It’s likely an attempt to get Spec# style annotations into a non-Spec# build. The ! annotation in Spec# means the value is not-null. The author is likely trying to indicate that both the return values, args array and all of the elements in args are always non-null values.
Spec# Link:
- http://research.microsoft.com/en-us/projects/specsharp/
Quick Spec# Overview:
Spec# is a .Net language created via a Microsoft Research Project. It is an extension of the C# language which attempts to embed code contracts into the type system. The most prominent are non-nullable types (indicated with ! after the type name), checked exceptions and pre/post conditions.