In our example project we want to validate the state field is a valid Australian state if the country field is “Australia”.

IValidatableObject

Any model that implements IValidatableObject in MVC3 (note this doesn’t work in MVC2) will get it’s Validate method called when validating the object.

public abstract class ModelBase : IValidatableObject
...
public IEnumerable Validate(ValidationContext validationContext)

ConditionalValidationAttribute

To define the conditional validation we’ll use a ConditionalValidationAttribute which identifies a method to call to perform the validation. The method must be static, accept at least one parameter (the value of the property to be validated), optionally accept a second context parameter (this will be the object whose property is being validated) and return a ValidationResult.

[ConditionalValidation(typeof(User), "ValidateState")]
public string State { get; set; }
...
public static ValidationResult ValidateState(string state, object context)

ModelBase

The ModelBase class brings this all together. The constructor builds a dictionary of ConditionalValidationAttributes for each property and it implements the IValidatableObject.Validate method to loop through all the ConditionalValidationAttributes and validate them.

Note that the IValidatableObject.Validate method is called after all data annotations have been successfully validated. So if you have some properties with a RequiredAttribute, the required field error message will show and the user will need to fix this before our model’s Validate messages are displayed.

Here’s the full code for ModelBase.cs:

public abstract class ModelBase : IValidatableObject
{
  /// <summary>
  /// The list of validations for each property
  /// </summary>
  protected Dictionary<string, ConditionalValidationAttribute[]> Validations { get; private set; }

  protected ModelBase()
  {
    Validations = (from property in GetType().GetProperties()
             let validations = GetValidations(property)
             where validations.Length > 0
             select new {PropertyName = property.Name, Validations = validations})
      .ToDictionary(a => a.PropertyName, a => a.Validations);
  }

  /// <summary>
  /// Validates all ConditionalValidationAttributes
  /// </summary>
  /// <param name="validationContext"></param>
  /// <returns></returns>
  public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
  {
    return from property in Validations
         from validator in property.Value
         select validator.IsValid(GetPropertyValue(property.Key), this)
         into validationResult
         where validationResult != ValidationResult.Success
         select validationResult;
  }

  #region Private & Protected Methods

  /// <summary>
  /// Gets the validation attributes attached to a property.
  /// </summary>
  /// <param name="property">The property.</param>
  /// <returns>An array of validation attributes.</returns>
  private static ConditionalValidationAttribute[] GetValidations(PropertyInfo property)
  {
    return property.GetCustomAttributes(typeof(ConditionalValidationAttribute), true)
      .Cast<ConditionalValidationAttribute>().ToArray();
  }

  /// <summary>
  /// Gets the value for a property.
  /// </summary>
  /// <param name="propertyName">The property name.</param>
  /// <returns>The value of the property as an object.</returns>
  protected object GetPropertyValue(string propertyName)
  {
    try
    {
      return GetType().GetProperty(propertyName).GetValue(this, null);
    }
    catch (TargetInvocationException exception)
    {
      if (exception.InnerException != null)
        throw exception.InnerException;
      throw;
    }
  }

  #endregion
}

Note that when we call the ConditionalValidationAttribute.IsValid method we pass in ‘this’ as the context so we have a reference to the object being validated.

ConditionalValidationAttribute.cs:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Property)]
public class ConditionalValidationAttribute : Attribute
{
  #region Fields

  private readonly MethodInfo _methodInfo;
  private readonly bool _isSingleArgumentMethod;
  private readonly Type _valuesType;

  #endregion

  #region Properties

  public string ErrorMessage { get; set; }
  public string Method { get; private set; }
  public Type ValidatorType { get; private set; }

  #endregion

  #region Constructor

  /// <summary>
  /// Performs validation by calling a specified static method with one of the following signatures:
  /// ValidationResult Method(object value)
  /// ValidationResult Method(object value, object context)
  /// The value parameter may be strongly typed, if a process passes a value of a different type, type conversion will be attempted.
  /// </summary>
  /// <param name="validatorType">The type that contains the method that performs custom validation.</param>
  /// <param name="method">The method that performs custom validation.</param>
  public ConditionalValidationAttribute(Type validatorType, string method)
  {
    if (string.IsNullOrEmpty(method))
      throw new ArgumentException("method");

