Validation via data annotations does not support dependency injection / resolution

[reddogaw]

In a custom data annotation, the ValidationContext typically supports acting as an IServiceProvider to resolve dependencies required for the validation action.

This support is typically provided by the ValidationContext constructor overload which allows you to pass the IServiceProvider used by the application. However, that's currently not being passed by Cocona, so it's leading to unregistered dependency exceptions when resolving within a custom validation attribute's IsValid method.

Example which is silly but indicative:

    class IsEvenUsingDependencyInjectionAttribute : ValidationAttribute
    {
        protected override ValidationResult? IsValid(object? value, ValidationContext validationContext)
        {
            if (value is not int number)
            {
                return new ValidationResult($"Could not validate value, values's type is {value?.GetType()}");
            }

            var calculator = validationContext.GetRequiredService<Calculator>();
            return calculator.IsEven(number)
                ? ValidationResult.Success
                : new ValidationResult("Value is an uneven number.");
        }
    }

    class Calculator
    {
        public bool IsEven(int number) => number % 2 == 0;
    }