TDD: How to test result when a method my return different result depending on the input?
public class MyClass
{
   public int MyMethod(int? i)
   {
       throw new NotImplementedException();
   }
}

[TestClass]
public class MyClassTest
{
  Public void Retur_Int_Greater_Than_Zero_When_Input_Is_Not_Null_And_Zero_Otherwise()
  {
     // Arrange
     var myVar = new MyClass();
     int? i = 3;

    // Act
    var result = myVar.MyMethod(i)

   //Assert
   //
  }
}

if(i.HasValue)
{
  //Define the Assert statement inside a If, else constructions...
}

// Act
var result = myVar.MyMethod(null)
//Assert
Assert.That(result, Is.EqualTo(0));

// Act
var result = myVar.MyMethod(0)
//Assert
Assert.That(result, Is.EqualTo(0));

// Act
int graterThatZero = 1;
int absoluteValue = //define it here;
var result = myVar.MyMethod(graterThatZero)
//Assert
Assert.That(result, Is.EqualTo(absoluteValue));

[TestCase(null,0)]
[TestCase(0,0)]
[TestCase(1, 1 /* absolute value */)]
[TestCase(-1, 1)]
public void MyMethod_should_return_absolute_value(int? argument, int expectedResult)
{
   // Arrange
   var myVar = new MyClass();
   // Act
   var result = myVar.MyMethod(argument)
   // Assert
   Assert.That(result, Is.EqualTo(expectedResult));
}

[TestCase(null,0)]
[TestCase(0,0)]
[TestCase(10,10)]
[TestCase(-10,10)]
public void TestItWorksForInput(int? input, int expectedOutput)
{
   var myVar = new MyClass();
   var output = myVar.MyMethod(argument);
   Assert.That(output, Is.EqualTo(expectedOutput));
}