An example of why not to use class level variables in unit tests

UnitTesting.NoClassLevelVariables.BadExample.cs

public class Cache
{
	private Dictionary<string, object> cache;

	public Cache()
	{
		this.cache = new Dictionary<string, object>();
	}

	public int Count { get { return cache.Count; } }

	public void AddObject(string key, object data)
	{
		this.cache.Add(key, data);
	}
}

UnitTesting.NoClassLevelVariables.BadExample.Tests.cs

[TestClass]
public class CacheTests
{
	private Cache testCache;

	public CacheTests()
	{
		this.testCache = new Cache();
	}

	[TestMethod]
	public void TestCountReturnsTwoWhenTwoOjectsAdded()
	{
		var testData1 = new object();
		var testData2 = new object();

		this.testCache.AddObject("testData1", testData1);
		this.testCache.AddObject("testData2", testData2);

		Assert.AreEqual(2, this.testCache.Count);
	}

	[TestMethod]
	public void TestCountReturnsThreeWhenThreeOjectsAdded()
	{
		var testData1 = new object();
		var testData2 = new object();
		var testData3 = new object();

		this.testCache.AddObject("testData1", testData1);
		this.testCache.AddObject("testData2", testData2);
		this.testCache.AddObject("testData3", testData3);

		Assert.AreEqual(3, this.testCache.Count);
	}
}