MockFileStream unexpected behavior when used via BinaryWriter

[RThomasHyde]

Using the latest stable NuGet version (7.0.7)

If I initialize a BinaryWriter with a MockFileStream (obtained e.g. by using MockFileSystem.File.Create), and then manipulate it via BinaryWriter.BaseStream, I get unexpected behavior. For instance, examine the following:

var fileSystem = new MockFileSystem();
using (var s = fileSystem.File.Create(@"C:\testfile.bin"))
using (var writer = new System.IO.BinaryWriter(s))
{
    writer.Write(new byte[400], 0, 400);
    writer.BaseStream.Seek(200, System.IO.SeekOrigin.Begin);
    Console.WriteLine(writer.BaseStream.Position);   // prints 400, expect 200
    Console.WriteLine(s.Position); // prints 400, expect 200
}

Presumably this is because BinaryWriter.BaseStream calls Flush on the underlying stream before returning it, though it's not immediately clear to me why this should be a problem. Avoiding accessing BaseStream by calling e.g. writer.Seek directly does avoid the problem, but the point is that MockFileStream does not replicate the behavior of a real FileStream in this regard:

using (var s = System.IO.File.Create(@"C:\testfile.bin")
using (var writer = new System.IO.BinaryWriter(s))
{
    writer.Write(new byte[400], 0, 400);
    writer.BaseStream.Seek(200,  System.IO.SeekOrigin.Begin);
    Console.WriteLine(writer.BaseStream.Position);   // prints 200 as expected
    Console.WriteLine(s.Position); // prints 200 as expected
}

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[fgreinacher]

Thanks for reporting! Might have the same root cause as #464.

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