If you are going to call Marshal.GetLastWin32Error, the function whose error you're retrieving had better have SetLastError=true

A customer reported that their p/invoke to a custom DLL was failing, and the error code made no sense.

// C#
using System;
using System.Runtime.InteropServices;
using System.Diagnostics;

class Program
{
  [DllImport("contoso.dll", CallingConvention=CallingConvention.Cdecl)]
  public static extern int Fribble();

  public static void Main()
  {
    Console.WriteLine("About to call Fribble");

    var result = Fribble();
    if (result >= 0) {
      Console.WriteLine("succeeded {0}", result);
    } else {
      Console.WriteLine("failed {0}, last error = {1}",
                        result, Marshal.GetLastWin32Error());
    }
  }
}

// C++

int __cdecl Fribble()
{
 HANDLE hEvent = OpenEvent(EVENT_MODIFY_STATE, FALSE,
                           TEXT("FribbleEvent"));
 if (hEvent == nullptr)
  return -1;
 }

 if (!SetEvent(hEvent)) {
  CloseHandle(hEvent);
  return -2;
 }

 CloseHandle(hEvent);
 return 1;
}

The customer reported that their Fribble function was returning −1, indicating a failure to open the event, but the error code returned by Marshal.Get­Last­Win32­Error is 87, "The parameter is incorrect." But all of the parameters to Open­Event look correct. Why are we getting this strange error code?

My psychic powers tell me that if the customer had taken the time to troubleshoot their problem by writing a C++ program that calls the Fribble function, Get­Last­Error would have returned the more reasonable error 2, meaning that the event does not exist.

That's because Get­Last­Error is working fine. The last error code is 2.

The problem is with the p/invoke declaration.

The documentation for the Marshal.Get­Last­Win32­Error function says as its very first line

Returns the error code returned by the last unmanaged function that was called using platform invoke that has the DllImportAttribute.SetLastError flag set.

(Emphasis mine.)

This reminder about Dll­Import­Attribute.Set­Last­Error is repeated in the Remarks.

You can use this method to obtain error codes only if you apply the System.Runtime.Interop­Services.Dll­Import­Attribute to the method signature and set the Set­Last­Error field to true.

Observe that the Set­Last­Error field was not set in the p/invoke declaration. Therefore, what you are actually getting when you call Marshal.Get­Last­Win32­Error is whatever error was lying around after the previous call to a p/invoke function that did specify Set­Last­Error = true.

Changing the p/invoke to

[DllImport("contoso.dll", SetLastError=true,
           CallingConvention=CallingConvention.Cdecl)]
public static extern int Fribble();

fixed the problem.