Question

I’m trying to find a simple way of binding a single control (eg. TextBlock or ListBox) to list all the validation errors on a WPF form. Most source code examples I have been able to find just bind a control to (Validation.Errors)[0].ErrorContent which only shows a single validation error.

I’m currently using ValidationRule classes, though I’m open to using IDataErrorInfo, or building a custom validator. I’m just not sure how to accomplish what I would expect to be a common use-case.

How do I list all validation errors in one control on a WPF form?

Answer 1

I don’t think you can do anything like this using (Validation.Errors) binding. The reason is that Validation attached property provides one-to-one relationship between a bound control and an adorner site, so you just cannot combine validation errors from different controls in one adorner – the last one would always “take over” the site. BTW, I have no idea why Validation.Errors is an array – maybe for multiple errors from the same control?

But there is still hope – you have at least two ways to solve this, without using validation adorners.

The first one is simple as a nail – if you use IDataErrorInfo, you have some mechanism for checking the bound values of your object for validity. You could then write something along the lines of

public IEnumerable<string> CombinedErrors
{
    get { 
           if (FirstValError) yield return "First value error"; 
           if (SecondValError) yield return "Second value error"; 
        }
}

and bind some ItemsControl to the CombinedErrors property

The second one would involve setting NotifyOnValidationError=True on each binding (to raise Validation.Error routed event) and catch this event on the top container:

public List<ValidationError> MyErrors { get; private set; }

private void Window_Error(object sender, 
    System.Windows.Controls.ValidationErrorEventArgs e)
{
    if (e.Action == ValidationErrorEventAction.Added)
        MyErrors.Add(e.Error);
    else
        MyErrors.Remove(e.Error);
}

then you can bind these similarly to any ItemsControl.