Unit test for If Switch stament

I'm trying to write a test for the code below and tried a lot of options, but cannot get it right. I've now managed to write something without errors, but ofcourse this doesn't test my method. Some of my method:

  protected virtual Expression Visit(Expression exp)
    {
        if (exp == null)
        {
            return exp;
        }
        switch (exp.NodeType)
        {"Some case expressions")
        default:
                throw new Exception(string.Format("Unhandled expression type: '{0}'", exp.NodeType));

And this is my testcase so far, and ofcourse this fails..

    [TestClass]
  public class ExpressionVisitorTests : ExpressionVisitor
  {
    // public Statements.Solvers.ExpressionVisitor _instance;
    public new Expression Visit(Expression exp)
    {
        return base.Visit(exp);
    }

    //Expression expectedException = null;
    private string expected = "Hello World";
    private string[] _expected = new string[] { "Hello World" };

    private TestContext testContextInstance;
    public TestContext TestContext
     {
         get
         {
             return testContextInstance;
         }
         set
         {
             testContextInstance = value;
         }
     }


    [TestInitialize]
    public void Initialize()
    {
        // var _formatText = new Statements.Solvers.ExpressionVisitor.Expression();
    }


    [TestMethod]
    public void CreateShouldThrowIfModelTestIsNull()
    {

        Assert.AreEqual("Hello World.", expected);
    }
  }

Can someone send me in the right direction please.