Public Shared Function GetRelativeUri ( _
    sourcePartUri As Uri, _
    targetPartUri As Uri _
) As Uri

Dim sourcePartUri As Uri
Dim targetPartUri As Uri
Dim returnValue As Uri

returnValue = PackUriHelper.GetRelativeUri(sourcePartUri, _
    targetPartUri)

public static Uri GetRelativeUri(
    Uri sourcePartUri,
    Uri targetPartUri
)

public:
static Uri^ GetRelativeUri(
    Uri^ sourcePartUri, 
    Uri^ targetPartUri
)

public static function GetRelativeUri(
    sourcePartUri : Uri, 
    targetPartUri : Uri
) : Uri

You cannot use methods in XAML.