Add a separate sanity check for SMX.dll existing, since Windows gives the same error if a DLL is missing or one of its dependencies is missing.

smx-config/App.xaml.cs

@@ -15,9 +15,16 @@ namespace smx_config
         {
             AppDomain.CurrentDomain.UnhandledException += UnhandledExceptionEventHandler;
             
+            if(!SMX.SMX.DLLExists())
+            {
+                MessageBox.Show("SMXConfig encountered an unexpected error.\n\nSMX.dll couldn't be found:\n\n" + Helpers.GetLastWin32ErrorString(), "SMXConfig");
+                Current.Shutdown();
+                return;
+            }
+            
             if(!SMX.SMX.DLLAvailable())
             {
-                MessageBox.Show("SMXConfig encountered an unexpected error:\n\nSMX.dll failed to load.", "SMXConfig");
+                MessageBox.Show("SMXConfig encountered an unexpected error.\n\nSMX.dll failed to load:\n\n" + Helpers.GetLastWin32ErrorString(), "SMXConfig");
                 Current.Shutdown();
                 return;
             }

smx-config/Helpers.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Windows.Media;
 using System.Windows.Threading;
 using SMXJSON;
@@ -79,6 +80,15 @@ namespace smx_config
             return false;
         }
 
+        // Return the last Win32 error as a string.
+        public static string GetLastWin32ErrorString()
+        {
+            int error = Marshal.GetLastWin32Error();
+            if(error == 0)
+                return "";
+            return new System.ComponentModel.Win32Exception(error).Message;
+        }
+
         // Work around Enumerable.SequenceEqual not checking if the arrays are null.
         public static bool SequenceEqual<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second)
         {

smx-config/SMX.cs

@@ -198,8 +198,27 @@ namespace SMX
 
     public static class SMX
     {
+        [System.Flags]
+        enum LoadLibraryFlags : uint
+        {    
+              None = 0, 
+              DONT_RESOLVE_DLL_REFERENCES = 0x00000001,
+              LOAD_IGNORE_CODE_AUTHZ_LEVEL = 0x00000010,
+              LOAD_LIBRARY_AS_DATAFILE = 0x00000002,
+              LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE = 0x00000040,
+              LOAD_LIBRARY_AS_IMAGE_RESOURCE = 0x00000020,
+              LOAD_LIBRARY_SEARCH_APPLICATION_DIR = 0x00000200,
+              LOAD_LIBRARY_SEARCH_DEFAULT_DIRS = 0x00001000,
+              LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR = 0x00000100,
+              LOAD_LIBRARY_SEARCH_SYSTEM32 = 0x00000800,
+              LOAD_LIBRARY_SEARCH_USER_DIRS = 0x00000400,
+              LOAD_WITH_ALTERED_SEARCH_PATH = 0x00000008
+        }
+
         [DllImport("kernel32", SetLastError=true)]
         static extern IntPtr LoadLibrary(string lpFileName);
+        [DllImport("kernel32.dll", SetLastError = true)]
+        static extern IntPtr LoadLibraryEx(string lpFileName, IntPtr hReservedNull, LoadLibraryFlags dwFlags);
         [DllImport("SMX.dll", CallingConvention = CallingConvention.Cdecl)]
         private static extern void SMX_Start(
             [MarshalAs(UnmanagedType.FunctionPtr)] InternalUpdateCallback callback,
@@ -229,11 +248,21 @@ namespace SMX
         private static extern bool SMX_ReenableAutoLights();
 
         // Check if the native DLL is available.  This is mostly to avoid exceptions in the designer.
+        // This returns false if the DLL doesn't load.
         public static bool DLLAvailable()
         {
+            
             return LoadLibrary("SMX.dll") != IntPtr.Zero;
         }
 
+        // Check if the native DLL exists.  This will return false if SMX.dll is missing entirely,
+        // but not if it fails to load for another reason like runtime dependencies.  This just lets
+        // us print a more specific error message.
+        public static bool DLLExists()
+        {
+            return LoadLibraryEx("SMX.dll", (IntPtr)0, LoadLibraryFlags.LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE) != IntPtr.Zero;
+        }
+        
         public delegate void UpdateCallback(int PadNumber, SMXUpdateCallbackReason reason);
 
         // The C API allows a user pointer, but we don't use that here.