在 WPF 的 ValidationRules 中设置自定义错误消息

Question

I have set everything perfectly. If I set some string in ErrorMessage then it shows without error.

What I want is, I want to set ErrorMessage dynamically/programmatically. something

MyValidation.ErrorMessage = "some new message";
username.Update() //something

XAML Code

<TextBox Margin="5" Name="userName">
   <TextBox.Text>
      <Binding RelativeSource="{RelativeSource Self}" Path="Tag" Mode="OneWayToSource" UpdateSourceTrigger="PropertyChanged">
         <Binding.ValidationRules>
            <local:MyValidation ErrorMessage="Static String" />
         </Binding.ValidationRules>
      </Binding>
   </TextBox.Text>
</TextBox>

C# Class Code

public class MyValidation : ValidationRule {
   public string ErrorMessage { get; set; }

   public override ValidationResult Validate(object value, CultureInfo cultureInfo) {
      if (ErrorMessage.Length > 0) {
         return new ValidationResult(false, ErrorMessage);
      }
      return ValidationResult.ValidResult;
   }
}

Answer 1

You can implement INotifyDataErrorInfo in your viewmodel. Implement GetErrors(string) so it returns different error messages based on your condition. You can even return multiple messages at once and they will be displayed below each other.

Here's a nice tutorial , but feel free to implement it on your own. Keep in mind that there's not just one correct approach and the interface gives you a lot of freedom.

Answer 2

If you give the ValidationRule a name in the XAML markup:

<Binding.ValidationRules>
    <local:MyValidation x:Name="val" ErrorMessage="Static String" />
</Binding.ValidationRules>

...you could set its ErrorMessage property directly and then just explicitly update the binding:

val.ErrorMessage = "some new message";
userName.GetBindingExpression(TextBox.TextProperty)?.UpdateSource();