Introduction

A normal requirement in any ASP.Net application is to sum/average up the values in a DataGrid column to the Footer. In this article we will compare the normal methods and later create our own custom DataGrid column "SumColumn", derived from BoundColumn to avoid repetition of the same code.

Background

Let us consider the example of a DataGrid showing the list of employees. Columns are Name, Salary and Bonus. Normal solutions are

• Query database for the sum:
  
  One way is to compute the sum of the data through a SQL query. For example, to compute the sum of all salaries in Employee Table, the following SQL statement could be used:
  
  SELECT SUM(Salary) FROM Employees
  
  After firing the query using ADO.Net, we will save the result of query in a local variable e.g. internalSum. Dividing internalSum by the no of Items in DataGrid will yield average. Then we have to loop through the DataGrid Items Collection to find the Footer and set the value.
  
  

  For Each dgItem In Me.dgEmployee.Items

      If dgItem.ItemType = ListItemType.Footer Then

            values = "Sum : " & internalSum & "<br>"

            values += "Average : " & (internalSum / Me.dgEmployee.Items.Count)

            dgItem.Cells(0).Text = values

      End If

  Next

  
  Disadvantages:
  
  
  • Two round trips to the database server; one for records and one for sum.
    
    
  • Also, if you want to compute sum for Bonus as well, then one will need one more SQL query i.e. no of queries will be (n+1) where n is the number of columns you want to sum up.
    
    
  • To show the sum in the Footer, one has to loop through the Items collection of the DataGrid, checking the Footer ItemType, setting the value. 
    
    
• Use DataTable  Compute() Method:
  
  Another way is to use the Compute() method of the DataTable i.e. to get the same output as in first approach, we have to do like
  
  

  internalSum = dtEmployee.Compute("SUM(Salary)", String.Empty).ToString
  internalAvg = dtEmployee.Compute("AVG(Salary)", String.Empty).ToString
  
  Now do the above described looping technique and set the value in DataGrid footer.
  
  Disadvantages:

  • The results of the SQL query, the one that's being bound to the DataGrid, must be retrieved in a DataTable; no custom collection, Array or ArrayList will work.
    
    
  • To show the sum in the Footer, one has to loop through the Items collection of the DataGrid.
    
    
• Use DataGrid ItemDataBound Event:
  
  The idea is to use the ItemDataBound event of the DataGrid, grabbing the corresponding item value of all the rows of the DataGrid and summing it up. We have to keep the count of Items as well to calculate average. Then we could display the values in the Footer.
  
  

  Private Sub dgEmployee_ItemDataBound(ByVal sender As Object, _

       ByVal e As System.Web.UI.WebControls.DataGridItemEventArgs)

    Handles dgEmployee.ItemDataBound

          Select Case e.Item.ItemType

              Case ListItemType.Item, ListItemType.AlternatingItem

                  internalSum += CType(e.Item.Cells(1).Text, Double)

                  internalCount += 1

              Case ListItemType.Footer

                  e.Item.Cells(0).Text = "Sum : " & Me.FormatDataValue(internalSum) & "<br>"

                  e.Item.Cells(0).Text += "Count : " & internalCount & "<br>"

                  e.Item.Cells(0).Text += "Average : " & _

                     Me.FormatDataValue(internalSum / internalCount)

          End Select

  End Sub
  
  ADVANTAGES:
  
  

  • The DataSource of the DataGrid can be of any type. It is not restricted to be DataTable as in second solution.
    
    
  • Only one query as compare to (n+1) queries in first solution.
    
    
  • No extra loop of DataGrid Item Collection to find Footer as in first and second solution. 

Problem

At this point you will be thinking that if the last solution does not have problems, what we are going to do. If it ain't broken, why fix it. Actually I am tired of repeating the same old code in code behind of pages containing DataGrid. Are you not tired as well? I want to do it in an easier way; a way that is elegant as well. What if we do not have to write a single line of code and it will be done automatically. I am sure you will enjoy it as I do. May be you have heard that quote too

Laziness is the mother of invention.

