Really simple WPF form data validation – how to?

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I would definitely recommend using IDataErrorInfo for WPF validation since WPF already understands how to use it, and its easy to implement.

To start with, add the interface to the class containing the data you want to validate. The required methods will probably look something like this:

public class Customer : IDataErrorInfo
{
    ...

    #region IDataErrorInfo Members

    string IDataErrorInfo.Error
    {
        get { return null; }
    }

    string IDataErrorInfo.this[string columnName]
    {
        get
        {
            if (columnName == "Name")
            {
                // Validate property and return a string if there is an error
                if (string.IsNullOrEmpty(Name))
                    return "Name is Required";
            }

            // If there's no error, null gets returned
            return null;
        }
    }
    #endregion
}

Next, you need to set ValidatesOnDataErrors=True in your TextBox binding so it runs the validation whenever the Name property changes:

<TextBox Text="{Binding Path=Customer.Name, ValidatesOnDataErrors=True}" ... />

And finally, create a Validation Template in your XAML to tell WPF how to draw a validation error. Here’s the style/template I usually use:

<!-- ValidatingControl Style -->
<Style TargetType="{x:Type FrameworkElement}" x:Key="ValidatingControl">
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="True">
            <Setter Property="ToolTip" Value="{Binding 
                Path=(Validation.Errors)[0].ErrorContent, 
                RelativeSource={x:Static RelativeSource.Self}}" />
        </Trigger>
    </Style.Triggers>
</Style>

Also, be sure your Customer class implements INotifyPropertyChanged so it correctly responds to UI updates. I don’t see that in your code, but often people leave that out for simplicity 🙂

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