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  Creating a custom validation attribute

Custom validation attributes can be created by deriving from the ValidationAttribute base class, then overriding virtual methods as needed.

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public class NotABananaAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        var inputValue = value as string;
        var isValid = true;

        if (!string.IsNullOrEmpty(inputValue))
        {
            isValid = inputValue.ToUpperInvariant() != "BANANA";
        }

        return isValid;
    }
}

This attribute can then be used like this:

public class Model
{
    [NotABanana(ErrorMessage = "Bananas are not allowed.")]
    public string FavoriteFruit { get; set; }
}
  Data Annotation Basics

Data annotations are a way of adding more contextual information to classes or members of a class. There are three main categories of annotations:

  • Validation Attributes: add validation criteria to data
  • Display Attributes: specify how the data should be displayed to the user
  • Modelling Attributes: add information on usage and relationship with other classes

Usage

Here is an example where two ValidationAttribute and one DisplayAttribute are used:

class Kid
{
    [Range(0, 18)] // The age cannot be over 18 and cannot be negative
    public int Age { get; set; }
    [StringLength(MaximumLength = 50, MinimumLength = 3)] // The name cannot be under 3 chars or more than 50 chars
    public string Name { get; set; }
    [DataType(DataType.Date)] // The birthday will be displayed as a date only (without the time)
    public DateTime Birthday { get; set; }
}

Data annotations are mostly used in frameworks such as ASP.NET. For example, in ASP.NET MVC, when a model is received by a controller method, ModelState.IsValid() can be used to tell if the received model respects all its ValidationAttribute. DisplayAttribute is also used in ASP.NET MVC to determine how to display values on a web page.
  Manually Execute Validation Attributes

Most of the times, validation attributes are use inside frameworks (such as ASP.NET). Those frameworks take care of executing the validation attributes. But what if you want to execute validation attributes manually? Just use the Validator class (no reflection needed).

Validation Context

Any validation needs a context to give some information about what is being validated. This can include various information such as the object to be validated, some properties, the name to display in the error message, etc.

ValidationContext vc = new ValidationContext(objectToValidate); // The simplest form of validation context. It contains only a reference to the object being validated.

Once the context is created, there are multiple ways of doing validation.

Validate an Object and All of its Properties

ICollection<ValidationResult> results = new List<ValidationResult>(); // Will contain the results of the validation
bool isValid = Validator.TryValidateObject(objectToValidate, vc, results, true); // Validates the object and its properties using the previously created context.
// The variable isValid will be true if everything is valid
// The results variable contains the results of the validation

Validate a Property of an Object

ICollection<ValidationResult> results = new List<ValidationResult>(); // Will contain the results of the validation
bool isValid = Validator.TryValidatePropery(objectToValidate.PropertyToValidate, vc, results, true); // Validates the property using the previously created context.
// The variable isValid will be true if everything is valid
// The results variable contains the results of the validation

And More

To learn more about manual validation see:
  EditableAttribute (data modeling attribute)

EditableAttribute sets whether users should be able to change the value of the class property.

public class Employee
{
    [Editable(false)]
    public string FirstName { get; set; }
}

Simple usage example in XAML application

<Window x:Class="WpfApplication.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:wpfApplication="clr-namespace:WpfApplication"
        Height="70" Width="360" Title="Display name example">

    <Window.Resources>
        <wpfApplication:EditableConverter x:Key="EditableConverter"/>
    </Window.Resources>

    <StackPanel Margin="5">
        <!-- TextBox Text (FirstName property value) -->
        <!-- TextBox IsEnabled (Editable attribute) -->
        <TextBox Text="{Binding Employee.FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                 IsEnabled="{Binding Employee, Converter={StaticResource EditableConverter}, ConverterParameter=FirstName}"/>
    </StackPanel>
    
</Window>


namespace WpfApplication
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private Employee _employee = new Employee() { FirstName = "This is not editable"};

        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }

        public Employee Employee
        {
            get { return _employee; }
            set { _employee = value; }
        }
    }
}


namespace WpfApplication
{
    public class EditableConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            // return editable attribute's value for given instance property,
            // defaults to true if not found
            var attribute = value.GetType()
                .GetProperty(parameter.ToString())
                .GetCustomAttributes(false)
                .OfType<EditableAttribute>()
                .FirstOrDefault();

            return attribute != null ? attribute.AllowEdit : true;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

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  Manually Execute Validation Attributes

Most of the times, validation attributes are use inside frameworks (such as ASP.NET). Those frameworks take care of executing the validation attributes. But what if you want to execute validation attributes manually? Just use the Validator class (no reflection needed).

Validation Context

Any validation needs a context to give some information about what is being validated. This can include various information such as the object to be validated, some properties, the name to display in the error message, etc.

ValidationContext vc = new ValidationContext(objectToValidate); // The simplest form of validation context. It contains only a reference to the object being validated.

Once the context is created, there are multiple ways of doing validation.

Validate an Object and All of its Properties

ICollection<ValidationResult> results = new List<ValidationResult>(); // Will contain the results of the validation
bool isValid = Validator.TryValidateObject(objectToValidate, vc, results, true); // Validates the object and its properties using the previously created context.
// The variable isValid will be true if everything is valid
// The results variable contains the results of the validation

Validate a Property of an Object

ICollection<ValidationResult> results = new List<ValidationResult>(); // Will contain the results of the validation
bool isValid = Validator.TryValidatePropery(objectToValidate.PropertyToValidate, vc, results, true); // Validates the property using the previously created context.
// The variable isValid will be true if everything is valid
// The results variable contains the results of the validation

And More

To learn more about manual validation see:
  DisplayNameAttribute (display attribute)

DisplayName sets display name for a property, event or public void method having zero (0) arguments.

public class Employee
{
    [DisplayName(@"Employee first name")]
    public string FirstName { get; set; }
}

Simple usage example in XAML application

<Window x:Class="WpfApplication.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:wpfApplication="clr-namespace:WpfApplication"
        Height="100" Width="360" Title="Display name example">

    <Window.Resources>
        <wpfApplication:DisplayNameConverter x:Key="DisplayNameConverter"/>
    </Window.Resources>

    <StackPanel Margin="5">
        <!-- Label (DisplayName attribute) -->
        <Label Content="{Binding Employee, Converter={StaticResource DisplayNameConverter}, ConverterParameter=FirstName}" />
        <!-- TextBox (FirstName property value) -->
        <TextBox Text="{Binding Employee.FirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
    </StackPanel>
    
</Window>


namespace WpfApplication
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private Employee _employee = new Employee();

        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }

        public Employee Employee
        {
            get { return _employee; }
            set { _employee = value; }
        }
    }
}


namespace WpfApplication
{
    public class DisplayNameConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            // Get display name for given instance type and property name
            var attribute = value.GetType()
                .GetProperty(parameter.ToString())
                .GetCustomAttributes(false)
                .OfType<DisplayNameAttribute>()
                .FirstOrDefault();

            return attribute != null ? attribute.DisplayName : string.Empty;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

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