Using .NET Generics to store ArcObjects?

Question

What are the implications of using Generic types in .NET to store ArcObjects, such as ILayer, IField, etc?

The compiler throws me a warning when using these kind of values

public class Foo
{
 private List<ILayer> fooLayers;
 public List<ILayer> FooLayers
 {
 get { ... }
 set { ... }
 }
}

Here is the warning:

Warning 15 Type library exporter warning processing 'ArcMemorialCore.Topography.IMemorialDocument.set_ProfessionalsEnvolved(value), ArcMemorialCore'. Warning: Type library exporter encountered a generic type instance in a signature. Generic code may not be exported to COM. ArcMemorialCore

My concern is not only the warning's existence, but of good design practices, performance, etc.

• Reed Copsey: What warning are you getting?
• Keith G: Please add some context to your question too. We need to know more about how you are using the objects. As far as I know there are no inherent problems with using ArcObjects classes or interfaces like this.
• Hugo Estrada: What do the warnings say?
• JC5577: Are you getting an error or a warning?
• George Silva: Warning. It compiles just fine. My interests are beyond the existence of the warning. It's more of a concern in terms of performance, good design practices, etc. The warning is: Warning 15 Type library exporter warning processing 'ArcMemorialCore.Topography.IMemorialDocument.set_ProfessionalsEnvolved(value), ArcMemorialCore'. Warning: Type library exporter encountered a generic type instance in a signature. Generic code may not be exported to COM.

Answer 1

The warning you received is because you have your class (or assembly) marked with:

[ComVisible(true)]

This causes the compiler to issue warnings when you use types that are not compatible with COM objects.

That being said, there is no problem with using generics with ArcObject types. You should, however, only use them for types that are internal to your application - ie: when you're working with a set of values.

• Dandy: Adding to this I believe it is only a problem for COM+ to refer to a generic type name, for example List<object>. Going from CLR to COM+ should not be a problem.
• Petr Krebs: You can avoid this sort of warnings by marking your property as [ComVisible(false)] while still allowing the class (or whole assembly, which is not recommended practice in .NET 2.0+) to be marked as [ComVisible(true)].
• Adam Armour: For reference: Interoperating Using Generic Types

Answer 2

I don't have an issue getting it to compile.

I am using C# 3.5

using System; 
using System.Collections.Generic; 
using System.Web;
using ESRI.ArcGIS.Carto; //Perhaps not having this reference is the issue?
public class Foo
 {
 private List<ILayer> _fooLayers;
 public List<ILayer> FooLayers
 {
 get { return _fooLayers; }
 set { _fooLayers = value; }
 }
 }