.NET and C# interview questions

Clients send an Idempotency-Key

Authenticate during the handshake, authorize per message when needed, and validate origin.

Sensitive actions re-check recent MFA or password confirmation.

Prefer System.Text.Json source generation, reuse buffers, and avoid ToList on huge graphs.

Domain has no infra references

I use a hosted service with the processor SDK, complete on success, and abandon or dead-letter on failure.

I create a Meter and counters or histograms for business events.

Interpolation is readable and compile-checked for expressions.

It holds local launch profiles, URLs, and env vars for Visual Studio and dotnet run.

Attributes inject the caller name for INotifyPropertyChanged or logging helpers.

A field EF maps that does not exist on your CLR type, like UpdatedAt.

Build the provider with validate-on-build and validate-scopes enabled in Development.

I use Microsoft.FeatureManagement or a remote flag service with filters by user and environment.

Helpers that return strongly typed IResult implementations for OpenAPI metadata.

It stops a dependency from flowing transitively to consumers.

An async-friendly timer that awaits ticks without callback reentrancy headaches.

HttpOnly, Secure, SameSite, short lifetimes, and sliding refresh with care.

It spins real Docker dependencies like Postgres for integration tests.

A value-type record with value equality semantics.

I batch with AddRange, raise MaxBatchSize, or use bulk extensions for millions of rows.