Sample.cs

using NUnit.Framework;

[TestFixture]
public class Sample : TestUtil
{
    [TestCaseSource("TestCases")]
    public void Test(int i, string expected)
    {
        Assert.That(i.ToString(), Is.EqualTo(expected));
    }

    static object[] TestCases =
    {
        Given(1).Returns("1"),
        Given(2).Returns("2"),
        Given(3).Returns("3"),
    };
}

TestUtil.cs

using System;
using System.Linq;

public class TestUtil
{
    public object[] Case(params object[] xs) { return xs; }
    public Expect Given(params object[] args)
    {
        return new Expect(args);
    }

    public class Expect
    {
        readonly object[] args;
        internal Expect(object[] args) { this.args = args; }
        public object[] Returns(object expected)
        {
            return args.Concat(new[] { expected }).ToArray();
        }
    }
}