How do i wipe string variable value from memory

usefull information:
Would it be good secure programming practice to overwrite a "sensitive" variable before deleting it? - Information Security Stack Exchange[^]

and this:
Address of a variable - Rosetta Code[^]

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How do I wipe string variable value from memory

VB have automatic memory management. You can consider that the memory/garbage management is doing it for you automatically. You have nothing to do.

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for the time i put a dummy value and the end of the procedure. But is this a good & effective way of doing?

Basically, no need, not necessary, because the new value will not overwrite the old one.

Do you have a special reason to want to do this ?

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Why Ii want to this? To protect data from being read in memory by ... unallowed persons/programs/viruses etc...

In this case, don't use VB, use a language that allow you to manage memory manually and wipe data before releasing the memory.