how to play from stream?

Question

var ms=new MemoryStream();
MediaSource.CreateFromStream(ms.AsRandomAccessStream(),"video/mp4");
while(true)
{
//here I will read from another stream to buffer.
ms.Write(buffer,0,buffer.Length);
}

but it will not play the video. and will throw exception that System.OutOfMemoryException but the length is only 130Mb of the stream.

why?