Question

I created and initialized quite a large Dictionary and got this error when starting the application:

An unhandled exception of type 'System.Windows.Markup.XamlParseException' occurred in      PresentationFramework.dll

Additional information: Zeilennummer "3" und Zeilenposition "9" von "Durch den Aufruf des     
Konstruktors für Typ "AbiZeitung.MainWindow", der den angegebenen Bindungseinschränkungen 
entspricht,    wurde eine Ausnahme ausgelöst.".

If there is a handler for this exception, the program may be safely continued.

Here is a part of the Dictionary:

Dictionary<string, string> teacherList = new Dictionary<string, string>()
    {
        {"Mr", "M"},
        {"Mr", "D"},
        {"Mr", "S"},
        {"Mr", "W"}
    };

Any guess why this happens?

Answer 1

A Dictionary<T,U> needs to have unique keys. Right now, your collection initializer will raise an exception (ArgumentException) when it tries to add {"Mr","D"}, as “Mr” is already a key.

For details, see Dictionary.Add, under exceptions:

ArgumentException: An element with the same key already exists in the Dictionary.

If your View creates this dictionary, even indirectly (ie: it’s in a ViewModel generated from the xaml), you’ll get an exception, which in turn will cause the XamlParseException to occur.

If you need a collection like this, I recommend making an ObservableCollection<T> (or List<T> if this won’t change) of a custom type which contains the two strings.