From e1981226914c92589ddbe4af77726d91c56c8b87 Mon Sep 17 00:00:00 2001
From: CasualPokePlayer <50538166+CasualPokePlayer@users.noreply.github.com>
Date: Mon, 29 Aug 2022 16:33:41 -0700
Subject: [PATCH] output a more helpful error message on windows for
 GetErrorMessage

---
 src/BizHawk.Common/OSTailoredCode.cs | 29 ++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/src/BizHawk.Common/OSTailoredCode.cs b/src/BizHawk.Common/OSTailoredCode.cs
index eec8a48fba..a51ef5d5dd 100644
--- a/src/BizHawk.Common/OSTailoredCode.cs
+++ b/src/BizHawk.Common/OSTailoredCode.cs
@@ -3,6 +3,7 @@
 using System;
 using System.Diagnostics;
 using System.Runtime.InteropServices;
+using System.Text;
 
 using BizHawk.Common.StringExtensions;
 
@@ -147,8 +148,24 @@ namespace BizHawk.Common
 			private static extern bool FreeLibrary(IntPtr hModule); // return type was annotated MarshalAs(UnmanagedType.Bool)
 
 			[DllImport("kernel32.dll")]
-			private static extern uint GetLastError();
+			private static extern uint GetLastError();
+
+			private enum FORMAT_MESSAGE : uint
+			{
+				ALLOCATE_BUFFER = 0x00000100,
+				ARGUMENT_ARRAY = 0x00002000,
+				FROM_HMODULE = 0x00000800,
+				FROM_STRING = 0x00000400,
+				FROM_SYSTEM = 0x00001000,
+				IGNORE_INSERTS = 0x00000200,
+			}
+
+			[DllImport("kernel32.dll")]
+			private static extern int FormatMessageA(FORMAT_MESSAGE flags, IntPtr source, uint messageId, uint languageId, out IntPtr outMsg, int size, IntPtr args);
 
+			[DllImport("kernel32.dll")]
+			private static extern IntPtr LocalFree(IntPtr hMem); // use this to free a message from FORMAT_MESSAGE.ALLOCATE_BUFFER
+
 			[DllImport("kernel32.dll", SetLastError = true)] // had BestFitMapping = false, ThrowOnUnmappableChar = true
 			private static extern IntPtr GetProcAddress(IntPtr hModule, string procName); // param procName was annotated `[MarshalAs(UnmanagedType.LPStr)]`
 
@@ -173,7 +190,15 @@ namespace BizHawk.Common
 				return ret != IntPtr.Zero ? ret : throw new InvalidOperationException($"got null pointer from {nameof(LoadLibrary)}, error code: {GetLastError()}");
 			}
 
-			public string GetErrorMessage() => $"Error Code 0x{GetLastError():X8}";
+			public string GetErrorMessage()
+			{
+				var errCode = GetLastError();
+				var sz = FormatMessageA(FORMAT_MESSAGE.ALLOCATE_BUFFER | FORMAT_MESSAGE.FROM_SYSTEM | FORMAT_MESSAGE.IGNORE_INSERTS,
+					IntPtr.Zero, errCode, 0, out var buffer, 1024, IntPtr.Zero);
+				var ret = Marshal.PtrToStringAnsi(buffer, sz);
+				_ = LocalFree(buffer);
+				return ret;
+			}
 		}
 
 		public enum DistinctOS : byte