Question

I’ve got a button that I need to be disabled when validation errors occur in my window. The items on which these errors can occur are all textboxes.

I’ve bound my Button’s datacontext as such:

DataContext="{Binding ElementName=txtEmail}"

Now with this, I can set the button style to disabled when validation errors occur in the email textbox, but I want to do it also when it occurs in other textboxes in my window?

How can I set this binding to multiple textboxes?

Answer 1

You can’t, at least not directly. You could use a MultiBinding with all of the desired text boxes as inputs, but you will need to provide an IMultiValueConverter to “combine” the various text boxes into one object (such as a list):

<Button>
  <Button.DataContext>
    <MultiBinding Converter="{StaticResource ListMaker}">
      <Binding ElementName="txtEmail" />
      <Binding ElementName="txtFirstName" />
      <Binding ElementName="txtLastName" />
    </MultiBinding>
  </Button.DataContext>
</Button>

And it is then that resulting list object that will be passed to your trigger, so you won’t be able to access the Validation.HasError property directly: your DataTrigger will also need to bring in a converter which converts the list object into a boolean indicating whether Validation.HasError is set for anything in the list. At this point you might as well just forget about triggers and bind IsEnabled using a MultiBinding:

<Button>
  <Button.IsEnabled>
    <MultiBinding Converter="{StaticResource AllFalse}">
      <Binding Path="(Validation.HasError)" ElementName="txtEmail" />
      <Binding Path="(Validation.HasError)" ElementName="txtFirstName" />
      <Binding Path="(Validation.HasError)" ElementName="txtLastName" />
    </MultiBinding>
  </Button.DataContext>
</Button>

(Here the AllFalse converter returns true if all inputs are false, and false if any input is true.)

A better approach, however, may be, instead of binding the Button directly to other UI elements, have your data object — the same object that your text boxes are binding to — expose an IsValid property (with suitable change notifications), and bind your Button.IsEnabled to that:

<Button IsEnabled="{Binding IsValid}" />

This moves you towards a MVVM-style solution which helps with things like testability (e.g. it’s easy to create tests for the IsValid property; it’s much harder to create tests for Button.IsEnabled).