gistfile1.txt

//
// if the textblock is in a managed control like the grid,
// its legal to set FontWeights with an int.
//

Grid grid = XamlReader.Load (@"<Grid xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
				    xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"">
                    		       	<TextBlock x:Name=""the_box"" FontWeight=""600"" />
			       </Grid>") as Grid;

Assert.IsNotNull (grid, "a1");

TextBlock block = grid.FindName ("the_box") as TextBlock;

Assert.IsNotNull (block, "a2");
Assert.AreEqual (FontWeights.SemiBold, block.FontWeight, "a3");


//
// But we can't just do this normally.
//

string bad_xaml = @"<TextBlock xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
			       xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"">
			FontWeight=""600"" />";
Assert.Throws<XamlParseException> (() => XamlReader.Load (bad_xaml));