调用了sqlserver系统存储过程 sp_pkeys 

private string getPK(string connStr, string tableName)
        {

            using (SqlConnection cn_badnewfish = new SqlConnection())
            {
                //连接字符串
                cn_badnewfish.ConnectionString = connStr;
                using (SqlCommand cmd_badnewfish = new SqlCommand())
                {
                    //在这里填存储过程的名字
                    cmd_badnewfish.CommandText = "sp_pkeys";
                    //告诉程序,你调用的是一个存储过程
                    cmd_badnewfish.CommandType = CommandType.StoredProcedure;
                    cmd_badnewfish.Connection = cn_badnewfish;
                    cmd_badnewfish.Parameters.Add("@table_name", SqlDbType.VarChar).Value = tableName;
                    cn_badnewfish.Open();
                    SqlDataReader reader = cmd_badnewfish.ExecuteReader();
                    while (reader.Read())
                    {
                       return reader.GetString(3);
                    }

                    return "error";

                }

            }