Is it possible to use DataAnnotations with Interfaces?

If you use a base class instead of an interface, the attributes will work fine.

I'm surprised no one mentioned MetadataTypeAttribute. But yes, this works.

[MetadataType(typeof(ICustomerMetaData))]
public partial class Customer
{
}


public interface ICustomerMetaData
{
  // Apply RequiredAttribute
  [Required(ErrorMessage = "Title is required.")]
  string Title { get; }
}

As for using an Interface directly (using Customer: ICustomerMetaData):

The product team does not want to implement this feature, for two main reasons:

• Consistency with DataAnnotations.Validator

• Consistency with validation behavior in ASP.Net MVC

• Tricky scenario: a class implements two interfaces that have the same property, but with conflicting attributes on them. Which attribute would take precedence?

While MVC automatically registers the MetaData with the TypeDescriptor, you may have to manually add it yourself:

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

  public class Program
  {
     public static void Main()
     {
        var customer = new Customer();

        TypeDescriptor.AddProviderTransparent(
          new AssociatedMetadataTypeTypeDescriptionProvider(typeof(Customer), 
            typeof(ICustomerMetaData)), 
            typeof(Customer));

        var context = new ValidationContext(customer);
        var validationResults = new List<ValidationResult>();

        var isValid = Validator.TryValidateObject(
          customer, context, validationResults, true);
        Console.WriteLine($"is Valid = {isValid}");

        customer.Title = "I has Title";

        isValid = Validator.TryValidateObject(
          customer, context, validationResults, true);
        Console.WriteLine($"is Valid = {isValid}");


        Console.ReadKey();
     }

     [MetadataType(typeof(ICustomerMetaData))]
     public partial class Customer
     {
        public string Title { get; set;  }
     }

     public interface ICustomerMetaData
     {
        // Apply RequiredAttribute
        [Required(ErrorMessage = "Title is required.")]
        string Title { get; }
     }
  }

Output:

is Valid = False

is Valid = True