Exceptions and nullable types

C# signals failure with exceptions. Throw a specific type, catch only what you can handle, and use using statements so connections and files are closed even on error. Create your own exceptions for business errors, such as OrderNotFoundException.

ASP.NET Core can later turn those into a 404 in one place. Nullable reference types are the other half of this topic. Put <Nullable>enable</Nullable> in the project file.

Then string means "never null" and string? means "maybe null". The compiler warns you when you use something that might be null. Treat those warnings as errors.