How to bind DataTemplate datatype to interface?

Question

I am writing a composite loosely coupled MVVM WPF application and child VMs in a parent VM are interfaces rather than class instances, e.g.

public IChildViewModel { get; set; }

Now how do I render this property using a DataTemplate? like:

<DataTemplate DataType="{x:Type contracts:IChildViewModel}">

I understand due to the nature of interfaces (multiple inheritance etc.) WPF does not allow this direct binding. But as interfaces should be used widely in loosely coupled applications, is there any workaround to bind DataTemplate to interfaces? Thanks.

Answer 1

You can bind to interfaces by telling wpf explicitly that you are binding to an interface field:

(Please note that ViewModelBase is simply a base-class that implements the INotifyPropertyChanged interface)

public class Implementation : ViewModelBase, IInterface
{
    private string textField;

    public string TextField
    {
        get
        {
            return textField;
        }
        set
        {
            if (value == textField) return;
            textField = value;
            OnPropertyChanged();
        }
    }
}

public interface IInterface
{
    string TextField { get; set; }
}

Then on the ViewModel:

private IInterface interfaceContent;
public IInterface InterfaceContent
{
    get { return interfaceContent; }
}

And finally the Xaml that makes it possible:

<ContentControl Grid.Row="1" Grid.Column="0" Content="{Binding InterfaceContent}">
    <ContentControl.ContentTemplate>
        <DataTemplate DataType="{x:Type viewModels:IInterface}">
            <TextBox Text="{Binding Path=(viewModels:IInterface.TextField)}"/>
        </DataTemplate>
    </ContentControl.ContentTemplate>
</ContentControl>

As you can see, the binding refers explicitly to the 'IInterface' definiton.

Answer 2

You can convert your interface to an equivalent abstract class. It works in this way.