Should I use the targetType parameter of the IValueConverter

Question

Every time I am writing a value converter I'm wondering how to handle the targetType parameter specified at the Convert and ConvertBack method of the IValueConverter :

object Convert(object value, Type targetType, object parameter, CultureInfo culture)
object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)  

I noticed that many implementations are using the targetType simply by checking it ( if (targetType == typeof(RequiredType)) ) and throwing some exceptions or returning a null value if the type is not like expected:

• Extended WPF Toolkit
• Silverlight 2 Recipes
• Infragistics

But also many of them (I think it's the majority) are totally ignoring the parameter:

• WPF tutorial
• Dot net tricks
• Code project

I tried to find some guidlines or tips using the parameter but unfortunately I found absolutely nothing. Both approaches are doing their jobs, but is one of the approaches more correct? What are the pros and cons of the approaches? Should I ignore or use the parameter?

My personal view is not to check the parameter to allow the usage of the converter more flexible but I'm excited to get your opinions.

Answer 1

The caller will be telling you what is the desired type to convert the value:

1. If you know you always will convert to one type, eg true/false to Visibility , then you can just ignore the targetType . It's obvious how to use the converter, there is no need for trowing exceptions.
2. Converting the value based on the targetType , eg using the converter to convert to ImageSource or Brush . In the first case you want to generate an image (eg error image), in the other you want a simple color (eg red).
3. If you know that someone might use your converter with a targetType you don't want to and it's not obvious if it's supported or not, then you can warn the developer by throwing an exception.

I think most common scenario is the 1).