Manually invoking ModelState validation

ValidateModel and TryValidateModel

You can use ValidateModel or TryValidateModel in controller scope.

When a model is being validated, all validators for all properties are run if at least one form input is bound to a model property. The ValidateModel is like the method TryValidateModel except that the TryValidateModel method does not throw an InvalidOperationException exception if the model validation fails.

ValidateModel - throws exception if model is not valid.

TryValidateModel - returns bool value indicating if model is valid.

class ValueController : Controller
{
    public IActionResult Post(MyModel model)
    {
        if (!TryValidateModel(model))
        {
            // Do something
        }

        return Ok();
    }
}

Validate Models one-by-one

If you validate a list of models one by one, you would want to reset ModelState for each iteration by calling ModelState.Clear().

Link to the documentation

            //
            var context = new ValidationContext(model);

            //If you want to remove some items before validating
            //if (context.Items != null && context.Items.Any())
            //{
            //    context.Items.Remove(context.Items.Where(x => x.Key.ToString() == "Longitude").FirstOrDefault());
            //    context.Items.Remove(context.Items.Where(x => x.Key.ToString() == "Latitude").FirstOrDefault());
            //}

            List<ValidationResult> validationResults = new List<ValidationResult>();
            bool isValid = Validator.TryValidateObject(model, context, validationResults, true);
            if (!isValid)
            {
                //List of errors 
                //validationResults.Select(r => r.ErrorMessage)
                //return or do something
            }

I found this to work and do precisely as expected.. showing the ValidationSummary for a freshly retrieved object on a GET action method... prior to any POST

Me.TryValidateModel(MyCompany.OrderModel)

You can call the ValidateModel method within a Controller action (documentation here).