与XAML相比，有必要指定完整的

In contrast to XAML, it is necessary to specify the full Resource File Pack URI in code behind:

mDiag.Icon = new System.Windows.Controls.Image
{
    Source = new BitmapImage(new Uri(
        "pack://application:,,,/assets/checked-32-context.png"))
};

还请注意，这不是相对URI.

Note also that this is not a relative URI.