public static readonly DependencyProperty ValueProperty =
    DependencyProperty.Register
    ( "Value"
    , typeof(T)
    , typeof(ValidatingTextBox<T,C>)
    , new FrameworkPropertyMetadata(default(T), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault)
    );

public static T MyDefault<T>(){

    switch(typeof(T)){
        case (typeof(String)) : return "";
        case (typeof(Foo)) : return new Foo();
    }

    }

DefaultGenerator<T>.Default

private static Dictionary<Type,Func<object>> factory = new Dictionary<Type,Func<object>>{
        {typeof(string), ()=> String.Empty },
        {typeof(MyClass), ()=> new MyClass() },
    };
public static T MyDefault<T>()
{

    Type t = typeof(T);
    Func<object> func = null;

    if(factory.TryGetValue(t, out func))
        return (T)func();
    else
        return default(T);
}