Error messages for model validation using data annotations

This is the only way I know of that, but it's far from clean. It involves using subclassing and MetaData classes to "override" the error message.

public class Book
{
    public PrimaryContact PrimaryContact { get; set; }
    public SecondaryContact SecondaryContact { get; set; }

    [Required(ErrorMessage = "Book name is required")]
    public string Name { get; set; }
}

public class Contact
{
    [Required(ErrorMessage = "Name is required")]
    public string Name { get; set; }
}

[MetadataType(typeof(PrimaryContactMD))]
public class PrimaryContact : Contact
{
    class PrimaryContactMD
    {
        [Required(ErrorMessage = "Primary Contact Name is required")]
        public string Name { get; set; }
    }
}

[MetadataType(typeof(SecondaryContactMD))]
public class SecondaryContact : Contact
{
    class SecondaryContactMD
    {
        [Required(ErrorMessage = "Secondary Contact Name is required")]
        public string Name { get; set; }
    }
}

The Below code in c# to format data annotations error and format in append format in single string

  public class ValidateModelAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (actionContext.ModelState.IsValid == false)
            {
                if (actionContext.ModelState.IsValid) return;
                string errors = actionContext.ModelState.SelectMany(state => state.Value.Errors).Aggregate("", (current, error) => current + (error.ErrorMessage + ". "));
            }
        }
    }

You may want to look at using the CustomValidation attribute for such properties instead of relying on the Required attribute.

CustomValidation will allow you to more granularly tailor your validation messages to the property you are validating. I've used context.DisplayName to dynamically display the name of the property being validated just for brevity, but this can be customized further based on your needs.

If even further customization is needed, you can write different CustomValidation handlers for each individual property instead of just reusing the same one as I've done in my code example.

using System.ComponentModel.DataAnnotations;

public class Book {
    [CustomValidation(typeof(Book), "ValidateContact")]
    public Contact PrimaryContact { get; set; }

    [CustomValidation(typeof(Book), "ValidateContact")]
    public Contact SecondaryContact { get; set; }

    [Required(ErrorMessage = "Book name is required")]
    public string Name { get; set; }

    public static ValidationResult ValidateContact(Contact contact, ValidationContext context) {
        ValidationResult result = null;

        if (contact == null) {
            result = new ValidationResult($"{context.DisplayName} is required.");
        } else if (string.IsNullOrWhiteSpace(contact.Name)) {
            result = new ValidationResult($"{context.DisplayName} name is required.");
        }

        return result;
    }
}

public class Contact {
    [Required(ErrorMessage = "Name is required")]
    public string Name { get; set; }
}