Converting a fairly simple C# Class library into a COM object?

Question

Here's my problem: I have to call a web service with a secure header from a classic ASP page that returns a complex data type. For various reasons concerning a 3rd party tool it has to be classic ASP. We decided that I should create an external dll to do this - which I did (in c#) so it returns a dataset (Something ASP can understand). However now I need to expose that function to the ASP page. Because this is classic ASP I think the only straightforward way to do this is to expose this class library as a COM object. I need to know the down and dirty easiest way to accomplish this task. What do I have to do to my dll?

I have never created a COM object before only used. Somebody said my class has to be static and I can't have a constructor. Is this true? Can someone layout the steps for me?

HELP! (o:

Edit: This specific problem is now solved however as Robert Rossney noted I can't do anything with the DataSet in classic ASP. This has led me to post a second question here regarding implementing XmlTextWriter - Robert if you see this I think you could really help!

Answer 1

No, that (static/no ctor) isn't true. Quite the opposite, in fact, since COM will need to create an instance! You simply need to make the class COM visible. Mainly, this is just adding some attributes, and registering it as a COM dll (regasm).

http://msdn.microsoft.com/en-us/library/zsfww439.aspx

Answer 2

Creating a class that returns a DataSet is not so difficult:

using System;
using System.Data;
using System.Runtime.InteropServices;

namespace COMTest
{
   [Guid("AC4C4347-27EA-4735-B9F2-CF672B4CBB4A")]
   [ComVisible(true)]
   public interface ICOMTest
   {
       [ComVisible(true)]
       DataSet GetDataSet();
   }

   [Guid("CB733AB1-9DFC-437d-A769-203DD7282A8C")]
   [ProgId("COMTest.COMTest")]
   [ComVisible(true)]
   public class COMTest : ICOMTest
   {
       public DataSet GetDataSet()
       {
           DataSet ds = new DataSet("COMTest");
           return ds;
       }
   }

}

You'll need to check the "Register for COM Interop" box in the Project properties, you'll also need to sign the assembly, and you'll need to make sure that the IIS user can access your bin\Debug directory.

Once you've done this, you can create an instance from ASP just fine:

<%
Dim o
Set o = Server.CreateObject("COMTest.COMTest")
Response.Write("Server.CreateObject worked.")
Response.Write("<br/>")
Dim ds
Set ds = o.GetDataSet()
If Not ds is Nothing Then
  Response.Write("o.GetDataSet returned an object.  Can we use it?")
  Response.Write("<br/>")
  Response.Write("We have a DataSet, and its DataSetName is: ")
  Response.Write(ds.DataSetName)
End If
%>

And here you will be sad. For while the method that returns a DataSet is visible to COM, none of the DataSet's properties or methods are.

Answer 3

Somebody said my class has to be static and I can't have a constructor. Is this true?

A COM class needs to have a default constructor, which will be used by clients when they call CoCreateInstance. After the object is created, you can set properties on it, or call other methods.

This is similar to the way that .Net treats serializable objects... you construct it with a default constructor, then set all its properties. (If you like RAII, you're out of luck with both COM and .Net. Get over it;)

As to the static comment, no, that isn't true, as Marc pointed out.