Unit Testing with functions that return random results

Question

I don't think that this is specific to a language or framework, but I am using xUnit.net and C#.

I have a function that returns a random date in a certain range. I pass in a date, and the returning date is always in range of 1 to 40 years before the given date.

Now I just wonder if there is a good way to unit test this. The best approach seems to be to create a loop and let the function run i.e. 100 times and assert that every of these 100 results are in the desired range, which is my current approach.

I also realize that unless I am able to control my Random generator, there will not be a perfect solution (after all, the result IS random), but I wonder what approaches you take when you have to test functionality that returns a random result in a certain range?

Answer 1

Mock or fake out the random number generator

Do something like this... I didn't compile it so there might be a few syntax errors.

public interface IRandomGenerator
{
    double Generate(double max);
}

public class SomethingThatUsesRandom
{
    private readonly IRandomGenerator _generator;

    private class DefaultRandom : IRandomGenerator
    {
        public double Generate(double max)
        {
            return (new Random()).Next(max);
        }
    }

    public SomethingThatUsesRandom(IRandomGenerator generator)
    {
        _generator = generator;
    }

    public SomethingThatUsesRandom() : this(new DefaultRandom())
    {}

    public double MethodThatUsesRandom()
    {
        return _generator.Generate(40.0);
    }
}

In your test, just fake or mock out the IRandomGenerator to return something canned.