I Prefer Jim Developer James Schubert shares his code and his thoughts.

System.ComponentModel.DataAnnotations for ASP.NET Web Forms

Although I'm primarily an ASP.NET Web Forms developer, I regularly dabble in new and interesting technologies.  I've toyed with other Microsoft technologies such as ASP.NET MVC and Dynamic Data web sites.

ASP.NET MVC offers an interesting mechanism for validating view models called DataAnnotations.   Examples can be seen here and here.

Some of the more useful attributes for validation include:

• RequiredAttribute
• RegularExpressionAttribute
• RangeAttribute
• DataTypeAttribute

I recently decided to implement a data validation schema (for lack of a better term) for web forms similar to that of ASP.NET MVC. This would allow us to maintain validation of a model object via attributes at the class level, instead of dispersing these validation rules through the web form code or any of the business logic layers. However, there will be occasions where an object's validity depends upon some state or validity of another object. I've handled this by allowing the validation method to accept any action to be called after the object has been validated.

In order to make a domain object validatable, I'm going to implement the following interface:

IValidatable.cs

namespace Validatable
{
    interface IValidatable
    {
        System.Type EntityType { get; set; }
        System.Collections.Generic.List<object> Errors { get; set; }

        /// <summary>
        /// Determine whether the object is valid.  If invalid, errors are added to item.Errors
        /// </summary>
        /// <returns>true if valid, false if invalid</returns>
        bool IsValid();

        /// <summary>
        /// Validate this item against a supplied action
        /// </summary>
        /// <param name="action">The action to use for validation</param>
        void Validate(System.Action action);

        /// <summary>
        /// Validate this object against a Repository.
        /// <example>
        /// Item a = new Item();
        /// a.Validate(new ItemRepository());
        /// </example>
        /// Note: Repository must have a Validate(T item) method
        /// </summary>
        /// <param name="repository">The Repository to use for validation</param>
        void Validate(object repository);
    }
}

As you can see, this abstract class adds a property accessor for the entity's type, a list of validation errors, an IsValid method, and two Validate methods.

Validatable<T>

I'll take the code in chunks, since there is a lot more than I usually post.
First of all, the using directives required for this class are pretty sparse:

    using System;
    using System.Collections.Generic;
    using System.ComponentModel.DataAnnotations;
    using System.Linq;
    using System.Reflection;

The class definition is:

   [Serializable]
    public abstract class Validatable<T> : IValidatable where T : class, new()
    { /* class */ }

If you're unfamiliar with generic constraints, the where T : class, new() above enforces the T passed as in to be a class and to have a parameterless constructor (new())

The properties can be auto-implemented properties, or they can implement any backing logic you prefer. The constructor as I have it is:

 protected Validatable()
 {
     Errors = new List<object>();
     EntityType = typeof(T);
 }

Now for the methods and a slight explanation of each:

IsValid()

        public virtual bool IsValid()
        {
            Errors = new List<object>();
            PropertyInfo[] props = this.GetType().GetProperties();

            foreach (PropertyInfo property in props)
            {
                foreach (ValidationAttribute va in
                    property.GetCustomAttributes(true).OfType<ValidationAttribute>())
                {
                    var value = property.GetValue(this, null);
                    if (!va.IsValid(value))
                    {
                        Errors.Add(va.ErrorMessage);
                    }
                }
            }

            return Errors.Count <= 0;
        }

As a side note, all DataAnnotation attributes inherit from ValidationAttribute. You can create your own Validation Attributes by inheriting from this class. This method clears the list of errors and repopulates it by looping over any properties with validation attributes of this object, validates that property's value against the attribute, and adds the ErrorMessage associated with it to the list of Errors.

Validate Methods

        public virtual void Validate(Action action)
        {
            if (IsValid())
            {
                action();
            }
        }

        public virtual void Validate(object repository)
        {
            MethodInfo method = repository.GetType().GetMethods()
                .Where(x => x.Name.Equals("Validate")).FirstOrDefault();
            if (method != null)
            {
                object[] parameters = new object[] { this };
                Validate(() => method.Invoke(repository, parameters));
            }
        }

These two methods can be changed to throw an exception if the object is invalid. I'll leave that up to you. The way these are set up is to allow you to validate the object and perform either some unknown action or call Validate(T item) against some other object, arbitrarily called a repository. In actuality, you could have a Validate method on some other object that takes an object of this type and essentially chain validations.

The simplicity of the Validate(Action action) method is what makes it beautiful. For instance, you can have an item (a) and cause it save after validation by doing something like:

a.Validate(() => new ItemDAL().Save(a));

As you can see, a Save method in your Item's Data Access Layer taking Item as a parameter can be called only when a.IsValid() is true. Again, there are a number of ways to tweak this and change it to your liking, but I'll leave that up to you.

DataAnnotationValidator

Here is the wonderful part about all of this. You can create a custom validator that validates in a similar way as the Validatable class. I'll post the code and quickly explain what it does. I got the idea from this from another blog and tweaked it a little.

