csharp-nullable-reference-types

1# C# Nullable Reference Types
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3## When to Use
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5- Introducing nullable reference types (NRT) into a codebase that has not yet adopted them
6- Writing or refactoring C# code that uses `T?` / nullable annotations
7- Annotating APIs with `System.Diagnostics.CodeAnalysis` nullable attributes
8- Designing public/internal APIs where nullability contracts matter
9- Wrapping unannotated or legacy APIs so downstream callers still benefit from NRT
10- Reviewing code for correct null-state analysis, guard helpers, and the `field` keyword
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12## Core Goals
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14- Prevent `NullReferenceException` at runtime by making null intent explicit in signatures.
15- Express contracts the type system cannot represent directly using the official nullable attributes.
16- Adopt NRT incrementally in legacy codebases without a big-bang rewrite.
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18## Core Nullability Model
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20### Non-nullable vs nullable
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22- `string` — non-nullable reference. The compiler assumes instances are never `null`; assigning `null` or a maybe-null value produces a warning.
23- `string?` — nullable reference. The variable may be `null`; the compiler requires a null check before dereference.
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25```csharp
26string name = "Alice";
27name = null;          // Warning: assigning null to non-nullable.
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29string? nickname = null;
30Console.WriteLine(nickname.Length); // Warning: possible null dereference.
31```
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33### Null-state analysis (flow)
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35The compiler tracks whether a reference is *definitely non-null* or *maybe null*. Null checks and assignments update this state.
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