将参数传递给返回 Func 的方法

Question

I have the following static class

public static partial class ClassX
    {
        private static readonly Expression<Func<Customer, CustomerDetail>> _exp = DoX();
        private static readonly Func<Customer, CustomerDetail> _comp = DoX().Compile();

        static ClassX() { }

        public static IQueryable<CustomerDetail> ConvertDetail(this IQueryable<Customer> query)
        {
            query.Select(_exp);
        }

        public static CODEDistrictDTO ConvertDetail(this Customer customer)
        {
            return _comp(customer);
        }

        private static Expression<Func<CODEDistrictEntity, CODEDistrictDTO>> DoX()
        {

        }
    }

What I need to do is passing a parameter to "DoX" method through the method "ConvertDetail", so the "DoX" method build the logic base on the received parameter

public static partial class ClassX
    {
        private static readonly Expression<Func<Customer, CustomerDetail>> _exp = DoX(); //problem
        private static readonly Func<Customer, CustomerDetail> _comp = DoX().Compile();  //problem

        static ClassX() { }

        public static IQueryable<CustomerDetail> ConvertDetail(this IQueryable<Customer> query, List<int> mlist)
        {
            query.Select(_exp(mlist));
        }

        public static CODEDistrictDTO ConvertDetail(this Customer customer, List<int> mlist)
        {
            return _comp(customer,mlist);
        }

        private static Expression<Func<CODEDistrictEntity, CODEDistrictDTO>> DoX(List<int> mlist)
        {

        }
    }

The problem in the static readonly fields!!!

Answer 1

Take another look at the syntax for how a Func<> is defined:

https://docs.microsoft.com/en-us/dotnet/api/system.func-2

https://docs.microsoft.com/en-us/dotnet/api/system.func-3

public delegate TResult Func<in T,out TResult>(T arg);
public delegate TResult Func<in T1,in T2,out TResult>(T1 arg1, T2 arg2);

The definition you have applied at the top for Func<Customer, CustomerDetail> then means that you are providing a delegate function that takes in one parameter of type Customer and returns a result of type CustomerDetail . This following block calls the delegate function _comp with the customer value as that parameter.

public static CODEDistrictDTO ConvertDetail(this Customer customer)
{
   return _comp(customer); // <- Calls our delegate Func<Customer,CustomerDetail> with a Customer param, returns a CustomerDetail
}

In your updated code, you suddenly start trying to pass two parameters to that delegate function with _comp(customer,mlist) , but it is only defined as having one. To change the delegate to expect two parameters, with the second being of type List<int> , then you would need to update that to private static readonly Func<Customer, List<int>, CustomerDetail> _comp =...

Keep in mind this is only going to help you if your DoX.Compile() method will actually accept this signature - updating the Func does not mean your provided delegate will suddenly know what to do with the extra parameter if it doesn't already expect it.