What's the difference between using statement

Question

In Visual Studio, when do you have to add a reference to a dll? I always try to have a minimum of references in my projects, I try to only include the ones that are really necessary.

I would think that I only need a reference if I have a using statement in my source. But that's not always enough.

For instance, I have a very simple program that is using System and Microsoft.Practices.EnterpriseLibrary.Data:

using System;
using Microsoft.Practices.EnterpriseLibrary.Data;

public class SimpleConnection {
    private static void Main() {
        var database = DatabaseFactory.CreateDatabase();
        var command =
            database.GetSqlStringCommand(
                "select table_name from information_schema.tables");
        using (var reader = database.ExecuteReader(command)) {
            while (reader.Read()) {
                Console.WriteLine(reader.GetString(0));
            }
        }
    }
}

I would think I only have to reference System and Microsoft.Practices.EnterpriseLibrary.Data. But that's not true. If I don't reference System.Data, the code won't compile.

The type 'System.Data.Common.DbCommand' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

How can I know beforehand when I have to add a reference to something I'm not using?

Answer 1

Sometimes the references you add have a dependency on another library, therefore you must have that library in your references.