Validation attributes, fluent interface, and extensions to validate models and throw rich exceptions.
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Validation attributes, fluent interface, and extensions to validate models and throw rich exceptions.

How to install Espalier.Validate

You can install Espalier.Validate from Nuget:

PM> Install-Package Espalier.Validate

There are currently versions targeting .NETFramework 4.5, 4.5.1, 4.5.2, 4.6, and 4.6.1, as well as .NETStandard 1.6. Need more? Let me know and I'll create them if they make sense.

Goals of Espalier.Validate

  • Create a .NET validation library with minimal dependencies.
  • Make it provide a lot of detail as to why a given validation error was thrown.
  • Provide sensible default validations.
  • It should be easily extendable.
  • Allow consumption via attributes, or a more direct approach.
  • Push validation down into services and the domain to keep code DRY.
  • Things should run async and in parallel whenever possible to make it fast.

Why use Espalier.Validate instead of data annotations?

  • Espalier.Validate has no dependencies on ASP.NET and other web-related binaries. That makes it not icky to have it in your domain models and entities.
  • Espalier.Validate provides Validate.That for dynamically validing object's properties.
  • You can contribute to this repository and help make a library you will enjoy using for a long time instead of being pissed off when Microsoft throws out (yet another!) technology in their stack.
  • This library has a comprehensive set of unit tests, and if there's something that isn't tested to your liking, you can write a test and contribute it back to the cause.

Using the library

To return a JSON payload with errors from your Web API 2 project

Espalier.Validate.WebAPI2 has an exception handler that will create HTTP 400 responses with a JSON payload containing all the errors Espalier.Validate generates. Check it out!

When you have a well defined model structure up front

If you have a defined set of models up front, the best way to use Espalier.Validate is via validation attributes. If you look at the TestModel class in Espalier.Validate.Tests you can see how to use the attributes:

public class TestModel
    public const string NotRequiredEmailFriendlyName = "Not required email";
    public const string RequiredEmailFriendlyName = "Required email";
    public const string RequiredStringFriendlyName = "Required string";

    public string NotRequiredEmail { get; set; }

    public string RequiredEmail { get; set; }

    public string RequiredString { get; set; }

Then you can validate an instance of that model using the Validate extension:

var model = new TestModel
    NotRequiredEmail = "bad email",
    RequiredEmail = "",
    RequiredString = "I am required."

await model.Validate();

In this case, an EspalierValidationException will be thrown with the errors that were encountered. If you would rather receive an array of validation errors and do whatever you want with them, you can call Validate as such:

var model = new TestModel
    NotRequiredEmail = "bad email",
    RequiredEmail = "",
    RequiredString = "I am required."

var errors = await model.Validate(ErrorResponse.ValidationErrors);

When you do not have a defined structure or do not want to decorate your classes

Espalier.Validate also provides extensions to help you validate the properties of any object. To validate properties on the fly, use Validate.That and pass in as many ValidationContext instances as you like. There are extension methods for creating ValidationContext instances for the built-in validations. Here is an example:

var model = new
    Zip = "ABC",
    Phone = "394 934-9348",
    Required = ""

await Validate.That(
    model.IsUSPostalCode(m => m.Zip, "Zip code"),
    model.IsRequired(m => m.Zip, "Zip code"),
    model.IsPhoneNumber(m => m.Phone, "Phone number"),
    model.IsRequired(m => m.Required, "Required field"));

Validate.That will throw an EspalierValidationException containing all the validation errors that were found within the ValidationContext instances provided.

Validations currently in the project

There are several handy validations currently in the project. If there are more that you need, you can either fork the project, build them, and submit a pull request, or email me and I will probably build them for you. Here are the current validations:

  • Email
  • ValidateEmailAttribute
  • model.IsEmail
  • Phone Number (currently USA style phone number only)
  • ValidatePhoneNumberAttribute
  • model.IsPhoneNumber
  • Required
  • ValidateRequiredAttribute
  • model.IsRequired
  • US Postal Code
  • ValidatePostalCodeAttribute
  • model.IsUSPostalCode

How to create your own validations

The current set of validations is clearly not a comprehensive list of the validations that could exist. In addition to that, you may have some unique scenarios. To create your own validations, implement the IValidation interface, and then implement a ValidateAttribute or create a way to build a ValidationContext instance for it. Here is the implementation of the RequiredValidation:

internal class RequiredValidation : IValidation
    private const string RequiredErrorMessage = "{0} is required.";
    private static RequiredValidation _instance;

    private RequiredValidation()

    public static RequiredValidation Instance => _instance ?? (_instance = new RequiredValidation());

    public Task<string> RunValidation(object value, string propertyFriendlyName)
        var stringValue = value as string;
        return Task.FromResult(string.IsNullOrWhiteSpace(stringValue) ? string.Format(RequiredErrorMessage, propertyFriendlyName) : string.Empty);

public class ValidateRequiredAttribute : ValidateAttribute
    public override Task<string> GetError(object value, string propertyFriendlyName)
        return RequiredValidation.Instance.RunValidation(value, propertyFriendlyName);

public static class ValidationExtensions
    public static ValidationContext IsRequired<TModel>(this TModel toValidate, Expression<Func<TModel, object>> selector, string friendlyName = null)
        var property = (PropertyInfo)((MemberExpression)selector.Body).Member;

        return new ValidationContext
            FriendlyName = string.IsNullOrWhiteSpace(friendlyName) ? property.Name : friendlyName,
            PropertyName = property.Name,
            Validation = RequiredValidation.Instance,
            Value = property.GetValue(toValidate)


I would love to have your contributions to this library! If you create some validations that other people may appreciate, or have some ideas to improve the library, please let me know. Then go ahead and fork the repository, add your validations, and submit a pull request.