Code Inspection: '??' condition is known to be null or not null

If you want to assign a value, pass an argument, or return from a method based on the nullability of an identifier, the clearest syntax you can use in these cases is the ?? (null-coalescing) operator.

A null-coalescing expression works as follows. The left-hand operand is evaluated first, and if it is null, the right-hand operand is evaluated and the result becomes the result of the whole expression.

However, redundant null-coalescing expressions produce dead code and impede readability. As it follows from the logic of the ?? operator, using null as the right-hand operand does not make sense. Below, JetBrains Rider suggests removing the right-hand operand null together with the ?? operator, because if the newCategory is null, then null will be assigned to the Category anyway:

            
public class Customer
{
  public string Category { get; set; }
  private void ChangeCategory(string newCategory)
  {
    Category = newCategory ?? null;
  }
  /* … */
}


        

            
public class Customer
{
  public string Category { get; set; }
  private void ChangeCategory(string newCategory)
  {
    Category = newCategory;
  }
  /* … */
}

        

Another situation when the null-coalescing operator is redundant is when the left-hand operand can never be null. In this case, the right-hand operand never gets reached and JetBrains Rider suggests removing the unreachable code:

            
string name = "John";
Console.WriteLine(name ?? "empty");

            
string name = "John";
Console.WriteLine(name);

Last modified: 08 March 2021