How much null checking is enough?

Question

What are some guidelines for when it is not necessary to check for a null?

A lot of the inherited code I\'ve been working on as of late has null-checks

Answer 1

Part of this depends on how the code is used -- if it is a method available only within a project vs. a public API, for example. API error checking requires something stronger than an assertion.

So while this is fine within a project where it's supported with unit tests and stuff like that:

internal void DoThis(Something thing)
{
    Debug.Assert(thing != null, "Arg [thing] cannot be null.");
    //...
}

in a method where you don't have control over who calls it, something like this may be better:

public void DoThis(Something thing)
{
    if (thing == null)
    {
        throw new ArgumentException("Arg [thing] cannot be null.");
    }
    //...
}