memorystream（byte []）vs memorystream.write（byte []）

Question

I needed to put a byte to a memory stream so initially, I used:

byte[] Input;
using (MemoryStream mem = new MemoryStream())
{
    mem.Write(Input, 0, (int)Input.Length);
    StreamReader stream = new StreamReader(mem);
    ...
}

I wanted to use the Streamreader to read lines from a text file.

It didn't work.

Then I used

using (MemoryStream mem = new MemoryStream(Input))

instead and removed

mem.Write(Input, 0, (int)Input.Length);

It worked. I don't know why. Why did it work?

Answer 1

In your first approach, you use mem.Write(Input, 0, (int)Input.Length); . Note that MemoryStream.Write sets the stream read/write position behind the written data. In your example case this is equivalent with a position signifying the end of the stream. Trying to read from the MemoryStream again will not return any data, as the MemoryStream read/write position is at the end of the stream.

In your second approach, you passed the Input byte array as argument to the MemoryStream constructor. Providing the byte array through the constructor not only will make MemoryStream use this byte array, but more importantly it keeps the initial stream position of zero. Thus, when trying to read from the MemoryStream initialized in this way, the data contained in the input byte array will be returned as expected.

How to fix the problem with the first approach?

You can make the first approach with MemoryStream.Write working by simply setting the MemoryStream position back to the intended/original value (in your example case it would be zero) after writing the data to the MemoryStream:

byte[] Input;
using (MemoryStream mem = new MemoryStream())
{
    mem.Write(Input, 0, (int)Input.Length);

    mem.Position = 0;

    using (StreamReader stream = new StreamReader(mem))
    {
        ...
    }
}