MVVM binding command to contextmenu item

问题:

I'm trying to bind a command to a menuitem in WPF. I'm using the same method that's been working for all my other command bindings, but I can't figure out why it doesn't work here.

I'm currently binding my commands like this:

Command = "{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserControl}}, Path=DataContext.MyCommand}" 

This is where it goes wrong (this is inside a UserControl)

             ... 

The first command binding works like it should, but the second one refuses to do anything. I've tried changing the ancestor level and naming my Control to access it through ElementName instead of RelativeSource, but still no change. It keeps saying "Cannot find source for binding with reference..."

What am I missing?

回答1:

(Edit) Since you mentioned this is in an ItemsControl's template, things are different:

1) Get the BindingProxy class from this blog (and read the blog, as this is interesting information): How to bind to data when the DataContext is not inherited.

Basically the elements in the ItemsControl (or ContextMenu) are not part of the visual or logical tree, and therefore cannot find the DataContext of your UserControl. My apologies for not writing more on this here, but the author has done a good job explaining it step by step, so there's no way I could give a complete explanation in just a few lines.

2) Do something like this: (you may have to adapt it a bit to make it work in your control):

a. This will give you access to the UserControl DataContext using a StaticResource:

b. This uses the DataContextProxy defined in (a):

This has worked for us in things like trees and datagrids.

回答2:

ContextMenu is in different logical tree, that's why RelativeSource doesnt work. But context menu inherit DataContext from its "container", in this case it is Button. It is enough in common case but in your case you need two "data contexts", of ItemsControl item and of ItemsControl itself. I think you have no other choice but combine your view models into one, implement custom class to be used as ItemsControl item data context and contain both "Name" and "Remove command" or your item's view model can define RemoveCommand "proxy", that would call parent command internally

EDIT: I slightly changed Baboon's code, it must work this way:

回答3:

koshdim is spot on, it works like a charm!! Thanks Koshdim

I modified his code to fit in my context menu

回答4:

That's a tricky issue, sure marginally you will find a quick workaround, but here is a no-magic-solution:

           ... 

It boils down to using the Tag of the PlacementTarget (the Button here).

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