What are nullable reference types in C#?

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**Nullable reference types** — compiler annotation to track nullability. **Enable in .csproj:** ```xml enable ``` **Syntax:** ```csharp string name = "Alice"; // Non-nullable string? nickname = null; // Nullable void Process(string input) { } // Expects non-null void Process(string? input) { } // Accepts null ``` **Operators:** - **!** — null-forgiving operator (suppress warning). - **?.** — null-conditional operator. - **??** — null-coalescing operator. ```csharp string name = GetNull(); // Warning! string name = GetNull()!; // No warning string name = GetNull() ?? "default"; // Handle null string? name = GetNull(); // Explicit nullable ``` **Benefits:** - Compile-time null checking. - Reduces NullReferenceException. - Documents API contracts. **Note:** Reference types always nullable at runtime — annotations are compiler-only.

C#: What are nullable reference types in C#?