app-win: implement dll-import resolver

Like the previous commit but for the windows loader.

This is generally not required on Windows, but might help with
fuse-x/studio#20.

src/runtime/win/Program.cs

@@ -1,5 +1,8 @@
 using System;
 using System.Globalization;
+using System.IO;
+using System.Reflection;
+using System.Runtime.InteropServices;
 using System.Threading;
 using System.Windows.Forms;
 
@@ -29,12 +32,55 @@ static Program()
             Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;
             Thread.CurrentThread.CurrentUICulture = CultureInfo.InvariantCulture;
             AppDomain.CurrentDomain.UnhandledException += UnhandledExceptionTrapper;
+
+            AppDomain.CurrentDomain.AssemblyLoad += (sender, ev) => {
+                try
+                {
+                    if (!UsesUnoPInvoke(ev.LoadedAssembly))
+                        return;
+
+                    // Set DllImportResolver for loading native libraries in assemblies generated by Uno
+                    NativeLibrary.SetDllImportResolver(ev.LoadedAssembly, (libraryName, assembly, searchPath) => {
+                        if (!libraryName.Contains(".dll") || libraryName.StartsWith("/"))
+                            return IntPtr.Zero;
+
+                        var dir = Path.GetDirectoryName(assembly.Location);
+                        var lib = Path.Combine(dir, libraryName);
+
+                        if (File.Exists(lib))
+                            return LoadLibrary(lib);
+
+                        Console.Error.WriteLine("Native library not found: " + lib);
+                        return IntPtr.Zero;
+                    });
+                }
+                catch (Exception e)
+                {
+                    Console.Error.WriteLine("NativeLibrary.SetDllImportResolver: " + e);
+                }
+            };
+        }
+
+        static bool UsesUnoPInvoke(Assembly asm)
+        {
+            foreach (var attr in asm.GetCustomAttributes())
+            {
+                var metadata = attr as AssemblyMetadataAttribute;
+
+                if (metadata?.Key == "Uno.PInvoke")
+                    return bool.Parse(metadata.Value);
+            }
+
+            return false;
         }
 
         static void UnoGenerated()
         {
-            // Uno compiler will replace this.
+            // Uno compiler will replace this function
             new DummyApp();
         }
+
+        [DllImport("kernel32.dll")]
+        static extern IntPtr LoadLibrary(string filename);
     }
 }