How do I enforce null checking?

This isn't a technical solution, but a social one. Simply make it unacceptable in your environment to access a reference type without checking for null when the reference type has been modified in any way by outside code (another method call, etc). Unit Testing doesn't replace good old-fashioned code review.

You should look into Code Contracts. The static checker is only available for the higher-end VS editions, but that's basically what you're after.

There are plenty of resources online, and <plug> you can also read a prerelease version of the chapter on Code Contracts from the 2nd edition of C# in Depth - download chapter 15 for free. </plug> (The chapter is slightly out of date with respect to the latest and greatest build of Code Contracts, but nothing huge.)

100% code coverage means nothing.

It is a false sense of security.

The only thing you're measuring is that you're executing all the lines of code.

Not:

• That those lines of code are all the lines of code that should've been there
• That those lines of code are operating correctly (are you testing all edge cases?)

For instance, if your procedure to deal with a fire contains 1 step "run out of the building", then even if that happens in 100% of the cases, perhaps a better procedure would be to "alert the fire department, try to stop the fire, then run out if all else fails".

There is nothing built into C# that will help you with this without you specifically going in and adding code, either code contracts (.NET 4.0) or specific IF-statements (<4.0).

C# 8 has introduced Non-nullable reference types.

A .Net project can be modified to have the Nullable option enabled:

<LangVersion>8.0</LangVersion>
<Nullable>enable</Nullable>

The compiler will be able distinguish

• string and string?

• NonNullableClass and NullableClass?