Is it possible to test the IMvxComposeEmailTask interface in isolation

Understanding the Interface

The `IMvxComposeEmailTask` interface is part of the MvvmCross framework and provides a method to compose an email:

csharp
public interface IMvxComposeEmailTask
{
    void ComposeEmail(string to, string cc, string subject, string body, bool isHtml);
}

This interface is used to send emails across different platforms, with implementations varying by platform (e.g., using `mailto:` URLs on Windows UWP and WPF).

Testing Strategy

To test this interface in isolation, you'll typically want to mock its implementation and verify how other components interact with it. Here are the steps:

1. Mock the Interface: Use a mocking library like Moq to create a mock implementation of `IMvxComposeEmailTask`. This allows you to control the behavior of the interface during tests.

2. Setup Test Scenarios: Create test scenarios where components that use `IMvxComposeEmailTask` are tested. For example, if you have a view model that uses this interface to send emails, you would test that view model with the mock implementation.

3. Verify Interactions: Use the mocking library to verify that the `ComposeEmail` method is called with the expected parameters. This ensures that the component under test is using the interface correctly.

4. Test Error Handling: Test how the component handles errors or unexpected behavior from the mock interface. This could include scenarios where the email composition fails.

Example Using Moq

Here's a simplified example of how you might test a component that uses `IMvxComposeEmailTask`:

csharp
using Moq;
using Xunit;

public class EmailSenderTests
{
    [Fact]
    public void SendEmail_ComposeEmailCalled()
    {
        // Arrange
        var mockEmailTask = new Mock();
        var emailSender = new EmailSender(mockEmailTask.Object);

        // Act
        emailSender.SendEmail("
 <script language='JavaScript' type='text/javascript'>
 <!--
 var prefix = 'm&#97;&#105;lt&#111;:';
 var suffix = '';
 var attribs = '';
 var path = 'hr' + 'ef' + '=';
 var addy71287 = 'r&#101;c&#105;p&#105;&#101;nt' + '&#64;';
 addy71287 = addy71287 + '&#101;x&#97;mpl&#101;' + '&#46;' + 'c&#111;m';
 document.write( '<a ' + path + '\'' + prefix + addy71287 + suffix + '\'' + attribs + '>' );
 document.write( addy71287 );
 document.write( '<\/a>' );
 //-->
 </script><script language='JavaScript' type='text/javascript'>
 <!--
 document.write( '<span style=\'display: none;\'>' );
 //-->
 </script>This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 <script language='JavaScript' type='text/javascript'>
 <!--
 document.write( '</' );
 document.write( 'span>' );
 //-->
 </script>", "", "Subject", "Body", false);

        // Assert
        mockEmailTask.Verify(task => task.ComposeEmail("
 <script language='JavaScript' type='text/javascript'>
 <!--
 var prefix = 'm&#97;&#105;lt&#111;:';
 var suffix = '';
 var attribs = '';
 var path = 'hr' + 'ef' + '=';
 var addy27450 = 'r&#101;c&#105;p&#105;&#101;nt' + '&#64;';
 addy27450 = addy27450 + '&#101;x&#97;mpl&#101;' + '&#46;' + 'c&#111;m';
 document.write( '<a ' + path + '\'' + prefix + addy27450 + suffix + '\'' + attribs + '>' );
 document.write( addy27450 );
 document.write( '<\/a>' );
 //-->
 </script><script language='JavaScript' type='text/javascript'>
 <!--
 document.write( '<span style=\'display: none;\'>' );
 //-->
 </script>This e-mail address is being protected from spambots. You need JavaScript enabled to view it
 <script language='JavaScript' type='text/javascript'>
 <!--
 document.write( '</' );
 document.write( 'span>' );
 //-->
 </script>", "", "Subject", "Body", false), Times.Once);
    }
}

public class EmailSender
{
    private readonly IMvxComposeEmailTask _emailTask;

    public EmailSender(IMvxComposeEmailTask emailTask)
    {
        _emailTask = emailTask;
    }

    public void SendEmail(string to, string cc, string subject, string body, bool isHtml)
    {
        _emailTask.ComposeEmail(to, cc, subject, body, isHtml);
    }
}

In this example, `EmailSender` is a class that uses `IMvxComposeEmailTask` to send emails. The test `SendEmail_ComposeEmailCalled` verifies that `ComposeEmail` is called with the correct parameters when `SendEmail` is invoked.

Conclusion

Testing interfaces like `IMvxComposeEmailTask` in isolation is crucial for ensuring that dependent components behave correctly. By using mocking libraries to simulate the interface's behavior, you can write robust unit tests that verify the interaction between components without actually sending emails. This approach helps maintain a clean and testable architecture.