SumColumn for the Rescue

Introduction to Solution

Now it is the time to pull the cat out of the bag. Solution to this problem is to build the desired functionality right into a custom DataGridColumn that we can then use on any page! Confused, let me explain. We are going to create our own custom DataGrid column which we will refer as "SumColumn", inherited from DataGrid BoundColumn. Due to its base class, the new SumColumn class will have all of the built-in functionality already present in the BoundColumn class. We just need to implement our sum functionality. Inheriting powerful controls like the BoundColumn class and adding new functionality to them is one of the OO features of the .NET Framework.

Before we dive into creating our custom column class, let's first look at an example that simply uses a normal BoundColumn control to display the data. For this, we set up a simple DataGrid that displays the Name, Salary and Bonus from Employee Table.

<asp:datagridid="dgEmployee"runat="server"ShowFooter="True"AutoGenerateColumns="False" ID="Datagridid1" NAME="Datagridid1">
      <FooterStyleForeColor="#330099"></FooterStyle>
      <SelectedItemStyleForeColor="#663399" </SelectedItemStyle>
      <ItemStyleForeColor="#330099"></ItemStyle> 
      <HeaderStyleFont-Bold="True"ForeColor="#FFFFCC"></HeaderStyle>
      <Columns>
            < asp:BoundColumn DataField ="Name" HeaderText ="Name"></ asp:BoundColumn >
            < asp:BoundColumn DataField ="Bonus" HeaderText ="Bonus"></ asp:BoundColumn >
            < asp:BoundColumn DataField ="Salary" HeaderText ="Salary"></ asp:BoundColumn >
      </Columns>
</asp:datagrid>

And in the code behind, we would do in the page load event.

dgEmployee.DataSource = GetEmployees()
dgEmployee.DataBind()

Where GetEmployees() will return a DataTable containing the employee records. Normal output will be like (depending upon the data)

1st Pass: Creating the Initial Draft

If you have Visual Studio .NET, create a new VB.Net project of type Class Library named CustomWebControls. You will need to add the reference of System.Web.dll assembly in your project. Otherwise create a file SumColumn.vb. We will make the class step by step as we proceed.

Imports System
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.ComponentModel
Public Class SumColumn
     Inherits BoundColumn
End Class

BoundColumn class has two methods that are normally overridden to provide some custom functionality. One is FormatDataValue() and other is InitializeCell(). The FormatDataValue method is normally used along with the DataFormatString property to format numeric and date information. Therefore we are going to use the second one i.e. InitializeCell. This method is much like the DataGrid ItemCreated method. The signature of the method is

Public Overrides Sub InitializeCell(ByVal cell As System.Web.UI.WebControls.TableCell, _
ByVal columnIndex As Integer, ByVal itemType As System.Web.UI.WebControls.ListItemType)

We do not have the value of cell at this time because cell is just being created. We should somehow link ourselves with the binding of the cell. This can be done by attaching a handler on the fly to the cell DataBound event

Public Overrides Sub InitializeCell(ByVal cell As System.Web.UI.WebControls.TableCell, _
   ByVal columnIndex As Integer, ByVal itemType As System.Web.UI.WebControls.ListItemType)
        MyBase.InitializeCell(cell, columnIndex, itemType)
        Select Case itemType
            Case ListItemType.AlternatingItem, ListItemType.Item, ListItemType.Footer
                AddHandler cell.DataBinding, AddressOf CellItemDataBound
        End Select
End Sub

We have called the InitializeCell method of the base class so that it can do the routine work. We have also filter the ItemType using select case because we only want to deal with Item, AlternalteItem and Footer. Our method, CellItemDataBound has been attached with the DataBinding event of cell. Inside this method we will do our magic. Just recall the third solution described previously.

