For months now, I've been desperate for a decent excuse to make a custom ASP.NET server control, so I was secretly pleased today when a CustomValidator originally coded for use on a single page was required elsewhere.
It's surprisingly easy to do, although it took me a while to find a decent example online, which eventually came courtesy of the 4GuysFromRolla. In my particular scenario, I needed to check that the text entered into a TextBox did not equal any of a selection of "banned" strings (which was available via a static property on another class).
Having added a new custom web control, deriving from BaseValidator, my first task was therefore to ensure that the control being validated really was a TextBox - this was done by overriding the ControlPropertiesValid() method:
///
/// This method checks whether the control set in the
/// ControlToValidate property is valid.
///
/// Boolean indicating whether we have a valid ControlToValidate
protected override bool ControlPropertiesValid()
{
// Get the control we're trying to validate
Control ctrl = FindControl(ControlToValidate);
if (ctrl == null)
{
// If the control does not exist, throw an exception
throw new ApplicationException(string.Format(
"StringValidator - Control {0} could not be found to validate",
ControlToValidate));
}
else
{
// Try to cast the control to a Textbox
try
{
_textctrl = (TextBox)ctrl;
return true;
}
catch (InvalidCastException)
{
// Control doesn't appear to be a TextBox -
// throw an exception
throw new ApplicationException(string.Format(
"StringValidator - Control {0} is not a TextBox",
ControlToValidate));
}
}
}
That done, I just had to override the EvaluateIsValid() method, which does the actual validation:
///
/// Determine whether the text entered in the TextBox being
/// validated is allowed.
///
/// Boolean indicating whether the string
protected override bool EvaluateIsValid()
{
// Get dataview of invalid strings
DataView dvInvalidStrings = SiteTerms.InvalidStrings;
// Search dataview for the string entered in the TextBox
// control being validated
dvInvalidStrings.RowFilter = string.Concat("[InvalidString] =
'",_textctrl.Text.ToString().Trim().Replace("'","''"),"'");
// Return false if any rows were found, otherwise true.
if (dvInvalidStrings.Count==0)
{
return true;
}
else
{
return false;
}
}
...and that was it. Job done, pretty much. OK, so I spent a while making a cute icon image too. But the coding was easy and straightforward - inheriting from BaseValidator meant that most of the work was already done for me. In no time at all I was able to drop this custom control onto the required pages and user controls.
