.net core get user in ValidationAttribute

Question

I am trying to access the current user (i.e. ClaimsPrincipal from identity) in a custom ValidationAttribute, and I haven't figured out how I could do that.

public class UniqueTitleValidator : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
      // var user = ?
    }
}

I know that I could access the user from a HttpContext (but I don't know how to get to the latter).

The main problem is that I don't have access to the User. When I use a custom ValidatorAttribute on a property, when the property gets constructed the User (or also HttpContext) is null. Meaning that I can't e.g. give the User to the Validator's constructor. That's why I'm wondering how the Validator could be made to know something about the User (and/or its claims). If I can access any of e.g. HttpContext or User inside the validator, I can get to all the other runtime information as well. Is my question understandable?

Answer 1

ValidationContext has its own GetService method, which is preconfigured to use the ASP.NET Core Dependency Injection container, IServiceProvider, when resolving services. Here's an example:

public class UniqueTitleValidator : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var httpContextAccessor = (IHttpContextAccessor)validationContext.GetService(typeof(IHttpContextAccessor));
        var user = httpContextAccessor.HttpContext.User;

        ...
    }
}

To get to HttpContext, use IHttpContextAccessor, which is resolved here using GetService as described above. You'll need to make sure you've registered this with DI, using e.g. AddHttpContextAccessor.