Unit testing - should I split up tests or have a single test?

Question

I hope this doesn't come across as a stupid question but its something I have been wondering about. I wish to write unit test a method which contains some logic to check that certain values are not null.

public void MyMethod(string value1, string value2)
{
    if(value1 != null)
    {
     //do something (throw exception)
    }

    if(value2 != null)
    {
     //do something (throw exception)
    }

    //rest of method
}

I want to test this by passing null values into the method. My question is should I create a unit test for each argument or can I create one unit test which checks what happens if I set value1 to null and then checks what happens if I set value2 to null.

i.e.

[TestMethod]
public void TestMyMethodShouldThrowExceptionIfValue1IsNull()
{
    //test
}

[TestMethod]
public void TestMyMethodShouldThrowExceptionIfValue2IsNull()
{
    //test
}

or

[TestMethod]
public void TestMyMethodWithNullValues()
{
  //pass null for value1
  //check

  //pass null for value2
  //check
}

Or does it make any difference? I think I read somewhere that you should limit yourself to one assert per unit test. Is this correct?

Thanks in advance Zaps

Answer 1

You should write a unit test for each test case (assertion) to avoid Assertion Roulette.