c# - the namespace prefix "utilities" is not defined

Question

I am fairly new to WPFs... I was trying to bind a validation to a text box in WPF.

Here is my text box in the xaml:

<TextBox>
  <TextBox.Text>
    <Binding Source="{x:Static r:Settings.Default}" Path="ServerIP" UpdateSourceTrigger="PropertyChanged">
      <Binding.ValidationRules>
        <utilities:PingValidationRule />
      </Binding.ValidationRules>
    </Binding>
  </TextBox.Text>
</TextBox>

Here is my validation class:

    public class PingValidationRule : ValidationRule
    { 
        public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
        {
            ...
            if (ipCorrect) return new ValidationResult(true, null);
            else return new ValidationResult(false, "It's not an IP");
        }
    }

With this I have the following error:

Error The namespace prefix "utilities" is not defined.

Anyone knows why?

Answer 1

It is not defined because you did not define it. Look at the top of your XAML file, all the stuff like xmlns:foobar="clr-namespace:Foo.Bar". You need to define it there.