No coercion operator is defined between types 'System.String' and 'System.Nullable`1[System.DateTime]'

Question

I am using AutoMapper for object mapping. Sometime the source value is null so i want to substitute it with "" rather than putting NULL in database. The source object have many nullable data types like Date, Decimal and Int. Currently i am using

public class Invoice 
    {
        public Guid Id { get; set; }
        public string Number { get; set; }        
        public Contact Contact { get; set; }        
        public InvoiceType Type { get; set; }
        public InvoiceStatus Status { get; set; }
        public LineAmountType LineAmountTypes { get; set; }     
        public DateTime? Date { get; set; }        
        public DateTime? DueDate { get; set; }        
        public DateTime? ExpectedPaymentDate { get; set; }        
        public DateTime? PlannedPaymentDate { get; set; }        
        public decimal? SubTotal { get; set; }        
        public decimal? TotalTax { get; set; }        
        public decimal? Total { get; set; }        
        public decimal? TotalDiscount { get; set; 
        public decimal? CurrencyRate { get; set; }        
        public DateTime? FullyPaidOnDate { get; set; }        
        public decimal? AmountDue { get; set; }        
        public decimal? AmountPaid { get; set; }        
        public decimal? AmountCredited { get; set; }

    }

public partial class TempInvoice
    {
        public int RowId { get; set; }
        public System.Guid InvoiceID { get; set; }
        public string InvoiceNumber { get; set; }
        public Nullable<System.Guid> ContactID { get; set; }
        public string Type { get; set; }
        public string Status { get; set; }
        public string LineAmountType { get; set; }
        public Nullable<System.DateTime> InvoiceDate { get; set; }
        public Nullable<System.DateTime> DueDate { get; set; }
        public Nullable<System.DateTime> ExpectedPaymentDate { get; set; }
        public Nullable<System.DateTime> PlannedPaymentDate { get; set; }
        public Nullable<decimal> SubTotal { get; set; }
        public Nullable<decimal> TotalTax { get; set; }
        public Nullable<decimal> Total { get; set; }
        public Nullable<decimal> TotalDiscount { get; set; }
        public Nullable<decimal> CurrencyRate { get; set; }
        public Nullable<System.DateTime> FullyPaidOnDate { get; set; }
        public Nullable<decimal> AmountDue { get; set; }
        public Nullable<decimal> AmountCredited { get; set; }
    }


    public class InvoiceMapper : Profile
        {
            public InvoiceMapper()
            {
                CreateMap<Invoice, TempInvoice>()
                    .ForMember(des => des.PaymentID, map => map.MapFrom(src => src.Id))
                    .ForMember(des => des.InvoiceID, map => map.MapFrom(src => src.Invoice.Id))
                    .ForMember(des => des.PaymentDate, map => map.MapFrom(src => src.Date))
                    .ForAllMembers(src => src.NullSubstitute(""))//this doesn't work
                    ;
            }
        }

But i am getting the above error. Is there a way i can exclude the datatime type and set int or decimal to 0 and string to "" ? Is there a generic method i can use for all datatypes? Thanks

Solution

You could use a resolver function with ResolveUsing, using your own custom mapping function that returns an empty string if the destination is a string, otherwise null.

For example:

    private static object MyMapperResolver(object source, bool returnEmptyString = true) {
        return source ?? (returnEmptyString ? "" : source);
    }

    public static IMapper InitMappings() {
        Mapper.Initialize(cfg => {
            cfg.CreateMap<Invoice, TempInvoice>()
                .ForMember(des => des.PaymentID, map => map.ResolveUsing(src => MyMapperResolver(src.Id)))
                .ForMember(des => des.InvoiceID, map => map.ResolveUsing(src => MyMapperResolver(src.Invoice.Id)))
                .ForMember(des => des.PaymentDate, map => map.ResolveUsing(src => MyMapperResolver(src.Date, false)))
                ;
        });
        return Mapper.Instance;
    }