How to create a WPF validation rule to ensure two text fields have the same value?

Question

How would I create a validation rule to ensure Value2 has the same value as Value1? If a validation rule is not the best method then what would be better? I could have the TextChanged event handle this, but I'm wondering if there is something more elegant.

<TextBox Name="Value1TextBox">
    <TextBox.Text>
        <BindingPath Path="Value1" UpdateSourceTrigger="PropertyChanged" />
    </TextBox.Text>
</TextBox>

<TextBox Name="Value2TextBox">
    <TextBox.Text>
        <BindingPath Path="Value2" UpdateSourceTrigger="PropertyChanged" />
    </TextBox.Text>
</TextBox>

Answer 1

Looks like implementing the System.ComponentModel.IDataErrorInfo interface and adding ValidatesOnDataErrors did the trick.

<TextBox Name="Value2TextBox">    
    <TextBox.Text>        
        <BindingPath Path="Value2" UpdateSourceTrigger="PropertyChanged" ValidatesOnDataErrors="True" />
    </TextBox.Text>
</TextBox>

Answer 2

Validation rules are not too handy, if you need to combine multiple values for your validation. If you have access to the bound datatype, then implementing IDataErrorInfo there will be much easier, and you only need to enable validation on your Binding with ValidateOnDataError=True. If not, you will need to do some work to parametrize your ValidationRule: Either use this workaround Virtual branch pattern by Josh Smith or some binding-proxy as described here (very nice helper, by the way) Binding Proxy by Douglas Stockwell

With one of those you should manage to make one binding be a parameter for your validation rule, when validating the other one.