At first sight it may seem that Singleton and Unit Testing are not compatible since you cannot ensure the separation between the tests. We could solve the problem by instanciating a new "Singleton" instance for each test and apply test on this "local" instance. The problem is that a well implemented Singleton is sealed and it's constructor are private (see Singleton Pattern here) and thus you cannot create this instance.

using System;

public sealed class Singleton
{
   ...
   private Singleton() {}

   public static Singleton Instance
   {
      get { ...}
   }
}

Reflection to the rescue

An elegant way for bypassing the "creation problem" is to use Reflection to get the private constructor (ConstructorInfo) and then use this constructor info to create a new "Singleton" instance.

using System.Reflection;

[TestFixture]
public class SingletonTest
{
    private Singleton target = null;
    [SetUp]
    public void SetUp()
    {
        ConstructorInfo ci =
            typeof(Singleton).GetConstructor(
                BindingFlags.Instance |
                BindingFlags.NonPublic,
                null,
                Type.EmptyTypes,
                null
                );
         Assert.IsNotNull(ci);
         this.target = (Singleton)ci.Invoke(null);
    }
}

Now we have a new instance of the Singleton for each test and we are happy.