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

In C#, ?? operator is known as Null-coalescing operator. It will return the value of its left-hand operand if it is not null. If it is null, then it will evaluate the right-hand operand and returns its result. Or if the left-hand operand evaluates to non-null, then it does not evaluate its right-hand operand.

Syntax:

p ?? q

Here, p is the left and q is the right operand of ?? operator. The value of p can be nullable type, but the value of q must be non-nullable type. If the value of p is null, then it returns the value of q. Otherwise, it will return the value of p.

Important Points:

  • The ?? operator is used to check null values and you can also assign a default value to a variable whose value is null(or nullable type).
  • You are not allowed to overload ?? operator.
  • It is right-associative.
  • In ?? operator, you can use throw expression as a right-hand operand of ?? operator which makes your code more concise.
  • You are allowed to use ?? operator with value types and reference 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 Program {
        static void Main(string[] args)
        {
       
            // Reference types
            string item_1 = null;
            string item_2 = "GeeksforGeeks";
            string item_3 = "GFG";
       
            string item_4 = item_1 ?? item_2;
            item_3 = item_4 ?? item_2;
       
            Console.WriteLine("Value of item_4 is: {0} \n"+
                  "Value of item_3 is: {1}", item_4, item_3);
       
            // Value types
            int ? item_5 = null;
       
            Program obj = new Program();
       
            // Using ?? operator assigns
            // the value of a value type
            // and also you are allowed 
            // to use method with ?? operator
            int ? item_6 = item_5 ?? obj.Add(10, 30);
            Console.WriteLine("Value of item_6 is: {0}", item_6);
        }
       
        // Method
        public int Add(int a, int b)
        {
            int result = a + b;
            return result;
        }
    }
    }

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    Output:

    Value of item_4 is: GeeksforGeeks 
    Value of item_3 is: GeeksforGeeks
    Value of item_6 is: 40
    
  • With the help of ?? operator you can prevent InvalidOperationException.

    Example:

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    // C# program to illustrate how ?? 
    // operator prevent the 
    // InvalidOperationException
    using System;
       
    namespace example {
       
    class GFG {
          
        // Main Method
        static void Main(string[] args)
        {
            // Creating items of nullable types
            int ? item_1 = null;
       
             /*
             Here if you use this commented part,
             then this statement will give you an
             InvalidOperationException. So to 
             overcome this problem we use ?? operator 
             int? item_2 = item_1.Value;
             */
       
            // With the help of ?? operator we 
            // assign a default value to the item_2
            // And the value of item_1 is null.
            int ? item_2 = item_1 ?? 100;
            Console.WriteLine("Value of item_1 is: {0}", item_1);
            Console.WriteLine("Value of item_2 is: {0}", item_2);
        }
    }
    }

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    Output:

    Value of item_1 is: 
    Value of item_2 is: 100
    
  • With the help of ?? operator you can remove many redundant “if-else” conditions and make your code compact and readable.

    Example:

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    // C# program to illustrate how ?? 
    // operator replaces if-else statements
    using System;
       
    namespace example {
       
    class GFG {
          
        // Main Method
        static void Main(string[] args)
        {
            // Creating items of nullable types
            int ? item_1 = null;
       
            int ? item_2;
       
            if (item_1.HasValue) {
                item_2 = item_1;
            }
            else {
                item_2 = 200;
            }
            Console.WriteLine("Value of item_1 is: {0}", item_1);
            Console.WriteLine("Value of item_2 is: {0}", item_2);
        }
    }
    }

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    Output:

    Value of item_1 is: 
    Value of item_2 is: 200
    
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    // C# program to illustrate how ??
    // operator replaces if-else statements
    using System;
       
    namespace example {
       
    class GFG {
          
        // Main Method
        static void Main(string[] args)
        {
            // Creating items of nullable types
            int ? item_1 = null;
       
            // Using ?? operator
            int ? item_2 = item_1 ?? 200;
            Console.WriteLine("Value of item_1 is: {0}", item_1);
            Console.WriteLine("Value of item_2 is: {0}", item_2);
        }
    }
    }

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    Output:

    Value of item_1 is: 
    Value of item_2 is: 200
    
  • ?? operator can be nested. It will make your code more readable and also reduce multiple if-else conditions.

    Example:

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    // C# program to illustrate how 
    // we use nested ?? operator
    using System;
       
    namespace example {
       
    class GFG {
          
        // Main Method
        static void Main(string[] args)
        {
            // Creating items of nullable types
            int ? item_1 = null;
            int ? item_2 = null;
            int ? item_3 = 500;
       
            // Nested ?? operator
            int ? item_4 = item_1 ?? item_2 ?? item_3;
              
            Console.WriteLine("Value of item_4 is: {0} ", item_4);
        }
    }
    }

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    Output:

    Value of item_4 is: 500 



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