ضعى البرمتر بعد بعد جملة الSELECT المهم تكون قبل
adapter.Fill(dsTasks);
جربى كده
SqlConnection SqlConnection = new SqlConnection();
SqlConnection.ConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString2"].ConnectionString;
string sqlselect = "SELECT Tasks.Task_Id,Tasks.Task_Name,Groups.Group_Id,Groups.Group_Name, Members.Member_Id,Members.Member_Name,Members.Email FROM Groups INNER JOIN Members_Groups ON Groups.Group_Id = Members_Groups.Group_Id INNER JOIN Tasks ON Groups.Group_Id = Tasks.Group_Id INNER JOIN Members ON Members_Groups.Member_Id = Members.Member_Id where Tasks.End_Task=@End_Task";
DataSet dsTasks = new DataSet();
cmd.Parameters.AddWithValue("@End_Task", DateTime.Now.Date.Date.ToString());
SqlDataAdapter adapter = new SqlDataAdapter(sqlselect, SqlConnection);
adapter.Fill(dsTasks);
SqlCommand cmd = new SqlCommand();