我不能使用Visual Studio Designer，无效的标记？

Question

我使用Visual Studio 2017.IDE告诉我

The type or namespace name 'SalamanderWindow' does not exist in the namespace 'SalamanderWnmp.UserClass' 

SalamanderWindow.cs

public class SalamanderWindow : Window
{
    public SalamanderWindow()
    {
        this.Style = Application.Current.Resources["BaseWindow"] as Style;
        this.SourceInitialized += delegate (object sender, EventArgs e)
        {
            this._HwndSource = PresentationSource.FromVisual((Visual)sender) as HwndSource;
        };
    }
    //...
}