Gists for 2004 Blog Post "Inheriting From BaseValidator to Make Custom Validation Controls"

ControlPropertiesValid.cs

///
/// 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));
        }
    }
}

EvaluateIsValid.cs

///
/// 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;
    }
}