Null-Coalescing Assignment Operator in C# 8.0

C# 8.0 has introduced a new operator that is known as a Null-coalescing assignment operator(??=). This operator is used to assign the value of its right-hand operand to its left-hand operand, only if the value of the left-hand operand is null. If the left-hand operand evaluates to non-null, then this operator does not evaluate its right-hand operand.

Syntax:

p ??= q

Here, p is the left and q is the right operand of ??= operator. If the value of p is null, then ??= operator assigns the value of q in p. Or if the value of p is non-null, then it does not evaluate q.

Important Points:

• The left-hand operand of the ??= operator must be a variable, or a property, or an indexer element.
• It is right-associative.
• You cannot overload ??= operator.
• You are allowed to use ??= operator with reference types and value types.

  Example:

  // C# program to illustrate how to use
  // ??= operator with value types and
  // reference types
  using System;
  
  namespace example {
  
  class GFG {
  
      static void Main(string[] args)
      {
  
          // Reference types
          string item_1 = null;
          string item_2 = null;
          string item_4 = "GeeksforGeeks";
          string item_5 = "GFG";
  
          // Using ??= operator
          item_1 = item_1 ??= item_4;
          string item_6 = item_2 ??= item_5;
  
          Console.WriteLine("Value of item_1 is: {0}\n"+
             "Value of item_6 is:{1}", item_1, item_6);
  
          // Value types
          int ? ele_1 = null;
  
          GFG obj = new GFG();
  
          // Using ??= operator assigns
          // the value of ele_1
          // And also you are allowed to
          // use method with ??= operator
          ele_1 = ele_1 ??= obj.Add(10, 30);
          Console.WriteLine("Value of ele_1 is {0}", ele_1);
      }
  
      // Method
      public int Add(int a, int b)
      {
          int result = a + b;
          return result;
      }
  }
  }

  Output:

  Value of item_1 is: GeeksforGeeks
  Value of item_6 is:GFG
  Value of ele_1 is 40
  

• With the help of ??= operator you can remove many redundant “if-else” conditions and make your code more compact and readable. As shown in the below example:

  Example:

  // C# program to illustrate how to use
  // ??= operator to remove if statements
  using System;
  
  namespace example {
  
  class GFG {
  
      // Main Method
      static void Main(string[] args)
      {
          // Nullable variable
          int ? element = null;
  
          // Checking the element is null or not
          if (element is null) {
  
              // Assign a new value to the element
              // Using ??= operator
              int ? new_element = element ??= 400;
              Console.WriteLine("Element is null. "+
               "So the new element is: {0}", new_element);
          }
      }
  }
  }

  Output:

  Element is null. So the new element is: 400
  

  // C# program to illustrate how to use
  // ??= operator to remove if statements
  using System;
  
  namespace example {
  
  class Program {
      static void Main(string[] args)
      {
          // Nullable variable
          int ? element = null;
  
          // Using ??= operator
          // Assign values to the null variable
          element ??= 400;
          Console.WriteLine("Element is: {0}", element);
      }
  }
  }

  Output:

  Element is: 400

• You can also use ??= operator like a nested chain. It makes your code more readable.

  Example:

  // C# program to illustrate
  // how to nest ??= operator
  using System;
  
  namespace example {
  
  class GFG {
  
      // Main Method
      static void Main(string[] args)
      {
          // Nullable variables
          int ? ele1 = null;
          int ? ele2 = null;
          int ? ele3 = 45;
  
          // Using Nested ??= operator
          int ? result = ele1 ??= ele2 ??= ele3;
  
          Console.WriteLine("Element is: {0}", result);
      }
  }
  }

  Output:

  Element is: 45