Visual Studio Visualizer for IEnumerable<string>

Question

TL;DR
Does anyone know how to write a Debug Visualizer for Visual Studio 2012, in C#, so I can visualize IEnumerable<string>, string[] or similar objects?

More Info
Visual Studio Debug Visualizer are great, and I use some popular ones (Mole) regularly. However, now the time has come to roll out some custom visualizers. I started off with a simple visualizer for a string:

[assembly: System.Diagnostics.DebuggerVisualizer(typeof(My.Namespace.DebuggerSide),
                                            typeof(VisualizerObjectSource),
                                            Target = typeof(string),
                                            Description = "Awesome Visualizer")]

the code of DebuggerSide is basically the example from the template:

public class DebuggerSide : DialogDebuggerVisualizer
{
    protected override void Show(IDialogVisualizerService windowService, IVisualizerObjectProvider objectProvider)
    {
        if (windowService == null)
            throw new ArgumentNullException("windowService");
        if (objectProvider == null)
            throw new ArgumentNullException("objectProvider");


        var data = (string)objectProvider.GetObject();

        using (var displayForm = new VisualizerForm(data))
        {
            windowService.ShowDialog(displayForm);
        }
    }

    /// <summary>
    /// Tests the visualizer by hosting it outside of the debugger.
    /// </summary>
    /// <param name="objectToVisualize">The object to display in the visualizer.</param>
    public static void TestShowVisualizer(object objectToVisualize)
    {
        VisualizerDevelopmentHost visualizerHost = new VisualizerDevelopmentHost(objectToVisualize, typeof(DebuggerSide));
        visualizerHost.ShowVisualizer();
    }
}

VisualizerForm is the custom form with extra controls etc... when I build the project and put the dll in the My Documents/Visual Studio 11/Visualizers folder, and restart visual studio, I can indeed see the debugger appearing under the looking glass icon when a breakpoint is hit for a string object. Woohoo! So far so good.

Now I would like to, instead of visualizing string, visualize string[] or IEnumerable<string> or a similar object. However when I change the assembly attribute to IEnumerable<string>, this is not working, there is not even a looking glass icon displayed on the IEnumerable objects.

UPDATE
I can get it to work by changing the TargetType to List<> and then checking if I can cast to List<string>. However, this means I have to cast all my objects I want to debug to List and can't use IEnumerable<> or string[]

Answer 1

Visualizers are documented as

Support for generic types is limited. You can write a visualizer for a target that is a generic type only if the generic type is an open type.

Which means you cannot write a visualizer that uses a closed constructed type like IEnumerable<string>.

Have you tried setting the target type to IEnumerable<> then checking to see if the elements are of type string?