Need help understanding "Activator.CreateInstance" (ASP.NET 2.0 / C#)

I'm trying to get this concept because I want to use it. Bascially the end result is having an interchangable data access layer (Switching from SqlClient to OracleClient for example). Below is the core code that I'm trying to understand:

      static public ArticlesProvider Instance
      {
         get
         {
            _instance = (ArticlesProvider)Activator.CreateInstance(Type.GetType(DAL.SqlClient.SqlArticlesProvider));
            return _instance;
         }
      }

I found this snippet on the web but it's incomplete and couldn't find the rest of it. What I don't understand is what does Activator.CreateInstance do and is this really saying that it creating an instance of an entire namespace?
bemara57Asked:
Who is Participating?
 
adam_ingenitiConnect With a Mentor Commented:
I think you're mostly correct - you just need to use

typeof(Company.DAL.MdbClient.MdbConfig)

instead of

Type.GetType(Company.DAL.MdbClient.MdbConfig))


... GetType() is only a valid method on actual INSTANCES...  whereas typeof() works when you're referring to an actual TYPE.

Try it and see if your code works?



            typeof(DataBinder);
            (new DataBinder()).GetType();
0
 
anarki_jimbelCommented:
From msdn (http://msdn2.microsoft.com/en-us/library/system.activator.createinstance(vs.80).aspx)
Activator.CreateInstance Method

Creates an instance of the specified type using the constructor that best matches the specified parameters.

Not sure about ASP.

E.g., in Remoting it may means thta no server instance is created before first clien call. You do not create an object by calling constructor but rather ask activator to get an instance. Srver object may be different type so the problem is that the *type* of object to create isn't know until runtime.  Activator helps to solve the problem.
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bemara57Author Commented:
Thanks but I can't get it to work in my website. Here's what I did but don't understand what the problem is:

namespace Company.DAL
{
    public abstract class ConfigProvider
    {
        static private ConfigProvider _instance = null;
        static public ConfigProvider Instance
        {
            get
            {
                if (_instance == null)
                    _instance = (ConfigProvider)Activator.CreateInstance(
                       Type.GetType(Company.DAL.MdbClient.MdbConfig));
                return _instance;
            }
        }

        public abstract string GetConfigValue(ConfigXML site, string id, string name);
    }
}

Then in the "interchangable" data access layer I have this:
namespace Company.DAL.MdbClient
{
    public class MdbConfig : ConfigProvider
    {
        public override string GetConfigValue(ConfigXML site, string id, string name)
        {
            try
            {
                string connString = WebConfigurationManager.ConnectionStrings[site.SettingsTable.ConnectionStringName].ConnectionString;
                string sqlCmd = "SELECT Value FROM " + site.SettingsTable.TableName + " WHERE UCASE(id)='" + id.ToUpper() + "' AND UCASE(name)='" + name.ToUpper() + "'";

                using (OleDbConnection conn = new OleDbConnection(connString))
                {
                    OleDbCommand cmd = new OleDbCommand(sqlCmd, conn);
                    conn.Open();

                    object obj = cmd.ExecuteScalar();

                    if (obj.Equals(System.DBNull.Value))
                        return string.Empty;
                    else
                        return (string)obj;
                }
            }
            catch
            {
                return string.Empty;
            }
        }
    }
}

Then in my aspx page I'm calling the method like this:
ConfigProvider.Instance.GetConfigValue(domain, "company", "Address1");

But I get this error:
Compiler Error Message: CS0119: 'Company.DAL.MdbClient.MdbConfig' is a 'type', which is not valid in the given context
Source Error:
Line 21:                 if (_instance == null)
Line 22:                     _instance = (ConfigProvider)Activator.CreateInstance(
Line 23:                        Type.GetType(Company.DAL.MdbClient.MdbConfig));
Line 24:                 return _instance;
Line 25:             }
 
What am I doing wrong??
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bemara57Author Commented:
That worked! Thanks for the help, it's exactly how I needed it to work.
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