What does the two question marks together (??) mean in C#?

The null-coalescing operator (??) in C# returns the value of its left-hand operand if it isn't null; otherwise, it evaluates and returns the right-hand operand. This operator provides a concise way to handle null values and assign default values.

The ?? operator is particularly useful with nullable types, which can represent either a value from the type's domain or be undefined (null). It helps prevent InvalidOperationException exceptions and reduces the need for verbose null-checking code.

Syntax

Following is the syntax for the null-coalescing operator −

result = leftOperand ?? rightOperand;

If leftOperand is not null, result gets the value of leftOperand. If leftOperand is null, result gets the value of rightOperand.

How It Works

Using Null-Coalescing with Nullable Value Types

Example

using System;

class Program {
    static void Main(string[] args) {
        int? value1 = null;
        int value2 = value1 ?? 99;
        Console.WriteLine("Value2: " + value2);
        
        int? value3 = 42;
        int value4 = value3 ?? 99;
        Console.WriteLine("Value4: " + value4);
        
        // Multiple null-coalescing operators
        int? a = null;
        int? b = null;
        int? c = 100;
        int result = a ?? b ?? c ?? 0;
        Console.WriteLine("Result: " + result);
    }
}

The output of the above code is −

Value2: 99
Value4: 42
Result: 100

Using Null-Coalescing with Reference Types

Example

using System;

class Program {
    static void Main(string[] args) {
        string testString = "Null Coalescing";
        string resultString = testString ?? "Original string is null";
        Console.WriteLine("Result: " + resultString);
        
        string nullString = null;
        string defaultString = nullString ?? "Default value";
        Console.WriteLine("Default: " + defaultString);
        
        // With arrays
        int[] numbers = null;
        int[] result = numbers ?? new int[] { 1, 2, 3 };
        Console.WriteLine("Array length: " + result.Length);
    }
}

The output of the above code is −

Result: Null Coalescing
Default: Default value
Array length: 3

Advantages of Null-Coalescing Operator

• It is used to define a default value for a nullable item (for both value types and reference types).

• It prevents the runtime InvalidOperationException exception.

• It helps us to remove many redundant "if" conditions.

• It works for both reference types and value types.

• The code becomes well-organized and readable.

Comparison with Traditional Null Checking

Conclusion

The null-coalescing operator (??) in C# provides a clean and efficient way to handle null values by returning a default value when the left operand is null. It simplifies code by eliminating verbose null-checking statements and works with both nullable value types and reference types.