Minor improvement in understanding GxExternalCollection<T> (#1190)

* Change GXExternalCollection<T> to use a typed generic field internally, eliminating casts while preserving the existing public API.

* Instance cannot be a strongly typed List<T> because the external type is unknown.
What we do know is the type of the wrapper SDT, which is T.

* Instance cannot be a strongly typed List<T> because the external type is unknown.

* Minor comment.

* Improve comment.

---------

Co-authored-by: claudiamurialdo <c.murialdo@globant.com>

Author: claudiamurialdo

dotnet/src/dotnetframework/GxClasses/Domain/GxCollections.cs

@@ -2536,6 +2536,8 @@ public interface IGxExternalObject
 	}
 	public class GXExternalCollection<T> : IGxCollection where T : IGxExternalObject
 	{
+		// T represents the type of the SDT wrapper for the external instance. E.g SdtWorkflowUser
+		// Instance is a list of ExternalType, whose concrete type is unknown. E.g. GXflow.API.User
 		IList instance;
 
 		public string _containedType;
@@ -2572,7 +2574,7 @@ public int Add(object value)
 			else
 				return instance.Add(value);
 		}
-		public void Add(Object o, int idx)
+		public void Add(object o, int idx)
 		{
 			object exoValue;
 			IGxExternalObject x = o as IGxExternalObject;
@@ -2706,11 +2708,11 @@ public void AddObjectProperty(string name, object prop)
 		}
 		public string ToJSonString()
 		{
-			return "";
+			return string.Empty;
 		}
 		public string ToJSonString(bool includeState)
 		{
-			return "";
+			return string.Empty;
 		}
 		public bool FromJSonString(string s)
 		{