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 so

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));
    }
}