumerable<T> GetAll<T>()
    {
        return (IEnumerable<T>) GetAllInstances(typeof (T));
    }
}

Which gets populated in an app_start class in app.xaml. It was designed like this for design time purposes and if I make a simple view model locator class using an instanced object defined in the View’s xaml it does indeed get the correct values from DependencyLocator, it is only since I switched to Dependency Properties that this issue came up.

Answer 1

This code is just clarifying what you are trying to achieve:

// Use this to trigger break into debugger when debugging another instance of Visual Studio in order to debug the behaviour at design time.
System.Diagnostics.Debugger.Break();

if (dependencyObject != null)
{
    var view = GetLocateFor( dependencyObject ); // or instead of this access the changed value through the eventargs

    var viewModelName = view.GetType().AssemblyQualifiedName;

    if (! string.IsNullOrEmpty(viewModelName))
    {
        viewModelName = viewModelName.Replace("View", "ViewModel");
        var viewModel = DependencyLocator.GetInstance(Type.GetType(viewModelName));
        if (viewModel != null)
        {
           ((FrameworkElement) view ).DataContext = viewModel;
        }
    }
}