Template class names in weird forms (.NET)

I'm retrieving the names of all the classes in the current runtime in .NET, for the purpose of identifying object and function declarations in sourcecode that's given as input to my program, but templates seem a bit off, for example I've got this among the output:

hashset`1+elementcount[t]
hashset`1+slot[t]
hashset`1+enumerator[t]

which I obtain more or less from this simple code (there's some similar code that gets the referenced assemblies but essentially does the following for each of those instead of the executing assembly).

foreach (Type t in Assembly.GetExecutingAssembly().GetTypes())
            {
                types.Add(t.ToString().Split('.').Last().ToLower());
            }

For now I can of course just Split() the strings to get the first part, before the ` mark, but I was wondering if anyone knows exactly what this might be. The three lines above are consecutive, and a couple more entries for HashSet in my results also have this 1+something thing, so I'm positive it's the actual HashSet class. (note: I'm turning everything to lowercase currently but disabling it doesn't seem to change anything.)

So... anyone know what's this notation? Not sure how to google it, but copy-pasting some lines to Google and enclosing in quotes returns either no results or very random threads that don't end up having the line in them. Thanks in advance.