Unit testing and coding

Question

In many TDD tutorials, I see code like this:

public class MyClass
{
    public void DoSomething(string Data)
    {
      if (String.IsNullOrWhiteSpace(Data))
        throw new NullReferenceException();
    }
}


[Test]
public DoSomething_NullParameter_ThrowsException()
{
   var logic = MyClass();

   Assert.Throws<NullReferenceException>(() => logic.DoSomething(null));
}

All this makes sense, but at some point you will end up in a class that actually uses MyClass, and you want to check if the exception is being processed:

public class EntryPointClass
{
  public void DoIt(string Data)
  {
     var logicClass = new MyClass();

     try
     {
        logicClass.DoSomething(Data);
     }
     catch(NullReferenceException ex)
     {

     }
  }
}

[Test]
public DoIt_NullParameter_IsHandled()
{
  var logic = new EntryPointClass()

  try
  {
    logic.DoIt(null);
  }
  catch
  {
    Assert.Fail();
  }
}

So, why not put try / catch in MyClass first and not throw an exception and test the null value in the MyClass unit test class, and not in the EntryPointClass unit test class?

Answer 1

Typically, exception handling will look like this:

public class EntryPointClass
{
  Logger _logfile;
  // ...
  public void DoIt(string Data)
  {
     var logicClass = new MyClass();

     try
     {
        logicClass.DoSomething(Data);
     }
     catch(NullReferenceException ex)
     {
         _logfile.WriteLine("null reference exception occured in method 'DoIt'");
     }
  }
}

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 public class EntryPointClass 
 {
      // ....
      catch(NullReferenceException ex)
      {
          if(_logfile!=null)
              _logfile.WriteLine("null reference exception occured in method 'DoIt'");
      }    
      // ....
 }

Answer 2

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Answer 3

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---

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Answer 4

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Answer 5

You are actually testing two completely different conditions:

• Verify that MyClass throws an exception if the argument is invalid
• Check how EntryPointClass, as a MyClass user, deals with the exception thrown by MyClass