Is IntPtr.Zero equivalent to null?

Question

I am trying to setup ReadFile to run asynchronously and according to MSDN, I need to set lpNumberOfBytesRead to null ("Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.")
For example, if I have the following:

  [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
  public static extern bool ReadFile(
     IntPtr hFile,
     out byte[] aBuffer,
     int cbToRead,
     IntPtr cbThatWereRead,
     ref OVERLAPPED pOverlapped
  );

and I call it like this (with the intention of having the 4th parameter being null):

Win32API.ReadFile(readHandle, out data_read, Win32API.BUFFER_SIZE, IntPtr.Zero, ref over_lapped);

will that be the same as calling it with null? If not, what should I change in the declaration or in the function call itself?

I was also curious if I should be using SafeHandle or HandleRef instead of IntPtrl for the hFile reference? I know to make sure that I close the handle with CloseHandle(IntPtr) when I'm done with it, just not sure if there is any othe reason to use the other two options over IntPtr. I am also tryingn to avoid using unsafe code.

EDIT: As it turns out, I shouldnt be setting the fourth parameter to IntPtr.Zero anyway, because even though I am running asynchronously, it could still return right away. See Asynchronous Disk I/O. Ahh, I love contradicting stories.

Answer 1

For P/Invoke purposes like you've listed, you should use IntPtr.Zero for NULL (note that it is not equivalent to the C# null however.)

Answer 2

You cannot assign null to a value-type. A reference-type can be null, as in, not referring to an object instance, but a value-type always has a value.

IntPtr.Zero is just a constant value that represents a null pointer.