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Problem with readonly fields

0 votes
Having an example: 

  public class Base
  {
    public Base()
    { }
    public string NullError
    {
      get{ return nullError;}
    }
    private readonly string nullError = "Error";
  }
  public class Derived : Base
  {
    public Derived()
      : base()
    {
      NullError.ToString(); //NullReference Exception Here
    }
  }
  [TestClass()]
  public class TableColumnTest
  {
    [TestMethod]
    public void TestBaseInitializations()
    {
      Mock mock = MockManager.Mock(typeof(Derived),  Constructor.NotMocked);
      mock.ExpectUnmockedConstructor();
      mock.CallBase.ExpectConstructor();
      new Derived();
      mock.Verify();
    }
  }

I've got a NullReferenceException. It seems that readonly fields on base class are not initalized properly or is it my mistake?
asked by ppr
Report User
 (600 points)

2 Answers

0 votes
Hi
You are getting the Exception because the Base constructor was not called.
You can use use ExpectUnmockedConstructor for the base type or mock
the property NullError.
It all depends on what you need to do in the test. 

//Call base constructor
[Test]
public void TestBaseInitializations()
{
   Mock mock = MockManager.Mock(typeof(Derived), Constructor.NotMocked);
   mock.ExpectUnmockedConstructor();
   mock.CallBase.ExpectUnmockedConstructor();
   new Derived();
   mock.Verify();
}

//Mock NullError property
[Test]
public void TestBaseInitializations()
{
   Mock mock = MockManager.Mock(typeof(Derived), Constructor.NotMocked);
   mock.ExpectUnmockedConstructor();
   mock.CallBase.ExpectGet("NullError", "abc");
   new Derived();
   mock.Verify();
}
answered by ohad
Report User
 (35.4k points)
0 votes
Perhaps I can give a more detailed explaination.

Initializing instance fields e.g: 
private readonly string nullError = "Error";
is actually done when the constructor is called.
something like: 
public Base()
{ 
   // Hidden code
   nullError = "Error";
}


So if the constuctor is mocked, the field never gets initialized.

:idea: If you want more control over the fields, (e.g. when you want to mock the constructor but set the field), you can use the new Mocking Fields APIs (since version 4.2.2) to reset the field.
answered by eli
Report User
 (5.7k points)
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