How to unit test "console.readLine" for invalid input?

Question

I am scratching my head on this little problem.

I have this line and 3 other similar ones in a method

try
{
    int PhoneIMEINumber = int.Parse(Console.ReadLine());
}

{
catch(Exception)
{
    return null;
}

If the user enters "abcd" for input, this throws an exception and I can catch it and show an error message.

But how do I make a unit test for this? I can't simulate console input from the unit test ofcourse and I want to check from my unit test if a null was returned.

Thank you

Answer 1

If you want to Unit Test the Console then you'll probably need to create a wrapper interface.

public interface IConsole
{
    string ReadLine();
}

public class ConsoleWrapper : IConsole
{
    public string ReadLine()
    {
        return Console.ReadLine();
    }
}

This way you can create a Stub/Fake to test your business rules.

public class TestableConsole : IConsole
{
    private readonly string _output;

    public TestableConsole(string output)
    {
        _output = output;
    }

    public string ReadLine()
    {
        return _output;
    }
}

Inside the test:

public class TestClass
{
    private readonly IConsole _console;

    public TestClass(IConsole console)
    {
        _console = console;
    }

    public void RunBusinessRules()
    {
        int value;
        if(!int.TryParse(_console.ReadLine(), out value)
        {
            throw new ArgumentException("User input was not valid");
        }
    }
}

[Test]
public void TestGettingInput()
{
    var console = new TestableConsole("abc");

    var classObject = new TestClass(console);

    Assert.Throws<ArgumentException>(() => classObject.RunBusinessRules());
}

I would go with trying to avoid Unit Testing the Console and taking a dependency on it.

Answer 2

You can set Console.In to a given text reader using SetIn:

var sr = new StringReader("Invalid int");
Console.SetIn(sr);

int? parsed = MethodUnderTest();
Assert.IsNull(parsed);