Question

I’m using a TextBox control and want the characters to turn red after a certain number of characters to show users they have typed too much. I don’t want to truncate as the user might have typed that “really important thought” and if I truncated they would lose it. I have validation on my underlying business model which tells me when the input is invalid and I style my TextBox to show invalidity.

I want to Style the text as well. Can I do this using a TextBox, or do I need to go to a RichTextBox? My underlying value is just a straight string.

Answer 1

I assume you’re using the stock WPF validation mechanism – ValidationRules. If so, you should define a Trigger on Validation.HasError == true, and set TextBox properties as needed. For example, the following will highlight text with red if it’s invalid.

<TextBox>
  <TextBox.Text>
    <Binding ...>
      <Binding.ValidationRules>
        ...
      </Binding.ValidationRules>
    </Binding>
  </TextBox.Text>
  <TextBox.Style>
    <Style TargetType="{x:Type TextBox}">
      <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="True">
          <Setter Property="Foreground" Value="Red" />
        </Trigger>
      </Style.Triggers>
    </Style>
  </TextBox.Style>
</TextBox>