从数据表对象以数据表格式返回n行

Question

I am using following code to return n number of rows from DataTable to another DataTable object. It is working fine when in first datatable records comes from database but when I mannualy added records in datatable then it crash and a message displays no rows . I had debugeed and found that currentTable.DefaultView.ToTable().Rows.Cast<DataRow>() this will returns null value. I had written following code to select n records from DataTable object

currentTable = currentTable.DefaultView.ToTable().Rows.Cast<DataRow>().Take(numberOfRecord).CopyToDataTable();

I dont know why it is crashed when I manualy creating DataTable and working fine when records comes from DataReader to DataTable.

I am using following code to create DataTable manually

DataTable tblCurrent = new DataTable();
//For Adding columns
for (int i = 0; i < fieldCol.Length; i++)
   tblCurrent.Columns.Add(string.Format("Column{0}", i + 1));

//for adding rows.
for (int i = (count + globalClass.sourceConnectionInfo.RowsToSkip); i < lineRows.Length; i++)
{
   DataRow newRow = tblCurrent.NewRow();
   for (int j = 0; j < fieldCol.Length; j++)
      newRow[j] = fieldCol[j];
   tblCurrent.Rows.Add(newRow);
}


//Then for selecting n records
if (currentTable.Rows.Count > 0)
   tblCurrent = tblCurrent.DefaultView.ToTable().Rows.Cast<DataRow>().Take(numberOfRecord).CopyToDataTable();

Answer 1

Check the following code snippet, it is working fine without any error.

 void Main()
    {
    DataTable dt = new DataTable();
        if (dt.Columns.Count == 0)
                {
                    dt.Columns.Clear();
                    dt.Columns.AddRange(
                        new DataColumn[] 
                        {
                            new DataColumn("TStamp", typeof(DateTime)),
                            new DataColumn("C1", typeof(double)),
                            new DataColumn("C2", typeof(double)),
                            new DataColumn("C3", typeof(double))
                            //new DataColumn("Unit", typeof(string))
                        }
                        );
                }

                  Random ovValue = new Random();
                for (int i = 0; i < 10; i++)
                {
                    DataRow dnew = dt.NewRow();
                    dnew["TStamp"] = DateTime.Now.AddMonths(i);
                    dnew["C1"] = ovValue.Next(1, 100);
                    dnew["C2"] = ovValue.Next(1, 100);
                    dnew["C3"] = ovValue.Next(1, 100);
                    dt.Rows.Add(dnew);
                }
                try
                {
                int numberOfRecord = 100;
                int count = dt.Rows.Count;  
                if(numberOfRecord <= count)
                {
                dt = dt.DefaultView.ToTable().Rows.Cast<DataRow>().Take(numberOfRecord).CopyToDataTable();
                }
                else
                {
                /// No Erroe cause if you select more tested at LinqPad
                dt = dt.DefaultView.ToTable().Rows.Cast<DataRow>().Take(100).CopyToDataTable();
                }
                }
                catch(Exception ex)
                {
                Console.WriteLine("Error");
                }

     foreach(DataRow r in dt.Rows)
     {
     Console.WriteLine(r["TStamp"].ToString());
     }

    }

Check your following code block, may be this make the no rows in the datatable. put dt.Rows.Clear(); in my code block before the selection it will cause error.

//for adding rows.
for (int i = (count + globalClass.sourceConnectionInfo.RowsToSkip); i < lineRows.Length; i++)
{
   DataRow newRow = tblCurrent.NewRow();
   for (int j = 0; j < fieldCol.Length; j++)
      newRow[j] = fieldCol[j];
   tblCurrent.Rows.Add(newRow);
}