Why is !0 a type in Microsoft Intermediate Language (MSIL)?

In many MSIL listings, I have observed the following:

System.Nullable`1<!0> etc ...

or

class !0 etc ...

What's the meaning of !0 in these circumstances?

This is quirk of the decompiler you use to look at a .NET assembly. It is the behavior of ildasm.exe, other ones like Reflector or ILSpy get this right. The Microsoft programmer who wrote it took a shortcut, he generates a string from the IL that just displays the type argument the way it is encoded, without writing the extra code to lookup the type argument name in the metadata.

You need to read !n as the n-th type argument of the generic type. Where !0 means "first type argument", !1 means "second type argument", etcetera. For Nullable<>, you know that '!0` means 'T' from the MSDN article.

You may also encounter something like !!T. Two exclamation marks indicate a type argument for a generic method. This time, ildasm.exe does look up the type argument name instead of using !!0. Why the programmer took the shortcut on generic types but not on generic methods is hard to reverse-engineer. Ildasm is a pretty quirky program and is written in a C++ coding style that's very different from other C++ code in .NET. Not as disciplined, non-zero odds that this was an intern's assignment :)

The `1 suffix on "Nullable" is a normal encoding for generic type names, it indicates the generic type has one type argument. In other words, for Nullable<> you'll never see !1 being used.

So simply read !0 as "T". Or use a better decompiler.

That is a generic type parameter.

They are positional.

Decompile some generic code to see how they are used (compare IL vs C#).