What's the "'1" for in class names sent from Reflection/CodeDom/CLR?

Question

I can't remember exactly where I've seen this strange '1 (single-tick and the number 1) appearing next to classnames, but it's shown up when inspecting variable values while debugging and most recently in the answer to this question.

targetClass.BaseTypes.Add(new CodeTypeReference { BaseType = "DataObjectBase`1[Refund]", Options = CodeTypeReferenceOptions.GenericTypeParameter })

I'm curious: where does this come from and why is it there?

Answer 1

It's a generic type with 1 type parameter.

For example, List<T> is

System.Collections.Generic.List`1

and Dictionary<TKey, TValue> is

System.Collections.Generic.Dictionary`2

This allows generic types to be overloaded by the number of type parameters.

Answer 2

It's the number of generic type parameters in CLS-compliant class names. It is needed because types can be overloaded on number of parameters; e.g.:

class Foo { }
class Foo<T1> { }
class Foo<T1, T2> { }

To distinguish these cases, compiler generates the following distinct names:

Foo
Foo`1
Foo`2

Note that while this is a CLS provider requirement, it is not a CLR requirement or limitation. As far as CLR itself is concerned, a generic type can have any name, but no two types can have the same fully qualified name. Hence the need for the backtick mangling scheme.