ComboBox binding in Workflow activities

Today I was creating a simple activity with a couple of combo boxes, and came across a problem where the binding wasn’t working correctly (well, at all). I had an InArgument<string> property on my activity, and wanted to set this from a combo box on the design surface. I guess I haven’t done this before as it doesn’t work out of the box, and you need to write an IValueConverter to get it working. I’ve trodden this path before but this was a bit of a tricky beast.

After some wailing and gnashing I came up with the following base class converter…

public abstract class ComboBoxToModelItemConverter<T> : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        T retVal = default(T);

        ModelItem mi = value as ModelItem;

        if (null != mi)
        {
            var arg = mi.GetCurrentValue() as InArgument<T>;

            if (null != arg)
            {
                var expression = arg.Expression;
                var literal = expression as Literal<T>;

                if (null != literal)
                    retVal = literal.Value;
            }
        }

        return retVal;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return new InArgument<T>((T)value);
    }
}

I then created a concrete class to bind to InArgument<string>…

public class ComboBoxStringToModelItemConverter : ComboBoxToModelItemConverter<string>
{
}

With that created I could then get the binding to work by adding the converter to the binding. I have a class that contains TemplateId and Name, I wish to show the Name on screen but store the TemplateId (and this is mapped to an InArgument<string> on the activity). The XAML is as follows…

<local:ComboBoxStringToModelItemConverter x:Key="comboConverter"/>

<ComboBox ItemsSource="{Binding Templates}" 
  SelectedValue="{Binding ModelItem.TemplateId, Converter={StaticResource comboConverter}}" 
  SelectedValuePath="TemplateId" DisplayMemberPath="Name" />

With that done it all works as expected. I’m surprised that a converter such as this doesn’t exist in the Workflow libraries – and also in the many, many years I’ve been playing with Workflow that I’ve not needed one. Anyhow, I hope this is of use to someone!