设置动态生成的WPF画布的背景颜色？

Question

I have a dynamically created WPF Canvas element inside a ContentControl (the same thing happens in a UserControl .) Whenever I attempt to set the background, either upon creation or later on in the program, the background color won't paint (I am however able to set the background on a Label inside of the Canvas (also added dynamically.) The creation code I have looks like:

_rootGrid = new Grid();
_rootGrid.Name = "_rootGrid";
_rootGrid.Margin = new Thickness(0);
_rootGrid.HorizontalAlignment = HorizontalAlignment.Stretch;
_rootGrid.VerticalAlignment = VerticalAlignment.Stretch;
_rootGrid.RowDefinitions.Add(new RowDefinition());
_rootGrid.RowDefinitions.Add(new RowDefinition());
_rootGrid.ColumnDefinitions.Add(new ColumnDefinition());
_rootGrid.RowDefinitions[0].Height = new GridLength(24);
_rootGrid.RowDefinitions[1].Height = new GridLength(0, GridUnitType.Star);
_rootGrid.ColumnDefinitions[0].Width = new GridLength(0, GridUnitType.Star);

_headerBlock = new Canvas();
_headerBlock.Name = "_headerBlock";
_headerBlock.SetValue(Grid.RowProperty, 0);
_headerBlock.SetValue(Grid.ColumnProperty, 0);
_headerBlock.PreviewMouseLeftButtonUp += _headerBlock_PreviewMouseLeftButtonUp;
_headerBlock.Background = Brushes.Red;

_title = new Label();
_title.Name = "_title";
_title.Content = "Title";
_title.VerticalAlignment = VerticalAlignment.Center;
_title.HorizontalAlignment = HorizontalAlignment.Left;
_title.FontWeight = FontWeights.Bold;
_title.Background = Brushes.Blue;

_clientBlock = new ScrollViewer();
_clientBlock.Name = "_clientBlock";
_clientBlock.Margin = new Thickness(0);
_clientBlock.HorizontalScrollBarVisibility = ScrollBarVisibility.Auto;
_clientBlock.VerticalScrollBarVisibility = ScrollBarVisibility.Auto;
_clientBlock.SetValue(Grid.RowProperty, 1);
_clientBlock.SetValue(Grid.ColumnProperty, 0);

_clientArea = new Grid();
_clientArea.Name = "_clientArea";
_clientArea.HorizontalAlignment = HorizontalAlignment.Stretch;
_clientArea.VerticalAlignment = VerticalAlignment.Stretch;

_headerBlock.Children.Add(_title);
_rootGrid.Children.Add(_headerBlock);
_clientBlock.Content = _clientArea;
_rootGrid.Children.Add(_clientBlock);
base.Content = _rootGrid;

And is called inside of the ContentControl constructor. From that I would expect the header to contain a full row of Red with a Blue rectangle around the text, but all I get is the Blue rectangle around text with most of the row left Transparent (noticeable due to the Green background of the root Grid .) Any help on this would be appreciated as it is enormously frustrating. I'm using version 6.2 of the .NET framework on Windows 7 if that plays into it (I have noticed some other odd behaviors, but am going for dynamic generation mostly because these ContentControl s take lots of child elements and the VS 2017 XAML parser is too broken to allow them to be named - which makes them virtually useless.)

Answer 1

The solution is to use non-zero Width for ColumnDefinition:

_rootGrid.ColumnDefinitions[0].Width = new GridLength(1, GridUnitType.Star); // equal to Width="*" in xaml

When 0 is used, Canvas has 0 witdh. But is is possible to see blue Label because Canvas doesn't clip contents on its bounds.

If you try Grid ( var _headerBlock = new Grid(); ) with zero width column, there won't be anything displayed at all.