    Method = method;
    ValidatorType = validatorType;
    // Find a matching method - if multiple matching methods, pick the one with 2 parameters.
    var methods = from m in validatorType.GetMethods(BindingFlags.Public | BindingFlags.Static)
            let p = m.GetParameters().Length
            where m.Name == method &&
              m.ReturnType == typeof(ValidationResult) &&
              (p == 1 || p == 2)
            orderby p descending
            select m;

    if (methods.Count() == 0)
      throw new InvalidOperationException(string.Format("No method '{0}' found in class '{1}' with a valid signature.", method, validatorType));
    _methodInfo = methods.First();
    var parameters = _methodInfo.GetParameters();
    _isSingleArgumentMethod = parameters.Length == 1;
    _valuesType = parameters[0].ParameterType;
  }

  #endregion

  #region Public Methods

  public ValidationResult IsValid(object value, object context)
  {
    object convertedValue;
    var info = _methodInfo;
    if (!TryConvertValue(value, out convertedValue))
    {
      throw new InvalidOperationException(
        string.Format(
          "Unable to convert value from type {0} to type {1}",
          (value != null) ? value.GetType().ToString() : "null",
          _valuesType));
    }
    try
    {
      var parameters = _isSingleArgumentMethod ? new[] { convertedValue } : new[] { convertedValue, context };
      var result = (ValidationResult)info.Invoke(null, parameters);
      if (result != ValidationResult.Success && result.ErrorMessage == null)
        result.ErrorMessage = ErrorMessage;
      return result;
    }
    catch (TargetInvocationException exception)
    {
      if (exception.InnerException != null)
      {
        throw exception.InnerException;
      }
      throw;
    }
  }

  #endregion

  #region Private Methods

  private bool TryConvertValue(object value, out object convertedValue)
  {
    convertedValue = null;
    Type conversionType = _valuesType;
    if (value == null)
    {
      return (!conversionType.IsValueType || (conversionType.IsGenericType && (conversionType.GetGenericTypeDefinition() == typeof(Nullable<>))));
    }
    if (conversionType.IsAssignableFrom(value.GetType()))
    {
      convertedValue = value;
      return true;
    }
    try
    {
      convertedValue = Convert.ChangeType(value, conversionType, CultureInfo.CurrentCulture);
      return true;
    }
    catch (FormatException)
    {
      return false;
    }
    catch (InvalidCastException)
    {
      return false;
    }
    catch (NotSupportedException)
    {
      return false;
    }
  }

  #endregion
}

A few notes here:

• The constructor tries to find an appropriate method using reflection. If there are multiple overloads of the method then the one with 2 parameters will be chosen.
• The first parameter of the validation method can be of any type. Type conversion is attempted on the value to validate to match the method signature. I.e. the first (value) parameter can be object, string, int, etc.
• A generic error message can be supplied to the ConditionalValidationAttribute. If the validation method doesn’t set an error message when it returns a validation error, the ConditionalValidationAttribute.ErrorMessage property will be used.

And a sample model User.cs:

public class User : ModelBase
{
  [Required]
  public int UserId { get; set; }
  [Required]
  public string FirstName { get; set; }
  [Required]
  public string LastName { get; set; }
  [Required]
  public string Country { get; set; }
  [Required]
  [ConditionalValidation(typeof(User), "ValidateState")]
  public string State { get; set; }

  public static ValidationResult ValidateState(string state, object context)
  {
    var user = context as User;
    if (user == null)
      throw new ArgumentException("context is not of type User", "context");

    var australianStates = new[] {"VIC", "TAS", "NSW", "ACT", "QLD", "NT", "WA", "SA"};
    if (user.Country.Equals("Australia", StringComparison.CurrentCultureIgnoreCase))
    {
      if (!australianStates.Contains(state, StringComparer.CurrentCultureIgnoreCase))
        return new ValidationResult("Not a valid Australian state", new[] {"State"});
    }
    return ValidationResult.Success;
  }
}

Download the example by clicking the button below.

