Downloading Managed DLLs

Question

I have implemented a plugin arcitecture in my windows application where if the required plugin(DLL created in c#)is not present on client then it should be downloaded programmatically from server.If I directly copy the DLL then my code works fine but if it gets downloaded from the server if fails to load.And gives the System.BadImageFormatException with the message "An attempt was made to load a program with an incorrect format."
How can I download managed DLLs through my code?
Any pointers