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Unit Testing return value of method called from inside another method
public List<MyClass> DoSomething(string Name, string Address, string Email, ref string ErrorMessage)
{
  //Check for empty string parameters etc now go and get some data
  List<MyClass> Data = GetData(Name, Address, Email);

  /*************************************************************
  //How do I unit test that the data variable might be empty???
  *************************************************************/

  List<MyClass> FormattedData = FormatData(Data);
  return FormattedData;
}

public List<MyClass> DoSomething(string Name, string Address,
              string Email, ref string ErrorMessage, IDataProvider provider)
{
  //Check for empty string parameters etc now go and get some data
  List<MyClass> Data = provider.GetData(Name, Address, Email);

  List<MyClass> FormattedData = FormatData(Data);
  return FormattedData;
}

[Test]
public void When_user_forgot_password_should_save_user()
{
    // Arrange
    var errorMessage = string.Empty;
    var dataProvider = MockRepository.GenerateStub<IDataProvider>();
    dataProvider.Stub(x => x.GetData("x", "y", "z", ref errorMessage )).Return(null);

    // Act
    var result DoSomething("x","y","z", ref errorMessage, dataProvider);

    // Assert

    //...
}

public class Daddy {
    public List<MyClass> DoSomething(string Name, string Address,                  string Email, out string ErrorMessage, IDataProvider provider)
    {
      //Check for empty string parameters etc now go and get some data
      List<MyClass> Data = provider.GetData(Name, Address, Email);

      if (Data.Count == 0)
      {
          ErrorMessage = "Oh noes";
          return Enumerable.Empty<MyClass>();
      }

      List<MyClass> formattedData = FormatData(Data);
      return formattedData;
    }
}

[TestClass]
public class DaddyTest {
    [TestMethod]
    public void DoSomethingHandlesEmptyDataSet() {
        // set-up
        Daddy daddy = new Daddy();

        // test
        IList<MyClass> result = daddy.DoSomething("blah",
                                                  "101 Dalmation Road",
                                                  "bob@example.com",
                                                  out error,
                                                  new FakeProvider(new Enumerable.Empty<AcmeData>())); // a class we've written to act in lieu of the real provider

        // validate
        Assert.NotNull(result); // most testing frameworks provides Assert functionality
        Assert.IsTrue(result.Count == 0);
        Assert.IsFalse(String.IsNullOrEmpty(error));
    }
}

List<MyClass> Data = GetData(Name, Address, Email);

public List<MyClass> DoSomething(List<MyClass> data, ref string ErrorMessage)

class ThisClass
{
   [Import]
   public IDataService DataService {get; set;}

   public List<MyClass> DoSomething(string Name, string Address, string Email, ref string ErrorMessage)
   {
     //Check for empty string parameters etc now go and get some data
     List<MyClass> Data = IDataService.GetData(Name, Address, Email); // using dependency

     List<MyClass> FormattedData = FormatData(Data);
     return FormattedData;
   }
}

[TestMethod]
public void MyDoSomethingTest()
{
   string errorMessage = string.Empty;
   var actual = myClass.DoSomething(..., ref errorMessage)
   Assert.AreEqual("MyErrorMessage", errorMessage);
   Assert.AreEqual(0, FormattedData.Count);
}

errorMessage = "Empty parameters are not allowed";
return new List<MyClass>();