C# "CS8602: Dereference of a possibly null reference" in CI

With nullable reference types enabled, CS8602 warns that you dereference a value the compiler cannot prove is non-null. It only fails the build when warnings are treated as errors (common in CI), so it often passes locally and breaks the pipeline.

What this error means

Build fails (or warns) with CS8602 at a member access on a maybe-null value. It is deterministic, and turns fatal under TreatWarningsAsErrors or <Nullable>enable</Nullable> with strict CI settings.

dotnet

User.cs(27,16): error CS8602: Dereference of a possibly null reference.

Common causes

A nullable value is dereferenced without a check

The compiler's flow analysis cannot prove the value is non-null at the access point, so it warns.

Warnings are treated as errors in CI

A CS8602 that is merely a warning locally becomes fatal when CI promotes warnings to errors.

How to fix it

Guard or null-check before dereferencing

Add a null check, pattern match, or early return before the access.
Use the null-conditional operator where appropriate.
Rebuild.

.cs

if (user is null) return NotFound();
return Ok(user.Name);

Express non-null intent precisely

Use the null-forgiving operator ! only where you can guarantee non-null.
Annotate APIs with proper nullability so flow analysis is accurate.
Rebuild.

How to prevent it

Enable <Nullable>enable</Nullable> early so issues surface during development.
Fix null warnings rather than suppressing them broadly.
Run a strict build locally that mirrors CI warnings-as-errors.