NetworkStream.Read（）在第一次循环迭代后无法正确读取

Question

I'm having a problem where my code works as expected when I'm stepping through it but will read incorrect data when running normally. I thought the problem could have been timing however NetworkStream.Read() should be blocking and I also tested this by putting the thread to sleep for 1000ms (more than enough time and more time than I was giving whilst stepping through).

The purpose of the code (and what is does when stepping through) is to read a bitmap image into a buffer that is preceded by a string containing the image size in bytes followed by a carriage return and a new line. I believe the problem lies in the read statements, but I can't be sure. The following code is contained within a larger loop also containing Telnet reads, however I have not has a problem with those and they are only reading ASCII strings, no binary data.

List<byte> len = new List<byte>();
byte[] b = new byte[2];
while (!Encoding.ASCII.GetString(b).Equals("\r\n"))
{
    len.Add(b[0]);
    b[0] = b[1];
    b[1] = (byte)stream.ReadByte();
}
len = len.FindAll(x => x != 0);
len.Add((byte)0);
string lenStr = Encoding.ASCII.GetString(len.ToArray());
int imageSize = int.Parse(lenStr);
byte[] imageIn = new byte[imageSize];
stream.Read(imageIn, 0, imageSize);
using (MemoryStream g = new MemoryStream(imageIn))
{
    g.Position = 0;
    bmp = (Bitmap)Image.FromStream(g);
}

The actual problem that occurs with the code is that the first time it runs it correctly receives the length and image, however it does not seem to recognize the \\r\\n in consecutive reads, however this may only be a symptom and not the problem itself.

Thanks in advance!

EDIT: So I did narrow the problem down and manage to fix it by adding in some artificial delay between my Telnet call using NetworkStream.Write() to retrieve the image and the networkStream.Read() to retrieve it, however this solution is messy and I would still like to know why this issue is happening

Answer 1

The Read() operation returns the number of bytes actually read. It only blocks when there is no data to read, and it can return less bytes as specified by the count parameter.

You can easily fix this by putting this inside a loop:

byte[] imageIn = new byte[imageSize];
int remaining = imageSize;
int offset = 0;
while (remaining > 0)
{
    int read = stream.Read(imageIn, offset, remaining);
    if (read == 0) throw new Exception("Connection closed before expected data was read");
    offset += read;
    remaining -= read;
}