That's just the way that a nested type is represented. So for example:
namespace Foo
{
class Outer
{
class Nested {}
}
}
will create a type with a full name of Foo.Outer+Nested
in the compiled code. (So that's what typeof(Outer.Nested).FullName
would return, for example.)
It's not clear to me whether this is specified behaviour, or just what the Microsoft C# compiler chooses to use; it's an "unspeakable" name in that you couldn't explicitly declare a class with a + in it in normal C#, so the compiler knows it won't clash with anything else. Section 10.3.8 of the C# 3 spec doesn't dictate the compiled name as far as I can see.
EDIT: I've just seen that Type.AssemblyQualifiedName
specifies that "+" is used to precede a nested type name... but it's still not clear whether or not that's actually required or just conventional.
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