MockFileSystem doesn't handle FileMode.Truncate as expected

[ckarras]

Using System.IO.Abstractions.TestingHelpers 2.1.0.231

FileStream.Create with FileMode.Truncate doesn't correctly truncate existing files. If the new content written to the file is smaller than the old content, the smaller content will overwrite the begining of the file, but then the old content will remain.

Example:

            using (var stream = fileSystem.FileStream.Create("c:\\t\\test", FileMode.Create, FileAccess.Write))
            {
                using (var writer = new StreamWriter(stream))
                {
                    writer.Write("test1234TEST5678");
                }
            }

            using (var stream = fileSystem.FileStream.Create("c:\\t\\test", FileMode.Truncate, FileAccess.Write))
            {
                using (var writer = new StreamWriter(stream))
                {
                    writer.Write("ab");
                }
            }

            fileSystem.File.ReadAllLines("c:\\t\\test").Should().Equal("ab");

• If fileSystem = new FileSystem(), then the test passes.
• But if fileSystem = new MockFileSystem(), then the test fails because the file contents is: abst1234TEST5678 (first 2 characters overwritten, followed by characters from the original file that shouldn't be there anymore)

[robkeim]

I made a first attempt to fix this in #358, but I'm not familiar with Truncate mode so my concern is there are other behavior differences this naive, first implementation is missing.

[robkeim]

Thanks for reporting this @ckarras and for the detailed test case that reproduces the issue.