Save and load MemoryStream to/from a file

The stream should really by disposed of even if there's an exception (quite likely on file I/O) - using clauses are my favourite approach for this, so for writing your MemoryStream, you can use:

using (FileStream file = new FileStream("file.bin", FileMode.Create, FileAccess.Write)) {
    memoryStream.WriteTo(file);
}

And for reading it back:

using (FileStream file = new FileStream("file.bin", FileMode.Open, FileAccess.Read)) {
    byte[] bytes = new byte[file.Length];
    file.Read(bytes, 0, (int)file.Length);
    ms.Write(bytes, 0, (int)file.Length);
}

If the files are large, then it's worth noting that the reading operation will use twice as much memory as the total file size. One solution to that is to create the MemoryStream from the byte array - the following code assumes you won't then write to that stream.

MemoryStream ms = new MemoryStream(bytes, writable: false);

My research (below) shows that the internal buffer is the same byte array as you pass it, so it should save memory.

byte[] testData = new byte[] { 104, 105, 121, 97 };
var ms = new MemoryStream(testData, 0, 4, false, true);
Assert.AreSame(testData, ms.GetBuffer());

Assuming that MemoryStream name is ms.

This code writes down MemoryStream to a file:

using (FileStream file = new FileStream("file.bin", FileMode.Create, System.IO.FileAccess.Write)) {
   byte[] bytes = new byte[ms.Length];
   ms.Read(bytes, 0, (int)ms.Length);
   file.Write(bytes, 0, bytes.Length);
   ms.Close();
}

and this reads a file to a MemoryStream :

using (MemoryStream ms = new MemoryStream())
using (FileStream file = new FileStream("file.bin", FileMode.Open, FileAccess.Read)) {
   byte[] bytes = new byte[file.Length];
   file.Read(bytes, 0, (int)file.Length);
   ms.Write(bytes, 0, (int)file.Length);
}

In .Net Framework 4+, You can simply copy FileStream to MemoryStream and reverse as simple as this:

MemoryStream ms = new MemoryStream();
using (FileStream file = new FileStream("file.bin", FileMode.Open, FileAccess.Read))
    file.CopyTo(ms);

And the Reverse (MemoryStream to FileStream):

using (FileStream file = new FileStream("file.bin", FileMode.Create, System.IO.FileAccess.Write))
    ms.CopyTo(file);

You may use MemoryStream.WriteTo or Stream.CopyTo (supported in framework version 4.5.2, 4.5.1, 4.5, 4) methods to write content of memory stream to another stream.

memoryStream.WriteTo(fileStream);

Update:

fileStream.CopyTo(memoryStream);
memoryStream.CopyTo(fileStream);