How much of your C# code do you understand?

Reading your code, there is the line:

MemoryMappedFile.CreateOrOpen("Test", 1000, MemoryMappedFileAccess.Rea
+dWrite);
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This suggests that the code opens or creates a file named Test in the current directory, and as jcb suggests, you could use that as the filename.

If this is your code, maybe you want to use a different approach than a memory mapped file. If this is not your code, consider talking to the original author about using a different serialization scheme, like JSON, or a well-defined binary layout, or just not using a memory mapped file when a plain file will do just as well.