Guys this error was occur when i was creating a RSS Feed application.  The error was actually:

A first chance exception of type ‘System.Windows.Markup.XamlParseException’ occurred in System.Windows.ni.dll

Additional information: Cannot create instance of type ‘Lessons_by_Bilal.RssTextTrimmer’ [Line: 11 Position: 90]

As shown below:

XamlparseException occured

Believe me i spend a huge amount of time in resolving the problem and i was just speechless when i resolved it because this error was occurring only because of one missing identifier.

I was having a class with name RssTextTrimmer and in .cs file it was actually declared as:

class RssTextTrimmer : IValueConverter
{
}

and the solution to this error was just add Public before the class name because when i made it public it become accessible to all the files where i created its instances so that’s why the error was occurring.