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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 ?

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