想請教一下一般public class XXX這樣宣告，然後裏面在寫方法或屬性然後new class後使用
但發現有一個很奇特用法就是在public後用IDbConnection這個介面，然後在設定一個Connection1()方法，之後new Program類別然後使用Connection1()後面可以點出IDbConnection裏面的方法與屬性，這是什麼原理，是繼承嗎，但我沒有下任何繼承，一般public 後面不是都加類別，改介面也行?然後介面下的功能就能被使用?

class Program
{
    public IDbConnection Connection1()
    {
        var Conn = ConfigurationManager.AppSettings["ConnString"];
        return new SqlConnection(Conn);
    }

    static void Main(string[] args)
    {
        Program program = new Program();
        var _cn = program.Connection1().BeginTransaction();
        var _cn2 = program.Connection1().ConnectionTimeout;
    }
}

namespace System.Data
{
    //
    // 摘要:
    //     Represents an open connection to a data source, and is implemented by .NET Framework
    //     data providers that access relational databases.
    public interface IDbConnection : IDisposable
    {
        //
        // 摘要:
        //     Gets or sets the string used to open a database.
        //
        // 傳回:
        //     A string containing connection settings.
        string ConnectionString { get; set; }
        //
        // 摘要:
        //     Gets the time to wait (in seconds) while trying to establish a connection before
        //     terminating the attempt and generating an error.
        //
        // 傳回:
        //     The time (in seconds) to wait for a connection to open. The default value is
        //     15 seconds.
        int ConnectionTimeout { get; }
        //
        // 摘要:
        //     Gets the name of the current database or the database to be used after a connection
        //     is opened.
        //
        // 傳回:
        //     The name of the current database or the name of the database to be used once
        //     a connection is open. The default value is an empty string.
        string Database { get; }
        //
        // 摘要:
        //     Gets the current state of the connection.
        //
        // 傳回:
        //     One of the System.Data.ConnectionState values.
        ConnectionState State { get; }

        //
        // 摘要:
        //     Begins a database transaction.
        //
        // 傳回:
        //     An object representing the new transaction.
        IDbTransaction BeginTransaction();
        //
        // 摘要:
        //     Begins a database transaction with the specified System.Data.IsolationLevel value.
        //
        // 參數:
        //   il:
        //     One of the System.Data.IsolationLevel values.
        //
        // 傳回:
        //     An object representing the new transaction.
        IDbTransaction BeginTransaction(IsolationLevel il);
        //
        // 摘要:
        //     Changes the current database for an open Connection object.
        //
        // 參數:
        //   databaseName:
        //     The name of the database to use in place of the current database.
        void ChangeDatabase(string databaseName);
        //
        // 摘要:
        //     Closes the connection to the database.
        void Close();
        //
        // 摘要:
        //     Creates and returns a Command object associated with the connection.
        //
        // 傳回:
        //     A Command object associated with the connection.
        IDbCommand CreateCommand();
        //
        // 摘要:
        //     Opens a database connection with the settings specified by the ConnectionString
        //     property of the provider-specific Connection object.
        void Open();
    }
}