Simple In-Memory Repo for Testing

UrlRepository.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Collections.Generic;

namespace UnitTestProject1
{
    [TestClass]
    public class UrlRepositoryTest
    {
        [TestMethod]
        public void ShouldAddKeyAndRetrieveKey()
        {
            UrlRepository repository = new UrlRepository();
            int key = repository.Add("http://www.github.com");
            Assert.IsTrue(key == 1);
            UrlRepository repo2 = new UrlRepository();
            int key2 = repository.Add("https://gist.github.com/rockarts/0c2539a8c64dc81e1c903133ab4b3e05");
            Assert.IsTrue(key2 == 2);
            Assert.AreEqual("http://www.github.com", repo2.Get(1));
        }


    }

    public class UrlRepository
    {
        private static Dictionary<int, string> database = new Dictionary<int, string>();

        public int Add(string url)
        {
            int key = database.Count + 1;
            database.Add(key, url);
            return key;
        }

        public string Get(int key)
        {
            return database[key];
        }
    }
}