MVC 5 - An item with the same key has already been added

Question

I realize there are many questions about this issue. But I have yet been able to resolve this, after spending many hours trying to track this down.

My model looks as follows:

public class Guide
    {
        public int GuideId { get; set; }
        public int GuideTypeId { get; set; }

        [Required(ErrorMessage = "Please enter a business name")]
        [StringLength(80, ErrorMessage="The Business Name cannot exceed 80 characters")]
        public string BusinessName { get; set; }

        public System.DateTime? ActiveStartDate { get; set; }
        public System.DateTime? ActiveEndDate { get; set; }

        [DataType(DataType.Url, ErrorMessage="The website url entered is not a valid url")]
        [StringLength(80, ErrorMessage = "The Website Url cannot exceed 80 characters")]
        public string WebsiteURL { get; set; }

        [Required(ErrorMessage = "Please enter a your primary location")]
        [StringLength(80, ErrorMessage = "The Primary Location cannot exceed 80 characters")]
        public string BaseLocation { get; set; }

        [DataType(DataType.EmailAddress, ErrorMessage = "The email address entered is not a valid email address")]
        [Required(ErrorMessage="Please enter a contact email address")]
        [StringLength(80, ErrorMessage = "The Email Address cannot exceed 80 characters")]
        public string EmailAddress { get; set; }

        [DataType(DataType.PhoneNumber, ErrorMessage = "The phone number entered is not a valid phone number")]
        [StringLength(20, ErrorMessage = "The Phone Number cannot exceed 20 characters")]
        public string PhoneNumber { get; set; }

        [Required(ErrorMessage = "Please enter a description of your fees")]
        public string FeesDescription { get; set; }

        public string VehiclesAvailable { get; set; }
        public string AdditionalInformation { get; set; }
        public string AccountName { get; set; }
        public string Countries { get; set; }
        public string NearestCities { get; set; }
        public string Locations { get; set; }
        public byte[] ProfileImage { get; set; }
        public bool Approved { get; set; }
        public System.DateTimeOffset LastModifiedTime { get; set; }

        // These four properties are here so we can display all tour types, durations, languages and currencies in a checkbox layout on the page. These are not
        // what the guide actually supports. These just represent all possible values.
        public List<TourType>     TourTypeList { get; set; }
        public List<TourDuration> TourDurationList { get; set; }
        public List<TourLanguage> TourLanguageList { get; set; }
        public List<TourCurrency> TourCurrencyList { get; set; }

        // These four properties represent what the guide does actually support
        public virtual List<GuideTourType>     GuideTourType { get; set; }
        public virtual List<GuideTourDuration> GuideTourDuration { get; set; }
        public virtual List<GuideTourLanguage> GuideTourLanguage { get; set; }
        public virtual List<GuideTourCurrency> GuideTourCurrency { get; set; }

        public bool HasImage
        {
            get
            {
                return ProfileImage.Length > 0;
            }
        }

        public virtual GuideType GuideType { get; set; }
    }

I did find someone who attempted to debug this using the following binding code:

public class DebugModelBinder : DefaultModelBinder, IModelBinder
    {
        public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
        {
            Dictionary<string, ModelMetadata> d = new Dictionary<string, ModelMetadata>(StringComparer.OrdinalIgnoreCase);
            foreach (var p in bindingContext.ModelMetadata.Properties)
            {
                var propertyName = p.PropertyName;
                try
                {
                    d.Add(propertyName, null);
                }
                catch (ArgumentException ex)
                {
                    throw new ArgumentException(String.Format("The Item {0} as already been added", propertyName), ex);
                }
            }
            return base.BindModel(controllerContext, bindingContext);
        }
    }

I was able to get this code to work, but the exception is occuring when this line of code is executed:

return base.BindModel(controllerContext, bindingContext);

So it's happening in the base class.

This exception is being generated whenI do a post on a form in an MVC 5 page. My C# controller method is never actually getting called. The error is occurring in the binding, and I have no idea what property is getting added twice. Nor do I know how to find out.

If you need more information, I'll be glad to provide it. I've wasted several hours on this.

Answer 1

In my case, it occurred due duplicate attribute in a entity

public class Employee
{
public int roleID {get; set;}
...
...
public int RoleID {get; set;}
}

when I removed int Role , this error was gone.