[generate-type-forwarders] Fix issues with properties stubs (#10941)

Examples

* Fix visibility, e.g. `protected`

```diff
-		public virtual System.IntPtr RawJointLandmarks {
+		protected virtual System.IntPtr RawJointLandmarks {
```

* Remove non-visible properties, e.g. `internal`

```diff
-		public virtual System.IntPtr _AddressBook {
-			get { throw new global::System.PlatformNotSupportedException (global::Constants.UnavailableOnMacCatalyst); }
-			set { throw new global::System.PlatformNotSupportedException (global::Constants.UnavailableOnMacCatalyst); }
-		}
```

* Fix `abstract` members

```diff
-public abstract ARSession Session { get; }
+public virtual ARSession Session { get; }
```

src/generate-type-forwarders/Program.cs

@@ -18,7 +18,7 @@
  */
 
 namespace GenerateTypeForwarders {
- class MainClass {
+ static class MainClass {
  public static int Main (string [] args)
  {
  var forwardFrom = args [0];
@@ -27,6 +27,30 @@ public static int Main (string [] args)
  return Fix (forwardFrom, forwardTo, output);
  }
 
+ static bool IsVisible (this TypeDefinition type)
+ {
+ if (!type.IsNested)
+ return true;
+ return type.IsNestedPublic || type.IsNestedFamily;
+ }
+
+ static bool IsVisible (this MethodDefinition method)
+ {
+ if (method is null)
+ return false;
+ return method.IsPublic || method.IsFamily;
+ }
+
+ static bool IsVisible (this PropertyDefinition property)
+ {
+ return property.GetMethod.IsVisible () || property.SetMethod.IsVisible ();
+ }
+
+ static bool IsVisible (this FieldDefinition field)
+ {
+ return field.IsPublic || field.IsFamily;
+ }
+
  static void EmitTypeName (StringBuilder sb, TypeReference type)
  {
  if (type.FullName == "System.Void") {
@@ -357,10 +381,14 @@ static bool HasPropertyInTypeHierarchy (TypeDefinition type, MethodDefinition ac
 
  static void EmitProperty (StringBuilder sb, PropertyDefinition pd, int indent)
  {
- var strIndent = new string ('\t', indent);
- sb.Append (strIndent);
- sb.Append ("public ");
- var m = pd.GetMethod ?? pd.SetMethod;
+ sb.Append ('\t', indent);
+ var gm = pd.GetMethod;
+ var sm = pd.SetMethod;
+ var m = gm ?? sm;
+ if (m.IsPublic)
+ sb.Append ("public ");
+ else
+ sb.Append ("protected ");
  if (m.IsStatic) {
  sb.Append ("static ");
  if (HasPropertyInTypeHierarchy (pd.DeclaringType, m, out var _))
@@ -373,20 +401,32 @@ static void EmitProperty (StringBuilder sb, PropertyDefinition pd, int indent)
  sb.Append ("new ");
  }
  } else if (!pd.DeclaringType.IsSealed) {
- sb.Append ("virtual ");
+ if (m.IsAbstract)
+ sb.Append ("abstract ");
+ else
+ sb.Append ("virtual ");
  }
  }
  EmitTypeName (sb, pd.PropertyType);
  sb.Append (' ');
  sb.Append (pd.Name);
  sb.AppendLine (" {");
- if (pd.GetMethod != null) {
- sb.AppendLine ($"{strIndent}\tget {{ throw new global::System.PlatformNotSupportedException (global::Constants.UnavailableOnMacCatalyst); }}");
+ if (gm.IsVisible ()) {
+ sb.Append ('\t', indent + 1);
+ if (gm.IsAbstract)
+ sb.AppendLine ("get;");
+ else
+ sb.AppendLine ("get { throw new global::System.PlatformNotSupportedException (global::Constants.UnavailableOnMacCatalyst); }");
  }
- if (pd.SetMethod != null) {
- sb.AppendLine ($"{strIndent}\tset {{ throw new global::System.PlatformNotSupportedException (global::Constants.UnavailableOnMacCatalyst); }}");
+ if (pd.SetMethod.IsVisible ()) {
+ sb.Append ('\t', indent + 1);
+ if (sm.IsAbstract)
+ sb.AppendLine ("set;");
+ else
+ sb.AppendLine ("set { throw new global::System.PlatformNotSupportedException (global::Constants.UnavailableOnMacCatalyst); }");
  }
- sb.AppendLine ($"{strIndent}}}");
+ sb.Append ('\t', indent);
+ sb.AppendLine ("}");
  }
 
  static void EmitEvent (StringBuilder sb, EventDefinition ed, int indent)
@@ -425,7 +465,12 @@ static void EmitPNSE (StringBuilder sb, TypeDefinition type, int indent)
  EmitParameters (sb, invoke.Parameters);
  sb.AppendLine (");");
  } else {
- sb.Append ($"{strIndent}public ");
+ sb.Append (strIndent);
+ // other are filtered not to generate stubs
+ if (type.IsNestedFamily)
+ sb.Append ("protected ");
+ else
+ sb.Append ("public ");
  if (type.IsInterface) {
  sb.Append ("interface ");
  } else if (type.IsEnum) {
@@ -484,7 +529,7 @@ static void EmitPNSE (StringBuilder sb, TypeDefinition type, int indent)
  sb.Append (" {");
  sb.AppendLine ();
  foreach (var nestedType in type.NestedTypes) {
- if (nestedType.IsNestedPrivate)
+ if (!nestedType.IsVisible ())
  continue;
  EmitPNSE (sb, nestedType, indent + 1);
  }
@@ -498,18 +543,20 @@ static void EmitPNSE (StringBuilder sb, TypeDefinition type, int indent)
  bt = bt.BaseType?.Resolve ();
  }
  foreach (var method in type.Methods) {
+ // need .ctor(IntPtr) for chaining
+ if (!method.IsVisible () && !method.IsConstructor)
+ continue;
  EmitPNSE (sb, method, indent + 1, nsobject);
  }
 
  foreach (var field in type.Fields) {
- if (field.IsPrivate)
+ if (!field.IsVisible ())
  continue;
-
  EmitField (sb, field, indent + 1);
  }
 
  foreach (var prop in type.Properties) {
- if (prop.GetMethod?.IsPrivate != false && prop.SetMethod?.IsPrivate != false)
+ if (!prop.IsVisible ())
  continue;
  EmitProperty (sb, prop, indent + 1);
  }