How to test a MessageBox in wpf? - wpf

Question

I'm going to do some unit tests and I am struggling with a MessageBox. I have a MessageBox that is showing a text and an "Ok" button in my code. When I trying to unit test the method that contains the MessageBox.Show("Text"), it pops up in the unit test, too, and I have to click "Ok" before it can pass through, which is a bad.
Does anyone know how to go around it? I think I need some kind of code that fakes this MessageBox and clicking "Ok", but I dont know how to do this. I'm a junior programmer, so please explain it as easy as you can ;) and gladly with some code examples.
This is my code for the MessageBox:
public void GetPopUpWithErrorMessage()
{
MessageBox.Show("Error Message", "text",
MessageBoxButton.OK);
}
Edit
I just realised that Fluent Assertions is used in the project. Does anyone know how to implement that in the test code? is it the same way as #thatguy showed?

Answer 1

You have to create a service that implements an interface that you can mock in your tests.
public interface IMessageBoxService
{
void ShowMessageBox(string title, string message);
}
public class MessageBoxService : IMessageBoxService
{
public void ShowMessageBox(string title, string message)
{
MessageBox.Show(message, title, MessageBoxButton.OK);
}
}
In the class where you use it, you would pass this service by its interface, e.g. in the constructor, so the class only knows its contract, but not its implementation.
public class MyClass
{
private IMessageBoxService _messageBoxService;
public MyClass(IMessageBoxService messageBoxService)
{
_messageBoxService = messageBoxService;
}
public void GetPopUpWithErrorMessage()
{
_messageBoxService.ShowMessageBox("text", "Error Message");
}
}
In the test class, you need to use a mocking framework like Moq that creates a mock object from an interface, which is just a stub that you can use to inject the behavior for any method or property that you use in your test.
In this example using NUnit and Moq, messageBoxService is created as a mock object that implements the IMessageBoxService interface. All methods do nothing unless you specify what they should do or return via Setup, but that is another topic. The last line shows how you could check if a sepcific method on the mock was invoked during the test.
[TestFixture]
public class MyClassTest
{
[Test]
public void MyTest()
{
var messageBoxService = Mock.Of<IMessageBoxService>();
var myClass = new MyClass(messageBoxService);
// ...your test code.
// Checks if the "ShowMessageBox" method in the service was called with any strings
Mock.Get(messageBoxService).Verify(mock => mock.ShowMessageBox(It.IsAny<string>(), It.IsAny<string>()));
}
}
Creating a service and an interface is not just useful for mocking, but also for separating view and logic, as you can extract the calling of a message box to a service which has an implementation hidden behind an interface. Moreover you can easily implement dependency injection via constructors.