Asp Mvc 6 Model Validation with a service in custom ValidationAttribute - asp.net

Question

TLDR: In Asp Mvc 6 how do I perform model validation with a service using data annotations? What are the alternatives?
I have a very simple model
public class MyModel
{
[Required]
public string Name { get; set; }
}
I also have a service that exposes some simple validation methods
public interface IMyService
{
string[] ReservedWords { get; }
bool IsValidName(string name);
// Internally calls IsValidName and throws an Exception if the name is invalid
void Save(MyModel myModel);
// ... snip
}
And I have wired up my controller like so
public class MyController : Controller
{
private readonly IMyService _service;
public MyController(IMyService service)
{
_service = service;
}
// ... snip
public IActionResult Post(MyModel myModel)
{
if (!_service.IsValidName(input?.Name))
{
ModelState.AddModelError(nameof(MyModel.Name), "Invalid Name");
}
if (!ModelState.IsValid)
{
return View(myModel);
}
_service.Save(myModel);
return RedirectToAction(nameof(Index));
}
}
It feels a bit clucky to have 2 stages of validation - automatic model validation then manually performing service validation. I was hoping that something simialr to this would work
public class MyModel
{
[ServiceValidation(nameof(IMyService), nameof(IMyService.IsValidName)]
[Required]
public string Name { get; set; }
}
public ServiceValidationAttribute : ValidationAttribute
{
private readonly Type _interfaceOrClass;
private readonly string _methodOrProperty;
public ServiceValidationAttribute(Type interfaceOrClass, string methodOrProperty)
{
_interfaceOrClass = interfaceOrClass;
_methodOrProperty = methodOrProperty;
}
public override bool RequiresValidationContext => true;
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
var service = validationContext.GetService(_interfaceOrClass);
// Extension method in shared library to assist with reflection
bool isValid = _interfaceOrClass.ValueForMethodOrPropertyNamed<bool>(service, _methodOrProperty, value);
return isValid
? ValidationResult.Success
: new ValidationResult(ErrorMessage);
}
}
However var serivce is always null, is there any way around this? I have wired up the IMyService to an implementation in the Startup.cs as it is available in the Controller.
Alternatively is there a better way of adding to the ModelState with a service?