unit testing custom OnRender-Method

Question

I have a custom control MyControl which inherits from Canvas and which has its own logic inside the OnRender-Method. It should draw some rectangles and text into the drawing context.

public class MyControl : Canvas
{
    protected override void OnRender(DrawingContext dc)
    {
        // do something like dc.DrawRectangle(...);
        // do something like dc.DrawText(...);
    }
}

Since I have to develop test-driven, I want to unit test the OnRender-Method. I tried several solutions which did not work. All of them required an inherited class for testing purpose, which I will call TestingMyControl and which exposes the OnRender-Method in the follwing form:

public class TestingMyControl : MyControl
{
    public void Render(DrawingContext dc)
    {
        base.OnRender(dc);
    }
}

1. Since DrawingContext is an abstract class, I thought I could implement a test class which implements DrawingContext. I could then collect all the rectangles and text which it should draw, and make assertions against this. Problem: DrawingContext has an internal constructor, so I cannot inherit from it.

2. Since DrawingContext is an abstract class, I thought I could create a Mock of it. Problem: it has some internal abstract Methods, so Rhino-Mocks could not create a proxy, since it cannot implement these internal abstract Methods.

So I have the problem, that I cannot test the OnRender-Method, since I cannot create an Instance of DrawingContext...

Answer 1

The solution is, to create the DrawingContext out of an DrawingGroup.

public class TestingMyControl : MyControl
{
    public DrawingGroup Render()
    {
        var drawingGroup = new DrawingGroup();
        using (var drawingContext = drawingGroup.Open())
        {
             base.OnRender(drawingContext);
        }

        return drawingGroup;
    }
}

So the fixture will look like:

    [Test]
    public void Should_render()
    {
        var controlToTest = new TestingMyControl();

        var drawingGroup = controlToTest.Render();

        var drawing = drawingGroup.Children[0] as GeometryDrawing;
        Assert.That(drawing.Brush, Is.EqualTo(Brushes.Black));
        Assert.That(drawing.Pen.Brush, Is.EqualTo(Brushes.SeaGreen));
        Assert.That(drawing.Pen.Thickness, Is.EqualTo(0.6));
        Assert.That(drawing.Bounds.X, Is.EqualTo(5));
        Assert.That(drawing.Bounds.Y, Is.EqualTo(15));
        Assert.That(drawing.Bounds.Width, Is.EqualTo(25));
        Assert.That(drawing.Bounds.Height, Is.EqualTo(35));
    }

This requires the follwing production code:

public class MyControl : Canvas
{
    protected override void OnRender(DrawingContext dc)
    {
        dc.DrawRectangle(Brushes.Black, new Pen(Brushes.SeaGreen, 0.6), new Rect(5, 15, 25, 35));
    }
}