DataAnnotationValidator.cs

// DataAnnotationValidator.cs
namespace Validatable
{
    using System;
    using System.ComponentModel.DataAnnotations;
    using System.Linq;
    using System.Reflection;
    using System.Web.UI;
    using System.Web.UI.WebControls;

    [ToolboxData("<{0}:DataAnnotationValidator runat=\"server\" ControlToValidate=\"[Required]\" Display=\"Dynamic\" Text=\"*\" SourceTypeName=\"[FullyQualifiedTypeName]\" PropertyToValidate=\"[PropertyName]\" />")]
    public class DataAnnotationValidator : BaseValidator
    {
        /// <summary>
        /// THe Property that should be checked
        /// </summary>
        public string PropertyToValidate { get; set; }

        /// <summary>
        /// The object's type
        /// </summary>
        public string SourceTypeName { get; set; }

        protected override bool EvaluateIsValid()
        {
            Type source = GetValidatedType();
            PropertyInfo property = GetValidatedProperty(source);
            string value = GetControlValidationValue(ControlToValidate);

            foreach (ValidationAttribute va in property
                .GetCustomAttributes(typeof(ValidationAttribute), true)
                .OfType<ValidationAttribute>())
            {
                if (!va.IsValid(value))
                {
                    if (string.IsNullOrEmpty(ErrorMessage))
                    {
                        this.ErrorMessage = va.ErrorMessage;
                    }
                    return false;
                }
            }

            return true;
        }

        private Type GetValidatedType()
        {
            if (string.IsNullOrEmpty(SourceTypeName))
            {
                throw new InvalidOperationException("Null SourceTypeName can't be validated");
            }

            Type validatedType = Type.GetType(SourceTypeName);
            if (validatedType == null)
            {
                throw new InvalidOperationException(
                    string.Format("{0}:{1}", "Invalid SourceTypeName", SourceTypeName));
            }

            return validatedType;
        }

        private PropertyInfo GetValidatedProperty(Type source)
        {
            PropertyInfo property = source.GetProperty(PropertyToValidate,
              BindingFlags.Public | BindingFlags.Instance);

            if (property == null)
            {
                throw new InvalidOperationException(
                  string.Format("{0}:{1}", "Validated Property Does Not Exists", PropertyToValidate));
            }
            return property;
        }
    }
}

This creates a validator web control which has 'PropertyToValidate' and 'SourceTypeName'. When a Validator web control is added to a page, it must have ControlToValidate, PropertyToValidate, and SourceTypeName specified or an error will be thrown. The validation method verifies the type and the property, gets the value of the property and finally, much like the Validatable abstract class, it loops over all ValidationAttributes and validates the value against that attribute. You can use this with multiple attributes, so it doesn't hurt to add this validator to a property-- that just means you can add an attribute in the future and everything is already wired up! That's pretty cool.

Example

As an example, I'm going to create a single page to **input** a Customer object. The customer object is very simple:

 public class Customer : Validatable.Validatable<Customer>
    {
        [Required(ErrorMessage="UserId is Required")]
        public int UserId { get; set; }

        [DataType(DataType.PhoneNumber, ErrorMessage="Invalid Phone Number")]
        public string PhoneNumber { get; set; }

        [RegularExpression(@"(^\d{5}(-\d{4}){0,1}$)", ErrorMessage="Invalid Zip Code")]
        public string ZipCode { get; set; }
    }

