Trashing the content of managed variables

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C#

I have an application using cryptography, mostly in streams (file and memory) which I decrypt as needed into a new MemoryStream. All this works well.
Only thing is, I would like to trash the content of the MemoryStream when I am done with it (as well as strings etc I copy portions into) to prevent it ever getting paged to disk in decrypted form via the swap file.
In "good old fashioned" C++ I would just blitz the data with random rubbish before I deleted it - is there any way with managed objects?