Private Sub CellItemDataBound(ByVal sender As Object, ByVal e As EventArgs)
        Dim cell As TableCell = CType(sender, TableCell)
        Dim DGI As DataGridItem = CType(cell.NamingContainer, DataGridItem)
        Dim dValue As Decimal
        Dim dataItem As Object = DGI.DataItem
        Select Case DGI.ItemType
            Case ListItemType.AlternatingItem, ListItemType.Item
                dValue = DGI.DataItem(DataField)
                internalSum += dValue
                internalCount += 1
                cell.Text = Me.FormatDataValue(dValue)
            Case ListItemType.Footer
                cell.Text = "Sum : " & Me.FormatDataValue(internalSum) & "<br>"
                cell.Text += "Count : " & internalCount & "<br>"
                cell.Text += "Average : " & Me.FormatDataValue(internalSum / internalCount)
        End Select
End Sub

Code is pretty straight forward. We just grabbed the DataItem out of the sender object, and then check the ItemType. If it is Item or AlternateItem, we set the text of the cell; add to internalSum and increment the internalCount. If it is Footer, we just concatenate the values and set the text of cell. Please note that we have called FormatDataValue method of base class to format the output. No complications till yet.

Compile the class library project. Add the reference of its output to your web project. Register the tag at the top of aspx page.

If we run our web project, the output will be more or less like

2nd Pass: Bindable with all DataSources

Isn't it great? We did not write a single line of code in the code behind of WebForm and we are getting sum/count/average in the Footer. You can do the same with the bonus column by just changing the column type to SumColumn. But there is just one little problem. This control works well if the DataSource of DataGrid is DataTable or DataReader. If you try to use Array or ArrayList of custom objects, you will probably see the output like

We have to think of a way that can handle any type of DataSource. Hmmm.. What about using PropertyDescriptor class of System.ComponentModel namespace to get the value regardless of the underlying object. We will update the following line in  CellItemDataBound  

dValue = DGI.DataItem(DataField)

to the line below 

and add a new method GetUnderlyingValue in our class.

Instead of just pulling the value out of DataItem, we are depending on GetUnderlyingValue Method to get value of the DataItem for us which in turn use TypeDescriptor class to check whether the DataField exists in the underlying object. If succeed, it returns the value to the calling method otherwise throw Exception. Now you can check the output with (nearly) all kinds of DataSource.

3rd Pass: Customizable Output

All seems fine now but there is no control on the output. What I mean is may be you want to just show sum, no average, no count; someone else wants to show sum and average. There are different possibilities so there should be some way to customize the output as per needed. Here is what we can do.

We exposed three public properties i.e. ShowSum, ShowCount, ShowAverage in our SumColumn class. One can use these properties in aspx to customize the output e.g.

OR

End If

If you update the code and aspx, you may get the output as below

4th Pass: Tweaking the Design Time Output

Yes, I know that now you all are feeling sleepy but please give me just 5 more minutes. If you are not interested in changing the design time output (i.e. when we see the DataGrid in design time using Visual Studio), then you can skip this section. For those who are still reading, check the change in code below.

I think that the above code is quite self explanatory. We simply used the DesignMode property of the base class BoundColumn to tweak the design time output of the SumColumn .

Conclusion

Now we have it. Our own new custom column SumColumn derived from BoundColumn having the functionality of showing the sum/average/count of values of the column in the footer of the DataGrid. This is just one example of a reusable DataGrid column and it is up to you to examine your own applications and find out what could be neatly wrapped up into a custom DataGrid column.

Note

• The downloadable code is not of production quality. The whole sole purpose was to initiate a thought that repetitive code can be wrapped inside a module.
  
  
• The code contains a custom class Employee which return the records in different formats e.g. DataTable, ArrayList and Array. One can use the sample page Test.aspx to check with all three type of DataSource.
  
  
• This control has not been tested using XML DataSource.

References

• Creating a Custom DataGridColumn Class By John Dyer article for beginners.
  
  
• Creating Custom Columns for the ASP.Net DataGrid by DataGridGirl One advance but comprehensive article.
  
  
• Metabuilders BoundBooleanColumn I was inspired by Andy of Metabuilders to make custom columns.