Are MakeGenericType / generic types garbage collected?

To answer your first question:

Generic constructions of types are not collected.

However, if you construct C<string> and C<object>, the CLR actually generates the code for the methods only once; since reference to string and reference to object are guaranteed to be the same size, it can do so safely. It's pretty clever. If you construct C<int> and C<double> though, the code for the methods gets generated twice, once for each construction. (Assuming that the code for the methods is generated at all of course; methods are jitted on demand; that's why its called jitting.)

To demonstrate that generic types are not collected, instead create a generic type

class C<T> { public static readonly T Big = new T[10000]; }

C<object> and C<string> share any code generated for the methods, but each one gets its own static fields, and those fields will live forever. The more types you construct, the more memory will be filled up with those big arrays.

And now you know why those types cannot be collected; we have no way of knowing if someone is going to try to access a member of one of those arrays at any time in the future. Since we don't know when the last array access is going to be, they have to live forever, and therefore the type that contains it has to live forever too.

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To answer your second question: Is there a way to make dynamically-emitted assemblies that are collected?

Yes. The documentation is here:

http://msdn.microsoft.com/en-us/library/dd554932.aspx