How do Nullable Reference Types work in C#, and what is the role of Static Flow Analysis?

Short answer

They distinguish references that allow null (such as string?) from those that do not (string); the compiler uses static analysis to warn before a NullReferenceException occurs at runtime.

Explanation

Since C# 8, developers can enable <Nullable>enable</Nullable>. In this mode, the compiler assumes object references are non-null unless explicitly marked with ?. It performs Static Flow Analysis: after checking if (user != null), the compiler knows the variable is safe inside the block and allows property access without warnings.

Code example

string nonNull = "Hello"; // Does not accept null
string? canBeNull = null; // Allowed

// Console.WriteLine(canBeNull.Length); // Compiler warning!
if (canBeNull != null) {
    Console.WriteLine(canBeNull.Length); // Safe after flow analysis
}

Common mistakes

• Using the Null-forgiving operator (!) such as user!.Name to bypass a compiler warning without a real check, causing a runtime crash.

Follow-up questions

• What is the basic difference between Value Types and Reference Types in C#, and how are they stored in memory?

Sources

• Microsoft Learn — Nullable reference types