How to make the equivalent of objective-C mutableCopy in Monotouch?

I have code in objective-C that uses the mutableCopy method to create a mutable copy of an NSURLRequest object called query:

NSMutableURLRequest mutableRequest = [request mutableCopy];

How do I do this in Monotouch?

I tried to put this code in an objective-C dll monotouch binding dll, but when I call a function that has this code, I get a hard crash in the simulator and the monodevelop ide is unloaded from memory spontaneously. The failure appears to be due to an attempt to redirect the newly copied mutableRequest back to monotouch.

Any advice is appreciated.