无法使用BinaryWriter写入MemoryStream

Question

I've tried this code:

byte[] someData = new byte[] { 1, 2, 3, 4 };
MemoryStream stream = new MemoryStream(someData, 1, someData.Length - 1, true);
using (BinaryWriter writer = new BinaryWriter(stream))
{
    writer.Write(1);
}
stream.Dispose();

Everytime it's run, a NotSupportedException is thrown, telling me that the stream cannot be written to. Why is this the case? The last parameter of the initialization shown in line 2 clearly is true, so I should be able to write to the stream. It works if I don't specify the start index and count.

Why does this happen?

Answer 1

Always (almost always) create a memory stream without parameters in the constructor:

 using (MemoryStream stream = new MemoryStream())
        {
            using (BinaryWriter writer = new BinaryWriter(stream))
            {
                writer.Write(1);
            }
            stream.Flush();
            byte[] bytes = stream.GetBuffer();
            //use it
        }

This code works fine

Answer 2

From MSDN :

Initializes a new non-resizable instance of the MemoryStream class based on the specified region of a byte array, with the CanWrite property set as specified.

The BinaryWriter starts writing at the end of the stream, so it needs to resize it to be able to write, but this is not allowed. You can only write to the already allocated bytes of the stream.