wrote you an example of a call to .NET Core 2 for reference:

using System;
using System.Collections.Generic;

namespace GenericMethodDemo {
    class Program {
        static void Main (string[] args) {
            var trees = new List<Tree> { };

            //GetTreeJsonByList2

            // 1.filternull
            var result1 = GetTreeJsonByList (trees, null, "pn", "In");
            Console.WriteLine ($"result 1:{result1}");

            // 2.filternull
            var result2 = GetTreeJsonByList (trees, x => x.Id > 100, "pn", "In");
            Console.WriteLine ($"result 2:{result2}");

            Console.ReadKey ();
        }

        public static string GetTreeJsonByList<T> (List<T> list, Func<T, bool> filter, string pn, string In) {
            //
            return "output";
        }
    }

    public class Tree {
        public int Id { get; set; }
        public string Name { get; set; }
        public int Sort { get; set; }
    }
}

Note: the generic T in the above call example is: Tree this entity object.

if you have any questions related to .NET development, you can go to a community of programming enthusiasts who focus on .NET development-Tuxing.com to find the answer. In the development of .NET, help you find more beautiful and advanced solutions to difficult problems