文件路径问题，请教大神！

private void button1_Click(object sender, EventArgs e)
        {                     
            string baseFolder = AppDomain.CurrentDomain.BaseDirectory;
            string tF = this.folder; //这个路径没问题，已经定义好了。
            string sF = (baseFolder,folder1,"1.txt"); //①
            File.Copy(tF,sF, true);
        }

问题是：1.txt路径为：当前程序路径\folder\下，要写成相对路径，如何表达呢？即①处的路径怎么写呢？