What do two question marks together mean in C#?

The ?? in C# is called the null-coalescing operator. It’s used to provide a default value when the left-hand operand is null. This operator helps you simplify code that would otherwise require explicit null checks.

How It Works

string message = null;
string defaultMessage = "Hello World";

string result = message ?? defaultMessage;
Console.WriteLine(result); // Outputs "Hello World"

• Left operand: The value that might be null.
• Right operand: The fallback (or “coalesced”) value that will be used if the left operand is null.

Common Use Cases

1. Setting Defaults
   When you have a nullable reference type or a nullable value type, the null-coalescing operator lets you provide a default without an explicit if statement.

2. Parameter and Configuration Values
   If a parameter or configuration is optional, you can check and assign a default more concisely.

3. Chaining with Null-Conditional Operators
   Often used with the ?. (null-conditional) operator to handle deeply nested objects or properties without lengthy null checks.

Avoiding Over-Engineering

Sometimes developers place too many default checks using ??, which can obscure the code’s intent. Use it wisely to maintain readability and clarity.

Strengthen Your C# Skills

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• Grokking the Coding Interview: Patterns for Coding Questions
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