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Null-Coalescing Assignment Operator in C# 8.0
  • Last Updated : 11 Dec, 2019

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:



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    // 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;
        }
    }
    }

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    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:

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    // 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);
            }
        }
    }
    }

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    Output:

    Element is null. So the new element is: 400
    
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    // 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);
        }
    }
    }

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    Output:

    Element is: 400
  • You can also use ??= operator like a nested chain. It makes your code more readable.

    Example:

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    // 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);
        }
    }
    }

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    Output:

    Element is: 45



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