Validate two TextBox.Text values to be distinct - wpf

Question

Grab two TextBox and say that you need to validate that their content are distinct strings.
Example :
Correct result : prefix1, prefix2
Incorrect result : prefix1, prefix1
To do that task I thought about using a MultiBinding but two problems arises then :
Where should it be placed ? currently it is on a dummy TextBox
Even using that dummy TextBox, the ValidationRule is never called
Not sure whether this approach is correct, how would you do that ?
<MultiBinding Converter="{StaticResource MyConverter}">
<Binding Path="GradientPrefix"
Source="{StaticResource Indices}"
UpdateSourceTrigger="PropertyChanged" />
<Binding Path="ColorPrefix"
Source="{StaticResource Indices}"
UpdateSourceTrigger="PropertyChanged" />
<MultiBinding.ValidationRules>
<gpl2Xaml:DistinctStringValidationRule />
</MultiBinding.ValidationRules>
</MultiBinding>

Answer 1

Here's the solution using a BindingGroup !
Error at BindingGroup level :
Error at BindingGroup and field levels :
No errors :
Here's the code :
<Window>
<Window.Resources>
<gpl2Xaml:Indices x:Key="Indices"
ColorIndex="1"
ColorPrefix="MyColor"
GradientIndex="1"
GradientPrefix="MyColor" />
</Window.Resources>
<Grid DataContext="{StaticResource Indices}"
Style="{StaticResource gridInError}"
Validation.ErrorTemplate="{StaticResource validationTemplate}">
<Grid.BindingGroup>
<BindingGroup>
<BindingGroup.ValidationRules>
<gpl2Xaml:DistinctValidationRule />
</BindingGroup.ValidationRules>
</BindingGroup>
</Grid.BindingGroup>
<TextBox x:Name="TextBoxGradientPrefix"
Style="{StaticResource textBoxInError}"
TextChanged="TextBoxGradientPrefix_OnTextChanged"
Validation.ErrorTemplate="{StaticResource validationTemplate}">
<Binding Path="GradientPrefix"
Source="{StaticResource Indices}"
UpdateSourceTrigger="PropertyChanged">
<Binding.ValidationRules>
<gpl2Xaml:StringValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox>
<TextBox x:Name="TextBoxColorPrefix"
Style="{StaticResource textBoxInError}"
TextChanged="TextBoxColorPrefix_OnTextChanged"
Validation.ErrorTemplate="{StaticResource validationTemplate}">
<Binding Path="ColorPrefix"
Source="{StaticResource Indices}"
UpdateSourceTrigger="PropertyChanged">
<Binding.ValidationRules>
<gpl2Xaml:StringValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox>
</Grid>
</Window>
Extra code to trigger validation every time :
private void TextBoxGradientPrefix_OnTextChanged(object sender, TextChangedEventArgs e)
{
grid.BindingGroup.CommitEdit();
}
private void TextBoxColorPrefix_OnTextChanged(object sender, TextChangedEventArgs e)
{
grid.BindingGroup.CommitEdit();
}
And the validation rule :
public class DistinctValidationRule : ValidationRule
{
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
var bindingGroup = value as BindingGroup;
if (bindingGroup == null) return new ValidationResult(false, "Not a BindingGroup");
var o = bindingGroup.Items[0] as Indices;
if (o == null) return new ValidationResult(false, "Not an Indices");
if (o.ColorPrefix == o.GradientPrefix)
return new ValidationResult(false, "Color prefix and Gradient prefix must be distinct.");
return new ValidationResult(true, null);
}
}