Allow only OneWayToSource binding mode

Question

I have EntitiesUserControl responsible for EntitiesCount dependency property:

public static readonly DependencyProperty EntitiesCountProperty = DependencyProperty.Register(
    nameof(EntitiesCount),
    typeof(int),
    typeof(EntitiesUserControl),
    new FrameworkPropertyMetadata(1, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

public int EntitiesCount
{
    get { return (int)this.GetValue(EntitiesCountProperty); }
    set { this.SetValue(EntitiesCountProperty, value); }
}

Another (primary) control include EntitiesUserControl and read it property through binding:

<controls:EntitiesUserControl EntitiesCount="{Binding CountOfEntities, Mode=OneWayToSource}" />

CountOfEntities property in view model just store and process changing of count value:

private int countOfEntities;
public int CountOfEntities
{
    protected get { return this.countOfEntities; }
    set
    {
        this.countOfEntities = value;
        // Custom logic with new value...
    }
}

I need EntitiesCount property of EntitiesUserControl to be read-only (primary control must not change it, just read) and it works this way only because Mode=OneWayToSource declared explicitly. But if declare TwoWay mode or don't explicitly declare mode, then EntitiesCount could be rewritten from outside (at least right after binding initialization, because it happens after default dependency property value assigned).

I can't do 'legal' read-only dependency property due to binding limitations (best described in this answer), so I need to prevent bindings with mode other than OneWayToSource. It would be best to have some OnlyOneWayToSource flag like BindsTwoWayByDefault value in FrameworkPropertyMetadataOptions enumeration...

Any suggestions how to achieve this?

Answer 1

It's a „bit” hacky, but you can create a Binding-derived class and use that instead of Binding:

[MarkupExtensionReturnType(typeof(OneWayToSourceBinding))]
public class OneWayToSourceBinding : Binding
{
    public OneWayToSourceBinding()
    {
        Mode = BindingMode.OneWayToSource;
    }

    public OneWayToSourceBinding(string path) : base(path)
    {
        Mode = BindingMode.OneWayToSource;
    }

    public new BindingMode Mode
    {
        get { return BindingMode.OneWayToSource; }
        set
        {
            if (value == BindingMode.OneWayToSource)
            {
                base.Mode = value;
            }
        }
    }
}

In XAML:

<controls:EntitiesUserControl EntitiesCount="{local:OneWayToSourceBinding CountOfEntities}" />

The namespace mapping local might be something else for you.

This OneWayToSourceBinding sets the Mode to OneWayToSource and prevents setting it to anything else.