Create Custom Validator in Sitecore WFFM

Sample code snippet to create custom validator in Sitecore WFFM.

public class CustomCountryValidator : FormCustomValidator
    {
        protected override bool EvaluateIsValid()
        {
            var form = WebUtil.GetParent<SimpleForm>(this);
            var field = (CustomDropListCountryPicker)WebUtil.FindFirstOrDefault(form, c => c is CustomDropListCountryPicker && (c as CustomDropListCountryPicker).Result.FieldName == "ddlCountry");
           
            Control controlToValidate;
            string value;

            if (!String.IsNullOrEmpty(base.ControlToValidate))
            {
                controlToValidate = this.FindControl(base.ControlToValidate);
                value = HttpContext.Current.Request[controlToValidate.UniqueID];

                if(condition)
                    return true;
            }
            return false;
        }

    }