In Part 1 we examined how to truncate a DataGrid's column by
using a custom function and a TemplateColumn. In this part we'll look at how to provide the same functionality
by rolling our own DataGridColumn class.
Building our Own DataGridColumn Class
The solution in Part 1 works very well and does exactly what we
wanted it to do. But we may want to use this functionality again in other pages or customize the
function further. But each time we make a change, we don't want to have to copy our
Truncate(input, characterLimit) function to every page we use it in. A great solution to this problem is to
build the Truncate(input, characterLimit) function right into a custom DataGridColumn that we can then use on any
page! Inheriting powerful controls like the BoundColumn class and adding new functionality to
them is one of the greatest and most powerful features of the .NET Framework.
To accomplish this we're going to create a new DataGridColumn class called LimitColumn
that inherits the BoundColumn class. Due to its being derived from the BoundColumn class, the new
DataGridColumn class will have all of the built-in functionality already present in the BoundColumn
class. We just need to add one property to it called CharacterLimit and give it a
default of 0 so that if CharacterLimit is not set, the column will not do any truncating
(that is, it will behave just as if the developer had added a BoundColumn control).
To provide truncating ability, we are going to override the BoundColumn's FormatDataValue
method. The FormatDataValue method is normally used along with the
DataFormatString property to format
numeric and date information. Essentially we will create our own custom Data Format by using the
Truncate(input, characterLimit) function in place of the built-in FormatDataValue method.
Below is the code for the LimitColumn class. If you are using Visual Studio .NET, create a new
C# project of type Class Library and copy in the below code (you will need to add the System.Web.dll
assembly in the References section of your project). If you do not have VS.NET, simply create a text file
named LimitColumn.cs and copy the following text into it.
using System;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace myNameSpace
{
/// <summary>
/// Summary description for LimitColumn.
/// </summary>
public class LimitColumn : BoundColumn {
private int characterLimit = 0;
public int CharacterLimit {
get { return characterLimit; }
set { characterLimit=value; }
}
protected override string FormatDataValue(object dataValue) {
return Truncate(dataValue.ToString());
}
string Truncate(string input) {
string output = input;
// Check if the string is longer than the allowed amount
// otherwise do nothing
if (output.Length > characterLimit && characterLimit > 0) {
// cut the string down to the maximum number of characters
output = output.Substring(0,characterLimit);
// Check if the space right after the truncate point
// was a space. if not, we are in the middle of a word and
// need to cut out the rest of it
if (input.Substring(output.Length,1) != " ") {
int LastSpace = output.LastIndexOf(" ");
// if we found a space then, cut back to that space
if (LastSpace != -1) {
output = output.Substring(0,LastSpace);
}
}
// Finally, add the "..."
output += "...";
}
return output;
}
} // End LimitColumn
} // End myNameSpace
We're now ready to compile this code. If you are using VS.NET go to the Build menu and select Build Project.
If you are not using VS.NET you will need to use the compiler from the command prompt. Simply navigate to the
directory where the .cs file is and enter:
Regardless of if you are using VS.NET or not you will need to move the created DLL to your Web project's
/bin folder. Once the DLL is in the /bin folder we can use the DataGridColumn
class we created. First, we need to import the myNameSpace namespace by adding the line :
to the top of our ASP.NET Web page. Then we can replace the TemplateColumn code with
myControls:LimitColumn and set the CharacterLimit property accordingly.
Conclusion
There you have it: Your own custom column that only outputs the amount of text that you want. You
could change the truncate function to limit to a number of words or do any other custom formatting
that you'd like it to do.
For More on Custom DataBoundColumn Classes...
For more code examples on creating your own custom DataGridColumn classes, head on over to
MetaBuilders.com. There are a number of complete code examples
that can be used to improve the functionality of the DataGrid Web control. (Unfortunately there are
no articles or tutorials on the code, just source code comments...)
