I want to make a program to download files from a remote host.
So what i do is to send throw sockets all the binary information + a string used as end "mystyring".
To check if i have to continue writing the buffer to the file i first convert all to string, so if i find "mystring" it will finish, if not, i reconvert all to bytes and i copy data.
The problem im having is that, .txt files are writed well, but binaris not, it always show error at opening. I ve tried copying a pdf, and when i open it show all the pages, but in blank :S. Also if i check the size of the original file is always some bytes bigger than the copy.

FileStream fs = new FileStream("C:\\Users\\Mauri\\Desktop\\" + item.Name, FileMode.Append, FileAccess.Write);
            BinaryWriter writer = new BinaryWriter(fs);
            bool continua = true;
            while (continua)
                recibido = getSocket(numsock).socketRecv(); // This return the data converted to string.
                words = recibido.Split('|'); //We split "|mystring" to find the end of file.
                foreach (string s in words)
                    if ((String.Compare(s, "mystring") != 0))                        
                        writer.Write(StringToBytes(s)); // Write converted data. 
                        continua = false; // if we find "mystring", end the action.

// Convert string to bytes
public byte[] StringToBytes(String cadena)
            System.Text.ASCIIEncoding codificador = new System.Text.ASCIIEncoding();
            return codificador.GetBytes(cadena);

The strange thing is that .txt files are writed correctly.

I'm also working on something similar to this. I believe your problem is that you're writing a string representation of the data to the file. .txt files contain only text and nothing else. Right click a .txt file and open it with Notepad. It's just text. Now right click an .exe file and open it with Notepad. It's a bunch of garbage. That's the file's raw byte code.

Converting that to a string and writing it to a file is not going to work. You're going to have to write the raw byte code to a file with no changes to it after you receive it.

Use File.WriteAllBytes(Destination, data)

The problem of using writeallbytes is that it will write the "mystring", thats why i ve first converted all to string.

You can't convert a file's byte code to a string and then back to bytes. It will not contain the same data. That's why your copy is smaller than the original; data is lost. I can't explain exactly why this happens but it just does.

I can't think of anything right now that would work with how you have this set up. You have to write the raw data to file without converting it. But I'll try to think of or find something that will work for you. I don't want you to have to rewrite it! I know how that feels. :D

In the meantime could I see your code for 'socketRecv()'?

When/where do you append "mystring" to the end of the received data?

And why do you have to do that anyway? The data returned from the remote host will include the length of the content sent, so you'll know where the "end" of the file is.