string sqlstr = "select username,userid from tb_user";

                DataSet ds=new DataSet ();
                //执行上个语句把结果给 ds;
                //ddl 为你的 DropDownList 控件ID
                ddl.DataSource = ds.Tables[0];
                ddl.DataTextField = "username";
                ddl.DataValueField = "userid";
                ddl.DataBind();

------解决方案--------------------
DataSet ds=new DataSet ();
int s=ds.Table[0].Rows.count;//判断记录数
直接绑定，像楼上的就可以了啊！

------解决方案--------------------
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C# code                string sqlstr = "select username,userid from tb_user";

                DataSet ds=new DataSet ();
                //执行上个语句把结果给 ds;
                //ddl 为你的 DropDownList 控件ID
                ddl.DataSource = ds.Tables[0];
                ddl.DataTextField = "username";
                ddl.DataValueField = "userid";
                ddl.DataBind();