Question

FileStream stream = new FileStream("test",FileMode.Create);
BufferedStream buff = new BufferedStream(stream, 8);
BinaryWriter writer = new BinaryWriter(buff);
writer.Write(1);
writer.Write(2);
writer.Write(3);
Console.WriteLine(buff.Length);

As I understand content is flushed when it reaches bufer size in this code 8 bytes.
So why buff.Length returns 12 and why data appears in file only if I explicit call Dispose/Close?

Answer 1

The BufferedStream works fine – it doesn’t flush: stream.Length is 8 until it is closed; i.e.

// note these should really use "using" statements of similar
FileStream stream = new FileStream("test", FileMode.Create);
BufferedStream buff = new BufferedStream(stream, 8);
BinaryWriter writer = new BinaryWriter(buff);
writer.Write(1);
writer.Write(2);
writer.Write(3);
Console.WriteLine(stream.Length); // 8
Console.WriteLine(buff.Length); // 12

The reason that buff.Length is 12 is because that is the length of the underlying stream. Basically, BufferedStream just re-exposes the stream underneath, and to ensure it doesn’t miss the buffered data it adds a Flush() if there is buffered data:

public override long Length
{
    get
    {
        this.EnsureNotClosed();
        if (this._writePos > 0)
        {
            this.FlushWrite();
        }
        return this._stream.Length;
    }
}

Hence:

Console.WriteLine(stream.Length); // 8
Console.WriteLine(buff.Length); // 12
Console.WriteLine(stream.Length); // 12

Note that the FileStream can have its own buffering too