File.OpenWrite appends instead of wiping contents?

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This is the specified behavior for File.OpenWrite:

If the file exists, it is opened for writing at the beginning. The existing file is not truncated.

To do what you’re after, just do:

using(Stream fileStream = File.Open(FileName, FileMode.Create)) 
   fileStream.Write(Contents, 0, Contents.Length);

Your current call is equivalent to use FileMode.OpenOrCreate, which does not cause truncation of an existing file.

The FileMode.Create option will cause the File method to create a new file if it does not exist, or use FileMode.Truncate if it does, giving you the desired behavior. Alternatively, you can use File.Create to do this directly.

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