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:

   // 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; 
    } 
} 
} 
 
  

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:

   // 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); 
    } 
} 
} 
 
  

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:

   // 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); 
    } 
} 
} 
 
  

Output:

  Value of item_1 is:   Value of item_2 is: 200

   // 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); 
    } 
} 
} 
 
  

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:

   // 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); 
    } 
} 
} 
 
  

Output:

Value of item_4 is: 500

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Null-Coalescing Operator in C#

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