Why do I get a "key already added" error when using the key from a GroupBy in ToDictionary?

Question

I had thought that a Linq GroupBy would always produce unique keys, and yet when I project the results of a GroupBy into a Dictionary using .ToDictionary(), I get an error saying "an item with the same key has already been added":

Here's the code in question:

return DbContext.Responsibilities
    .GroupBy(r => r.RoleCode)
    .ToDictionary(g => g.Key, g => g.Count());

[Here Responsibilities is a DbSet of entities whose RoleCode member is a simple int property.]

If I change this to produce a sequence of anonymous types, and then create a dictionary from that, it runs without error:

return DbContext.Responsibilities
    .GroupBy(r => r.RoleCode)
    .Select(g => new { Code = g.Key, Count = g.Count() })
    .ToDictionary(i => i.Code, i => i.Count);

Why does that make such a difference?

FYI, if I create a temporary List variable to hold the anonymous types, here's what the values are:

So, no duplicate keys.

Answer 1

A simple Google search led me to this blog post:

http://code-ninja.org/blog/2014/07/24/entity-framework-never-call-groupby-todictionary/

The simple answer is that ToDictionary doesn't translate into an Entity Framework query, but it enumerates the result from Group By.