Even if you remember to set Set­Last­Error=true in your p/invoke signature, you still have to be careful with Marshal.Get­Last­Win32­Error because there is only one last-error code, and it gets overwritten each time.

So let’s try this program:

using System;
using System.Runtime.InteropServices;

class Program
{
  [DllImport("user32.dll", SetLastError=true)]
  public static extern bool OpenIcon(IntPtr hwnd);

  public static void Main()
  {
    // Intentionally pass an invalid parameter.
    var result = OpenIcon(IntPtr.Zero);
    Console.WriteLine("result: {0}", result);
    Console.WriteLine("last error = {0}",
                      Marshal.GetLastWin32Error());
  }
}

The expectation is that the call to Open­Icon will fail, and the error code will be some form of invalid parameter.

But when you run the program, it prints this:

result: False
last error = 0

Zero?

Zero means “No error”. But the function failed. Where’s our error code? We printed the result immediately after calling Open­Icon. We didn’t call any other p/invoke functions. The last-error code should still be there.

Oh wait, printing the result to the screen involves a function call.

That function call might itself do a p/invoke!

We have to call Marshal.Get­Last­Win32­Error immediately after calling Open­Icon. Nothing else can sneak in between.

using System;
using System.Runtime.InteropServices;

class Program
{
  [DllImport("user32.dll", SetLastError=true)]
  public static extern bool OpenIcon(IntPtr hwnd);

  public static void Main()
  {
    // Intentionally pass an invalid parameter.
    var result = OpenIcon(IntPtr.Zero);
    var lastError = Marshal.GetLastWin32Error();
    Console.WriteLine("result: {0}", result);
    Console.WriteLine("last error = {0}",
                      lstError);
  }
}

Okay, now the program reports the error code as 1400: “Invalid window handle.”

This one was pretty straightforward, because the function call that modified the last-error code was right there in front of us. But there are other ways that code can run which are more subtle.

• If you retrieve a property, the property retrieval may involve a p/invoke.
• If you access a class that has a static constructor, the static constructor will secretly run if this is the first time the class is used.