Binding to an internal property?

Question

I am trying some different things using MVVM. In our ViewModel properties which are bind to View are public. I am taking example of a button binding. Here is a simple sample.

View.xaml:

<Button Content="Test Button" Command="{Binding TestButtonCommand}" />

ViewModel.cs

private ICommand _testButtonCommand;
public ICommand TestButtonCommand
{
    get { return _testButtonCommand?? (_testButtonCommand= new RelayCommand(SomeMethod)); }
}

Here my question is that can we make TestButtonCommand internal instead of public? Internal means it is accessible to current project so their should not be any problem doing that? But when I tried to do that it didn't worked. Adding a breakpoint in getter was not hit. So why we cannot make it internal?

Here is the link from msdn.

http://msdn.microsoft.com/en-us/library/ms743643.aspx

The properties you use as binding source properties for a binding must be public properties of your class. Explicitly defined interface properties cannot be accessed for binding purposes, nor can protected, private, internal, or virtual properties that have no base implementation.

Why we cannot do this?

In case of access internal is same as public if working in the same project. Then why we cannot use internal here? There must be a reason that these should be public, and I am looking for that reason.

internal ICommand TestButtonCommand { ...... }

Answer 1

In case of access internal is same as public if working in the same project. Then why we cannot use internal here. There must be a reason that these should be public, and I am looking for that reason.

You have part of your answer in your question itself in the quote from Microsoft:

The properties you use as binding source properties for a binding must be public properties of your class.

Presumably / speculatively, the reason for this is that internals can only be accessed within the same assembly and not from outside. Binding to internals doesn't work because binding is resolved by the WPF binding engine which is in a separate assembly PresentationFramework.dll.

Answer 2

Binding is only supported for public properties. MSDN reference:

http://msdn.microsoft.com/en-us/library/ms743643.aspx

As quoted in the reference

The properties you use as binding source properties for a binding must be public properties of your class. Explicitly defined interface properties cannot be accessed for binding purposes, nor can protected, private, internal, or virtual properties that have no base implementation.