Add GetInstance path for singletons

Author: JordanL8

src/AST/Class.cs

@@ -83,6 +83,10 @@ public class Class : DeclarationContext
         // True if the record is a POD (Plain Old Data) type.
         public bool IsPOD;
 
+        // True if the class is a singleton and should be accessed as such
+        // from the generated GetInstance method.
+        public bool IsSingleton;
+
         // Semantic type of the class.
         public ClassType Type;
 
@@ -140,6 +144,7 @@ public Class()
             IsUnion = false;
             IsFinal = false;
             IsPOD = false;
+            IsSingleton = false;
             Type = ClassType.RefType;
             Layout = new ClassLayout();
             templateParameters = new List<Declaration>();

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -2478,16 +2478,37 @@ private void GenerateNativeConstructor(Class @class)
                     {
                         @new = @class.HasBase && HasVirtualTables(@class.Bases.First().Class);
 
-                        WriteLines($@"
+                        if (@class.IsSingleton)
+                        {
+                            WriteLines($@"
+private static {printedClass} singletonInstance;
 internal static{(@new ? " new" : string.Empty)} {printedClass} __GetInstance({TypePrinter.IntPtrType} native)
 {{
+    if (singletonInstance != null)
+        return singletonInstance
     if (!{Helpers.TryGetNativeToManagedMappingIdentifier}(native, out var managed))
         throw new global::System.Exception(""No managed instance was found"");
     var result = ({printedClass})managed;
     if (result.{Helpers.OwnsNativeInstanceIdentifier})
         result.SetupVTables();
+    singletonInstance = result;
     return result;
 }}");
+                        }
+                        // Not a singleton.
+                        else
+                        {
+                            WriteLines($@"
+internal static{(@new ? " new" : string.Empty)} {printedClass} __GetInstance({TypePrinter.IntPtrType} native)
+{{
+    if (!{Helpers.TryGetNativeToManagedMappingIdentifier}(native, out var managed))
+        throw new global::System.Exception(""No managed instance was found"");
+    var result = ({printedClass})managed;
+    if (result.{Helpers.OwnsNativeInstanceIdentifier})
+        result.SetupVTables();
+    return result;
+}}");
+                        }
                         NewLine();
                     }
                 }