Is your feature request related to a problem? Please describe.

Model validation is synchronous, but if there's a database call or other awaitable call, it would be nice to have an async option.

See aspnet/Mvc#786.

Describe the solution you'd like

Add virtual methods for ValidationAttribute:

Task<bool> IsValidAsync(object value);
Task<ValidationResult> IsValidAsync(object value, ValidationContext validationContext);

Describe alternatives you've considered

Currently working around by using synchronous DB calls.