I'm trying to use a DataTable to perform calculations on rows of data using the Expression property on DataColumns. msdn
I found that if there is an exception while applying the expression to the rows, the default value will be used. That's exactly what I want for the row that has the problem. The issue is that each subsequent row will use the default value instead of evaluating the expression.
In this example start with a base table consisting of two columns and 10 rows. The first column is an auto-incremented column with a range of -5 to 5. The second column is always 3.
I add a third column with an expression that divides the "3" column by the first column. That will cause a div0 exception for one row.
For that one row I want it to use the default value I supplied. It does, but also applies the default value to the 4 remaining rows.
Output:
-5 3 -0.6
-4 3 -0.75
-3 3 -1
-2 3 -1.5
-1 3 -3
0 3 -33 //Expected
1 3 -33 //I want it to continue evaluating the expression, not default
2 3 -33
3 3 -33
4 3 -33
How can I add handling so that if an individual row has problems it will use the default value while continuing to use the expression for subsequent rows?
Please note that I cant simple use an IIF as described here to check for problems like this because the Expressions used are dynamic and supplied from a user as a way to customize data.
Complete code to reproduce this:
using System;
using System.Data;
namespace DataSetExpressionTest
{
public class SO
{
private static DataTable table = null;
private static void Main(string[] args)
{
table = CreateBaseTable();
//If there is a problem only in some rows of the table, the rows before the error are calculated using the expression. All rows afrer use the default value
DataColumn onlyOneException = new DataColumn()
{
ColumnName = "One-Ex",
Expression = "third / index",
DefaultValue = -33
};
AddCalculationToTable(onlyOneException);
PrintDataTable(table);
Console.ReadLine();
}
private static void AddCalculationToTable(DataColumn calculationColumn)
{
//You can wrap individual stats in try/catch blocks. If there is an exception in a calc, it will use the default value specified
try
{
table.Columns.Add(calculationColumn);
}
catch (Exception e)
{
Console.WriteLine("Caught a calculation exception while adding column {0}. Will use default value instead!!", calculationColumn.ColumnName);
}
}
private static DataTable CreateBaseTable()
{
DataTable table = new DataTable();
// Create 3s column.
DataColumn threeColumn = new DataColumn
{
DataType = Type.GetType("System.Decimal"),
ColumnName = "third",
DefaultValue = 3
};
//Create icrementing index column
DataColumn indexColumn = new DataColumn
{
DataType = Type.GetType("System.Decimal"),
ColumnName = "index",
AutoIncrement = true,
AutoIncrementSeed = -5
};
// Add columns to DataTable.
table.Columns.Add(indexColumn);
table.Columns.Add(threeColumn);
for (var i = 0; i < 10; i++)
{
table.Rows.Add(table.NewRow());
}
return table;
}
public static void PrintDataTable(DataTable table)
{
foreach (DataColumn column in table.Columns)
{
Console.Write(column.ColumnName + " ");
}
Console.WriteLine();
foreach (DataRow dataRow in table.Rows)
{
foreach (var item in dataRow.ItemArray)
{
Console.Write(item + " ");
}
Console.WriteLine();
}
}
}
}