Set a ValidationError by Code

A ValidatioRule in a BindingExpression only triggers, when the target side of the binding changes. If you want to set a validation error by code you can use the following snippet.

Set the validation error

 
ValidationError validationError = 
    new ValidationError(regexValidationRule, 
    textBox.GetBindingExpression(TextBox.TextProperty));
 
validationError.ErrorContent = "This is not a valid e-mail address";
 
Validation.MarkInvalid(
    textBox.GetBindingExpression(TextBox.TextProperty), 
    validationError);
 
 

Clear the validation error