WPF-InputValidation

This repo demonstrates how to validate input and show the validation error for WPF applications.

Introduction

With the data binding, input validation can be implemented at the source property setter, where it is called when the control element is updating the property. If the ValidationOnExceptions binding property is set to true, WPF will catch any exception thrown when updating the source property, and add it to the Validation.Errors collection.

By default, WPF will set the border of the TextBox to red when a validation error occurs, but it gives little information about the error. This project demonstrates three other methods to show the extra validation error by using the tooltip, error template and a custom control element.

Getting started

This project is built on .NET 6. Change the TargetFramework setting in the WPF-InputValidation.csproj file to target other .NET versions. For example, targetting .NET 5:

<TargetFramework>net5.0-windows</TargetFramework>

• Run with the project in a terminal

  Navigate the terminal to the \src\WPF-InputValidation folder and execute command: dotnet run

• Open the solution (\src\WPF-InputValidation.VS2022.sln) with Visual Studio 2022.

Pitfalls

• Binding path to the validation error

  Initially, I used (Validation.Errors)[0].ErrorContent as the binding path to get the validation error information. A binding error (index out of range) occurred when the Validation.Errors collection becomes empty after the input error has been cleared.

  The correct binding path is (Validation.Errors)/ErrorContent, and the binding engine will take care of the empty collection scenario.

  Thanks to the answer at stackoverflow.

• Error template element overlapping

  Rendering of an adorner is independent from rendering of the UIElement that the adorner is bound to, there is no way for the TextBox to automatically adjusts its position when adorner is visible.

  

  This project adds some extra margin to the TextBox when a validation error occurs to avoid the overlapping.

  However, if a custom control element is used to display the validation error, WPF will handle its posistion gracefully.

  Thanks to the answer at stackoverflow.

Reference

This project is inspired by this bolg.