ValidatorEnable(val, enable)

This function takes the validator control (val) and a boolean to determine whether it should be enable or disabled. The problem with this function is that it triggers the validator to validate itself immediately, showing any validation error messages before the user has submitted the form, which may not always be want you want.  The alternative is to set the validator’s enabled property to trigger this validator to validate itself when the user submits the form, and by using the Page_IsValid variable you can test whether the user has triggered a validation or not, then use ValidatorEnable function only after the validation has occurred when the user expects to see validation messages.

Consider a scenario where you want the user to enter their address only if they select Australia as the country, you also need to validate the address fields. Two ways to approach this are:

1. For each address field you need to validate use a CustomValidator control, and in the ClientValidationFunction first check to see if the country is set to Australia and if it’s not then consider any response for this control as valid. The problem with this is you will have to recrete all of the validator functionality in your client validation function, which means more work.
2. Use standard validator controls for the address fields, and enable or disable them in javascript.  The code you call to enable or disable the validators could be a CustomValidation control on the country field, or a standard javascript call when the user selects a country. If the Page_IsValid variable is set to true then assume that the page has just been loaded and no validation has occured, in this case just set the validator controls enabled property so it will be validated (or not) when the user submits the form.
   If Page_IsValid is false then assume the user has tried to submit but failed, in this case we expect validation messages to show / hide as the user changes data on the form – this is the normal ASP.Net behaviour.

Here’s an example:

// The ClientValidationFunction of a custom validator for country
function validateCountry(source, args) {
    // We we haven't submitted the form yet
    if (Page_IsValid) {
        // Turn the postcode validator on / off, but don't validate
        requiredFieldValidatorPostcode.enabled = enable;
    } else {
        // Turn the postcode validator on / off, and immediately validate
        ValidatorEnable(requiredFieldValidatorPostcode, enable);
    }

    // Validate country is not empty
    args.IsValid = (args.Value.length > 0);
}

Don’t forget to include server side code to mimic this enabling / disabling of validators. To do this override the Validate() function, enable or disable validators depending on the country selected and call the base.Validate() function.

public override void Validate()
{
    // Check if we need to validate the postcode.
    bool requireAddress = (dropDownListCountry.Text == "Australia");
    requiredFieldValidatorPostcode.Enabled = requireAddress;

    // Call the normal validation methods.
    base.Validate();
}

protected void customValidatorCountry_ServerValidate(object source, ServerValidateEventArgs args)
{
    // Implement the server side custom validation for country.
    args.IsValid = (args.Value.Length > 0);
}

Microsoft offers SQL Server Reporting Services (SSRS) with SQL Server 2005 and later editions to generate and publish reports using the Report Definition Language (RDL), as an alternative to Crystal Reports. SSRS requires a Report Server to publish reports to so they can be useful, however they also offer a ASP.Net or Windows Forms component called the ReportViewer which we can use in our code without the need for any SQL database to generate reports, and these reports can be saved as Excel files, PDF’s or images.

The ReportViewer uses a modified version of RDL called RDLC, which does not save any information about the data source along with the report, which makes it perfect for generating reports from objects that aren’t a database. For instance, I recently needed to generate PDF letters with custom information in them, so this solution proved perfect.

I’m going to demonstrate how to pass parameters to an ASP.Net page and return a PDF with that information in it, this could just as easily be done in a Windows Forms project. I’m using Visual Studio 2005, in my preferred language of c#.

Start by creating a new Web Site in Visual Studio. We are going to use a collection of custom business objects as the data source for our report, so add a new class to our project and call it Person.cs. Copy the following code into your Person.cs file.

using System;
using System.Collections.Generic;

/// <summary>
/// The business object we want to use in our report.
/// </summary>
public class Person
{
    /// <summary>
    /// Some fields to hold information about this person.
    /// </summary>
    private string _title;
    private string _firstName;
    private string _lastName;