And the Page in full is collapsed below:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="Example._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <asp:Label ID="lblStatus" runat="server" />
    <asp:ValidationSummary ID="ValidationSummary1" runat="server" />
    <div>
        <asp:FormView ID="FormView1" runat="server" DataSourceID="srcRepository"
            Width="119px" AllowPaging="True">
            <EditItemTemplate>
                UserId:
                <asp:TextBox ID="UserIdTextBox" runat="server" Text='<%# Bind("UserId") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator1" runat="server" ControlToValidate="UserIdTextBox" Display="Dynamic"
                    PropertyToValidate="UserId" ErrorMessage="Invalid User Identification Number"
                    SourceTypeName="Example.Customer, Example, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
                    Text="*"/>
                <br />
                PhoneNumber:
                <asp:TextBox ID="PhoneNumberTextBox" runat="server"
                    Text='<%# Bind("PhoneNumber") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator2" runat="server" ControlToValidate="PhoneNumberTextBox" Display="Dynamic"
                    PropertyToValidate="PhoneNumber" OnInit="GetTypeName" SourceTypeName="Example.Customer, Example, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" Text="*"/>
                <br />
                ZipCode:
                <asp:TextBox ID="ZipCodeTextBox" runat="server" Text='<%# Bind("ZipCode") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator3" runat="server" ControlToValidate="ZipCodeTextBox" Display="Dynamic"
                    PropertyToValidate="ZipCode" SourceTypeName="Example.Customer, Example, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" Text="*"/>
                <br />
                EntityType:
                <asp:TextBox ID="EntityTypeTextBox" runat="server"
                    Text='<%# Eval("EntityType") %>' />
                <br />
                <asp:LinkButton ID="UpdateButton" runat="server" CausesValidation="True"
                    CommandName="Update" Text="Update" />
                &nbsp;<asp:LinkButton ID="UpdateCancelButton" runat="server"
                    CausesValidation="False" CommandName="Cancel" Text="Cancel" />
            </EditItemTemplate>
            <InsertItemTemplate>
                UserId:
                <asp:TextBox ID="UserIdTextBox" runat="server" Text='<%# Bind("UserId") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator1" runat="server" ControlToValidate="UserIdTextBox" Display="Dynamic"
                    PropertyToValidate="UserId"  OnInit="GetTypeName"  Text="*"/>
                <br />
                PhoneNumber:
                <asp:TextBox ID="PhoneNumberTextBox" runat="server"
                    Text='<%# Bind("PhoneNumber") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator2" runat="server" ControlToValidate="PhoneNumberTextBox" Display="Dynamic"
                    PropertyToValidate="PhoneNumber" OnInit="GetTypeName"  Text="*"/>
                <br />
                ZipCode:
                <asp:TextBox ID="ZipCodeTextBox" runat="server" Text='<%# Bind("ZipCode") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator3" runat="server" ControlToValidate="ZipCodeTextBox" Display="Dynamic"
                    PropertyToValidate="ZipCode" OnInit="GetTypeName"  Text="*"/>
                <br />
                EntityType:
                <asp:TextBox ID="EntityTypeTextBox" runat="server"
                    Text='<%# Eval("EntityType") %>' />
                <br />
                <asp:LinkButton ID="InsertButton" runat="server" CausesValidation="True"
                    CommandName="Insert" Text="Insert" />
                &nbsp;<asp:LinkButton ID="InsertCancelButton" runat="server"
                    CausesValidation="False" CommandName="Cancel" Text="Cancel" />
            </InsertItemTemplate>
            <ItemTemplate>
                UserId:
                <asp:Label ID="UserIdTextBox" runat="server" Text='<%# Bind("UserId") %>' />
                <br />
                PhoneNumber:
                <asp:Label ID="PhoneNumberTextBox" runat="server"
                    Text='<%# Bind("PhoneNumber") %>' />
                <br />
                ZipCode:
                <asp:Label ID="ZipCodeTextBox" runat="server" Text='<%# Bind("ZipCode") %>' />
                <br />
                EntityType:
                <asp:Label ID="EntityTypeLabel" runat="server"
                    Text='<%# Bind("EntityType") %>' />
                <br />
                <asp:LinkButton ID="EditButton" runat="server" CausesValidation="False"
                    CommandName="Edit" Text="Edit" />
                &nbsp;<asp:LinkButton ID="NewButton" runat="server" CausesValidation="False"
                    CommandName="New" Text="New" />
            </ItemTemplate>
        </asp:FormView>
        <asp:ObjectDataSource ID="srcRepository" runat="server"
            DataObjectTypeName="Example.Customer" InsertMethod="Save"
            oninserted="srcRepository_Saved" onupdated="srcRepository_Saved"
            SelectMethod="Get" TypeName="Example.CustomerRepository" UpdateMethod="Save"></asp:ObjectDataSource>
    </div>
    </form>
</body>
</html>

Of particular note here is in the EditTemplate, you'll see

                  UserId:
                <asp:TextBox ID="UserIdTextBox" runat="server" Text='<%# Bind("UserId") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator1" runat="server" ControlToValidate="UserIdTextBox" Display="Dynamic"
                    PropertyToValidate="UserId" ErrorMessage="Invalid User Identification Number"
                    SourceTypeName="Example.Customer, Example, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
                    Text="*"/>

in which the SourceTypeName must be the full Assembly Qualified Name. This is necessary if the Validatable and DataAnnotationValidator classes are in an external library. This is a rather tedious property to fill for each validator. Another way to get around this is to populate the SourceTypeName as seen in the InserItemTemplate, e.g.

                UserId:
                <asp:TextBox ID="UserIdTextBox" runat="server" Text='<%# Bind("UserId") %>' />
                <example:DataAnnotationValidator ID="DataAnnotationValidator1" runat="server" ControlToValidate="UserIdTextBox" Display="Dynamic"
                    PropertyToValidate="UserId"  OnInit="GetTypeName"  Text="*"/>

and the code in the code behind is very simple:

        protected void GetTypeName(object sender, EventArgs e)
        {
            DataAnnotationValidator validator = (DataAnnotationValidator)sender;
            validator.SourceTypeName = new Customer().EntityType.AssemblyQualifiedName;
        }

Also notice that the first validator (the one with the assembly qualified name) has an ErrorMessage property specified. Doing this allows you to override the error message returned from the validated object. We can do this if the model says "UserId" and you want the user to see this property referred to as "User Identification".

Conclusion

Sometimes dabbling in other technologies can open the door for new and simpler ways of doing things. I like this example because it allows you to build web applications quickly and easily, while making them maintainable in the future (new validations only have to be added to the model, not to every control where the model must be validated).

For further reference, this project is linked below. Please download and modify if necessary.
Validatable.zip