A favorite C# pattern

Nulls and Exceptions, a horrific yet most popular combination in C#:

public CookieValueProvider(HttpActionContext actionContext)
{
    if (actionContext == null)
    {
        throw new ArgumentNullException("actionContext");
    }

    // [...] Remaining code removed for clarity.
}

• null check, because the compiler is kind enough to allow the caller to pass in this non-value, and, who knows, the caller might actually pass in this no-value.
• Exception throwing (rather than returning an error), well, to make the caller’s life a bit more interesting (because why leverage compile-time guarantees :-P).