    /// <summary>
    /// The person properties are only able to be set at instantiation.
    /// After the object is created the properties are read only.
    /// </summary>
    public string Title
    {
        get { return _title; }
    }

    public string FirstName
    {
        get { return _firstName; }
    }

    public string LastName
    {
        get { return _lastName; }
    }

    /// <summary>
    /// The person constructor.
    /// </summary>
    /// <param name="title">The person's title</param>
    /// <param name="firstName">Their first name</param>
    /// <param name="lastname">Their last name</param>
    public Person(string title, string firstName, string lastname)
    {
        _title = title;
        _firstName = firstName;
        _lastName = lastname;
    }
}

/// <summary>
/// Creates a data source for our report.
/// </summary>
public class PersonDataSource
{
    /// <summary>
    /// This method creates a new person object and adds them to a new collection that
    /// can be used as a data source for the report.
    /// </summary>
    /// <returns>A collection of person objects.</returns>
    public static List<Person> GetPerson(string title, string firstName, string lastName)
    {
        List<Person> people = new List<Person>();
        people.Add(new Person(title, firstName, lastName));
        return people;
    }
}

Now that we have our business object we can create the report. Click add item on the project and select Report. Call this item Report.rdlc.

In the left hand toolbar you should see Website Data Sources (if not select Data > Show Data Sources from the file menu) and in the data sources you should see our Person object. You can now build a report using the fields from our person object and the tools in the toobox. For simplicity just drag some Textboxes onto the report from the toolbox and enter any static text in them that you want, then drag on the Person’s title, firstName and lastName and arrange them appropriately.

Finally we need to write the code which will create the person data source and generate the report. Before we do this we need a reference to the ReportViewer control, right click on the project and Add Reference, in the .Net tab, select Microsoft.ReportViewer.WebForms.Now copy the following code into the Default.aspx source.

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_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>Untitled Page</title>
</head>
<body>
	<form id="form1" runat="server">
	<div>
	<asp:Label ID="lblTitle" runat="server" Text="Title: "></asp:Label>
	<asp:TextBox ID="txtTitle" runat="server">Mr</asp:TextBox><br />
	<asp:Label ID="lblFirstName" runat="server" Text="First Name: "></asp:Label>
	<asp:TextBox ID="txtFirstName" runat="server">John</asp:TextBox><br />
	<asp:Label ID="lblLastName" runat="server" Text="Last Name: "></asp:Label>
	<asp:TextBox ID="txtLastName" runat="server">Smith</asp:TextBox><br />
	<asp:Button ID="Button1" runat="server" Text="Submit" /></div>
	</form>
</body>
</html>

And copy the following into the Code View of Default.aspx (Default.aspx.cs).

using System;
using System.Web;
using Microsoft.Reporting.WebForms;
using System.IO;
using System.Text;
using System.Collections.Generic;
public partial class _Default : System.Web.UI.Page
{
	protected void Page_Load(object sender, EventArgs e)
	{
		if (Page.IsPostBack)
		{
			LocalReport report = new LocalReport();
			string reportName = "CompletionCertificate";
			string deviceInfo =
				"<DeviceInfo>" +
				"  <OutputFormat>EMF</OutputFormat>" +
				"  <PageWidth>8.5in</PageWidth>" +
				"  <PageHeight>11in</PageHeight>" +
				"  <MarginTop>0.25in</MarginTop>" +
				"  <MarginLeft>0.25in</MarginLeft>" +
				"  <MarginRight>0.25in</MarginRight>" +
				"  <MarginBottom>0.25in</MarginBottom>" +
				"</DeviceInfo>";
			Warning[] warnings;
			string[] streamids;
			string mimeType;
			string encoding;
			string extension;

			report.ReportPath = "Report.rdlc";
			report.DataSources.Add(new ReportDataSource("Person", PersonDataSource.GetPerson(txtTitle.Text, txtFirstName.Text, txtLastName.Text)));
			byte[] bytes = report.Render("PDF", deviceInfo, out mimeType, out encoding, out extension, out streamids, out warnings);

			Response.Clear();
			Response.ContentType = mimeType;
			Response.AddHeader("Content-Disposition", "attachment; filename=" + reportName + "." + extension);
			Response.OutputStream.Write(bytes, 0, bytes.Length);
			Response.End();
		}
	}
}

Now run your project, enter some person data into the form and click submit. Open the PDF file and verify that the report is addressed to your person.