How to instantiate or mock a Window programmatically?

For some of my unit tests I have to call methods that require a Window as parameter. Unfortunately I can't pass null, since the method uses the window as a reference to update the status bar where I show what is actually loaded in the real application.

So I tried to call the constructor in the unit test:

MainWindow window = new MainWindow();

But this results in the following error on the InitializeComponent method of the MainWindow constructor:

System.Windows.Markup.XamlParseException occurred
  Message='Provide value on 'System.Windows.Baml2006.TypeConverterMarkupExtension' threw an exception.' Line number '9' and line position '42'.
  Source=PresentationFramework
  LineNumber=9
  LinePosition=42
  StackTrace:
       at System.Windows.Markup.XamlReader.RewrapException(Exception e, IXamlLineInfo lineInfo, Uri baseUri)
  InnerException: System.IO.IOException
       Message=Assembly.GetEntryAssembly() returns null. Set the Application.ResourceAssembly property or use the pack://application:,,,/assemblyname开发者_开发百科;component/ syntax to specify the assembly to load the resource from.
       Source=PresentationFramework
       StackTrace:
            at MS.Internal.AppModel.ResourceContainer.GetResourceManagerWrapper(Uri uri, String& partName, Boolean& isContentFile)
            at MS.Internal.AppModel.ResourceContainer.GetPartCore(Uri uri)
            at System.IO.Packaging.Package.GetPartHelper(Uri partUri)
            at System.IO.Packaging.Package.GetPart(Uri partUri)
            at System.IO.Packaging.PackWebResponse.CachedResponse.GetResponseStream()
            at System.IO.Packaging.PackWebResponse.GetResponseStream()
            at System.IO.Packaging.PackWebResponse.get_ContentType()
            at System.Windows.Media.Imaging.BitmapDecoder.SetupDecoderFromUriOrStream(Uri uri, Stream stream, BitmapCacheOption cacheOption, Guid& clsId, Boolean& isOriginalWritable, Stream& uriStream, UnmanagedMemoryStream& unmanagedMemoryStream, SafeFileHandle& safeFilehandle)
            at System.Windows.Media.Imaging.BitmapDecoder.CreateFromUriOrStream(Uri baseUri, Uri uri, Stream stream, BitmapCreateOptions createOptions, BitmapCacheOption cacheOption, RequestCachePolicy uriCachePolicy, Boolean insertInDecoderCache)
            at System.Windows.Media.Imaging.BitmapFrame.CreateFromUriOrStream(Uri baseUri, Uri uri, Stream stream, BitmapCreateOptions createOptions, BitmapCacheOption cacheOption, RequestCachePolicy uriCachePolicy)
            at System.Windows.Media.ImageSourceConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
            at System.Windows.Baml2006.TypeConverterMarkupExtension.ProvideValue(IServiceProvider serviceProvider)
            at MS.Internal.Xaml.Runtime.ClrObjectRuntime.CallProvideValue(MarkupExtension me, IServiceProvider serviceProvider)
       InnerException: 

So my question is, how can I instantiate or mock a window in unit testing?

Try the following:

if(Application.ResourceAssembly == null)
    Application.ResourceAssembly = typeof(MainWindow).Assembly;
var window = new MainWindow();

Short answer is that you shouldn't. Unit Testing is not done on UI.

Unit Tests should be run on your logical code, and that's where patterns like MVC and MVVM come to help. The UI should be tested using tools like CodedUI.

if your logic has a dependency on UI, then you are doing it wrong.