C#: Begin move to .NET Core

We're targeting .NET 5 for now to make development easier while
.NET 6 is not yet released.

TEMPORARY REGRESSIONS
---------------------

Assembly unloading is not implemented yet. As such, many Godot
resources are leaked at exit. This will be re-implemented later
together with assembly hot-reloading.

modules/mono/glue/GodotSharp/GodotPlugins/Main.cs

@@ -0,0 +1,197 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Runtime.Loader;
+using Godot.NativeInterop;
+
+namespace GodotPlugins
+{
+    public static class Main
+    {
+        private static readonly List<AssemblyName> SharedAssemblies = new();
+        private static readonly Assembly CoreApiAssembly = typeof(Godot.Object).Assembly;
+        private static Assembly? _editorApiAssembly;
+
+        private static readonly AssemblyLoadContext MainLoadContext =
+            AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly()) ??
+            AssemblyLoadContext.Default;
+
+        // Right now we do it this way for simplicity as hot-reload is disabled. It will need to be changed later.
+        [UnmanagedCallersOnly]
+        internal static unsafe godot_bool Initialize(godot_bool editorHint,
+            PluginsCallbacks* pluginsCallbacks, Godot.Bridge.ManagedCallbacks* managedCallbacks)
+        {
+            try
+            {
+                SharedAssemblies.Add(CoreApiAssembly.GetName());
+
+                if (editorHint.ToBool())
+                {
+                    _editorApiAssembly = Assembly.Load("GodotSharpEditor");
+                    SharedAssemblies.Add(_editorApiAssembly.GetName());
+                }
+
+                NativeLibrary.SetDllImportResolver(CoreApiAssembly, OnResolveDllImport);
+
+                *pluginsCallbacks = new()
+                {
+                    LoadProjectAssemblyCallback = &LoadProjectAssembly,
+                    LoadToolsAssemblyCallback = &LoadToolsAssembly,
+                };
+
+                *managedCallbacks = Godot.Bridge.ManagedCallbacks.Create();
+
+                return godot_bool.True;
+            }
+            catch (Exception e)
+            {
+                Console.Error.WriteLine(e);
+                *pluginsCallbacks = default;
+                *managedCallbacks = default;
+                return false.ToGodotBool();
+            }
+        }
+
+        [StructLayout(LayoutKind.Sequential)]
+        internal struct PluginsCallbacks
+        {
+            public unsafe delegate* unmanaged<char*, godot_bool> LoadProjectAssemblyCallback;
+            public unsafe delegate* unmanaged<char*, IntPtr> LoadToolsAssemblyCallback;
+        }
+
+        [UnmanagedCallersOnly]
+        internal static unsafe godot_bool LoadProjectAssembly(char* nAssemblyPath)
+        {
+            try
+            {
+                string assemblyPath = new(nAssemblyPath);
+
+                var assembly = LoadPlugin(assemblyPath);
+
+                var method = CoreApiAssembly.GetType("Godot.Bridge.ScriptManagerBridge")?
+                    .GetMethod("LookupScriptsInAssembly",
+                        BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public);
+
+                if (method == null)
+                {
+                    throw new MissingMethodException("Godot.Bridge.ScriptManagerBridge",
+                        "LookupScriptsInAssembly");
+                }
+
+                method.Invoke(null, new object[] { assembly });
+
+                return godot_bool.True;
+            }
+            catch (Exception e)
+            {
+                Console.Error.WriteLine(e);
+                return false.ToGodotBool();
+            }
+        }
+
+        [UnmanagedCallersOnly]
+        internal static unsafe IntPtr LoadToolsAssembly(char* nAssemblyPath)
+        {
+            try
+            {
+                string assemblyPath = new(nAssemblyPath);
+
+                if (_editorApiAssembly == null)
+                    throw new InvalidOperationException("The Godot editor API assembly is not loaded");
+
+                var assembly = LoadPlugin(assemblyPath);
+
+                NativeLibrary.SetDllImportResolver(assembly, OnResolveDllImport);
+
+                var method = assembly.GetType("GodotTools.GodotSharpEditor")?
+                    .GetMethod("InternalCreateInstance",
+                        BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public);
+
+                if (method == null)
+                {
+                    throw new MissingMethodException("GodotTools.GodotSharpEditor",
+                        "InternalCreateInstance");
+                }
+
+                return (IntPtr?)method.Invoke(null, null) ?? IntPtr.Zero;
+            }
+            catch (Exception e)
+            {
+                Console.Error.WriteLine(e);
+                return IntPtr.Zero;
+            }
+        }
+
+        private static Assembly LoadPlugin(string assemblyPath)
+        {
+            string assemblyName = Path.GetFileNameWithoutExtension(assemblyPath);
+
+            var sharedAssemblies = new List<string>();
+
+            foreach (var sharedAssembly in SharedAssemblies)
+            {
+                string? sharedAssemblyName = sharedAssembly.Name;
+                if (sharedAssemblyName != null)
+                    sharedAssemblies.Add(sharedAssemblyName);
+            }
+
+            var loadContext = new PluginLoadContext(assemblyPath, sharedAssemblies, MainLoadContext);
+            return loadContext.LoadFromAssemblyName(new AssemblyName(assemblyName));
+        }
+
+        public static IntPtr OnResolveDllImport(string libraryName, Assembly assembly, DllImportSearchPath? searchPath)
+        {
+            if (libraryName == "__Internal")
+            {
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                {
+                    return Win32.GetModuleHandle(IntPtr.Zero);
+                }
+                else if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
+                {
+                    return Linux.dlopen(IntPtr.Zero, Linux.RTLD_LAZY);
+                }
+                else if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+                {
+                    return MacOS.dlopen(IntPtr.Zero, MacOS.RTLD_LAZY);
+                }
+            }
+
+            return IntPtr.Zero;
+        }
+
+        // ReSharper disable InconsistentNaming
+        private static class MacOS
+        {
+            private const string SystemLibrary = "/usr/lib/libSystem.dylib";
+
+            public const int RTLD_LAZY = 1;
+
+            [DllImport(SystemLibrary)]
+            public static extern IntPtr dlopen(IntPtr path, int mode);
+        }
+
+        private static class Linux
+        {
+            // libdl.so was resulting in DllNotFoundException, for some reason...
+            // libcoreclr.so should work with both CoreCLR and the .NET Core version of Mono.
+            private const string SystemLibrary = "libcoreclr.so";
+
+            public const int RTLD_LAZY = 1;
+
+            [DllImport(SystemLibrary)]
+            public static extern IntPtr dlopen(IntPtr path, int mode);
+        }
+
+        private static class Win32
+        {
+            private const string SystemLibrary = "Kernel32.dll";
+
+            [DllImport(SystemLibrary)]
+            public static extern IntPtr GetModuleHandle(IntPtr lpModuleName);
+        }
+        // ReSharper restore InconsistentNaming
+    }
+}