2006-12-02

Adding runtime error messages to page validation

One of the requirements when writing a website I once created was the ability to model object constraints in OCL and then validating the current page against those constraints.

The problem with normal validators is that they are designed to validate individual controls rather than a list of constraints on an object. The approach I took was to create a validator which I can place on any web form and add as many errors to as I required.

The first step was to create a WebControl which supported IValidator

public class MultiValidator : WebControl, IValidator
{
}
I then added a list of strings to hold the error strings, and a method to add an error.
    private List Errors = new List();

public void AddError(string message)
{
Errors.Add(message);
}//AddError

When ASP .net validates a page it enumerates all IValidators within its own Validators property, and called IValidator.Validate(). To determine if the page is valid or not it then checks IValidator.IsValid.

To add custom error messages at runtime I decided to create a static validator class which always returns "false" from IsValidator.IsValid. For each error message in my MultiValidator I could then simply create an instance of one of these validators.
[ToolboxItem(false)]
internal class StaticValidator : IValidator
{
private string errorMessage;

#region IValidator
void IValidator.Validate()
{
}//Validate

bool IValidator.IsValid
{
get { return false; }
set { }
}//IsValid
#endregion

public string ErrorMessage
{
get { return errorMessage; }
set { errorMessage = value; }
}//ErrorMessage
}
Now that the StaticValidator was written, all I needed to do was to add the required IValidator implementations to my MultiValidator class.
#region IValidator
void IValidator.Validate()
{
isValid = (Errors.Count == 0);
foreach(string error in Errors)
{
StaticValidator validator = new StaticValidator();
validator.ErrorMessage = error;
Page.Validators.Add(validator);
Validators.Add(validator);
}//foreach errors
}//Validate

bool IValidator.IsValid
{
get { return isValid; }
set { isValid = value; }
}//IsValid
#endregion

Within a webform, I would now
  1. Set "CausesValidation" to false on my submit button
  2. Validate my object
  3. Call MultiValidator1.AddError() for each error encountered
  4. Call Page.Validate()
  5. Check Page.IsValid as normal
Using a ValidationSummary I could then display the broken constraints to the user for rectification. The whole source code is listed below....

MultiValidator.cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace DroopyEyes.Web.Controls
{
public class MultiValidator : WebControl, IValidator
{
#region Members
private bool isValid = true;
private string errorMessage = "";
private List Errors = new List();
private List Validators = new List();
#endregion

#region IValidator
void IValidator.Validate()
{
isValid = (Errors.Count == 0);
foreach(string error in Errors)
{
StaticValidator validator = new StaticValidator();
validator.ErrorMessage = error;
Page.Validators.Add(validator);
Validators.Add(validator);
}//foreach errors
}//Validate

bool IValidator.IsValid
{
get { return isValid; }
set { isValid = value; }
}//IsValid
#endregion

protected override void OnInit(EventArgs e)
{
base.OnInit(e);
Page.Validators.Add(this);
}

protected override void OnUnload(EventArgs e)
{
if (Page != null)
{
Page.Validators.Remove(this);
foreach(IValidator validator in Validators)
Page.Validators.Remove(validator);
}//Page != null

base.OnUnload(e);
}


public void AddError(string message)
{
Errors.Add(message);
}//AddError


#region Properties
[Bindable(true)]
[Category("Appearance")]
[DefaultValue("")]
public string ErrorMessage
{
get { return errorMessage; }
set { errorMessage = value; }
}//ErrorMessage
#endregion
}
}
StaticValidator.cs
using System;
using System.ComponentModel;
using System.Web.UI;

namespace DroopyEyes.Web.Controls
{
[ToolboxItem(false)]
internal class StaticValidator : IValidator
{
private string errorMessage;

#region IValidator
void IValidator.Validate()
{
}//Validate

bool IValidator.IsValid
{
get { return false; }
set { }
}//IsValid
#endregion

public string ErrorMessage
{
get { return errorMessage; }
set { errorMessage = value; }
}//ErrorMessage
}
}

1 comment:

Anonymous said...

Peter,

Now with ASP2.0 a ValidationSummary can have a ValidationGroup. Your great MultiValidator doesn't support this property an so the errors are not showed when this property is set.
Can you help me out to get to work please.

Thanks