"Simulate" command line argument in unit-tests

Question

I have some functionality, which depends on command line arguments, and different arguments should lead to different results.

I can't directly "simulate" this arguments, since there are some sort of chain dependencies - I need to unit-test some xaml control, which depends on view-model, which depends on certain additional class, which fetches command line arguments using Environment.GetCommandLineArgs, and I can't directly impact on this last class to set arguments manually instead of using GetCommandLineArgs.

So, I'd like to know, is there any way to make Environment.GetCommandLineArgs return value I want it to return, for certain unit-test.

Answer 1

You need to abstract Environment.GetCommandLineArgs or what ever is eventually calling it behind something you can mock

public interface ICommandLineInterface {
    string[] GetCommandLineArgs();
}

Which can eventually be implemented in a concrete class like

public class CommandInterface : ICommandLineInterface {
    public string[] GetCommandLineArgs() {
        return Environment.GetCommandLineArgs();
    }
}

And can be Tested using Moq and FluentAssertions

[TestMethod]
public void Test_Should_Simulate_Command_Line_Argument() {
    // Arrange
    string[] expectedArgs = new[] { "Hello", "World", "Fake", "Args" };
    var mockedCLI = new Mock<ICommandLineInterface>();
    mockedCLI.Setup(m => m.GetCommandLineArgs()).Returns(expectedArgs);
    var target = mockedCLI.Object;

    // Act
    var args = target.GetCommandLineArgs();

    // Assert
    args.Should().NotBeNull();
    args.Should().ContainInOrder(expectedArgs);

}

Answer 2

Since you are dealing with environment variables, why don't we wrap the outside dependencies into one EnvironmentHelper class, then inject the dependencies?

Here is my suggestion:

public class EnvironmentHelper
{
    Func<string[]> getEnvironmentCommandLineArgs; 

       // other dependency injections can be placed here

       public EnvironmentHelper(Func<string[]> getEnvironmentCommandLineArgs)
       {
            this.getEnvironmentCommandLineArgs = getEnvironmentCommandLineArgs;
       }

       public string[] GetEnvironmentCommandLineArgs()
       {
            return getEnvironmentCommandLineArgs();
       }
}

Here is the Mock method:

public static string[] GetFakeEnvironmentCommandLineArgs()
{
    return new string[] { "arg1", "arg2" };
}

In your source code:

EnvironmentHelper envHelper = new EnvironmentHelper(Environment.GetCommandLineArgs);
string[] myArgs = envHelper.GetEnvironmentCommandLineArgs();

In your unit test code:

EnvironmentHelper envHelper = new EnvironmentHelper(GetFakeEnvironmentCommandLineArgs);
string[] myArgs = envHelper.GetEnvironmentCommandLineArgs();