Firstly check very carefully you are closing (disposing) all files object, as otherwise internal file buffers will not be released until the GC discovers you have forgotten to close the file.
If you don’t need to copy the file, just rename it (FileInfo.Rename). This is the normal way of coping with log files.
If you don’t need to process the data, use the FileInfo.CopyTo or CopyTo(Stream) method, that why the text will be copied using a sensible small buffer and memory will never need to be allocated to hold all the text at the same time.
If you do need to process the text, read one line at a time, this will result in lots of small strings being created, rather than one very large string. The .net GC is very good at reclaiming small short lived object. A no point should you have the entire log file in memory at the same time. Creating a custom iterator that returns the lines in the file, would be one way of doing it.
