GetOrCreate.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication2
{
    class SomeStringObject
    {
        public string S { get; private set; }
        public SomeStringObject(string s) { this.S = s; }
    }

    class AnotherStringObject
    {
        public string S { get; private set; }
        public AnotherStringObject(string s) { this.S = s; }
    }

    class AnIntObject
    {
        public int I { get; private set; }
        public AnIntObject(int i) { this.I = i; }
    }

    enum CreatedObjectType { SomeStringObject, AnotherStringObject, AnIntObject };

    class Program
    {
        static SomeStringObject GetOrCreateSomeStringObject(string s, List<SomeStringObject> someStringObjects)
        {
            var existing = someStringObjects.FirstOrDefault(x => x.S == s);
            if (existing != null) return existing;
            var newObject = new SomeStringObject(s);
            someStringObjects.Add(newObject);
            return newObject;
        }

        static AnotherStringObject GetOrCreateAnotherStringObject(string s, List<AnotherStringObject> anotherStringObjects)
        {
            var existing = anotherStringObjects.FirstOrDefault(x => x.S == s);
            if (existing != null) return existing;
            var newObject = new AnotherStringObject(s);
            anotherStringObjects.Add(newObject);
            return newObject;
        }

        static AnIntObject GetOrCreateAnIntObject(int i, List<AnIntObject> anIntObjects)
        {
            var existing = anIntObjects.FirstOrDefault(x => x.I == i);
            if (existing != null) return existing;
            var newObject = new AnIntObject(i);
            anIntObjects.Add(newObject);
            return newObject;
        }

        static object GetOrCreate(CreatedObjectType t, object parameter, List<SomeStringObject> someStringObjects, List<AnotherStringObject> anotherStringObjects, List<AnIntObject> anIntObjects)
        {
            switch (t)
            {
                case CreatedObjectType.SomeStringObject:
                    return GetOrCreateSomeStringObject((string)parameter, someStringObjects);
                case CreatedObjectType.AnotherStringObject:
                    return GetOrCreateAnotherStringObject((string)parameter, anotherStringObjects);
                case CreatedObjectType.AnIntObject:
                    return GetOrCreateAnIntObject((int)parameter, anIntObjects);
                default:
                    throw new Exception();
            }
        }

        static void Main(string[] args)
        {
            List<SomeStringObject> someStringObjects = new List<SomeStringObject>();
            List<AnotherStringObject> anotherStringObjects = new List<AnotherStringObject>();
            List<AnIntObject> anIntObjects = new List<AnIntObject>();

            var paramsList = new List<Tuple<CreatedObjectType, object>>
            {
                Tuple.Create(CreatedObjectType.SomeStringObject, (object)"coucou"),
                Tuple.Create(CreatedObjectType.AnotherStringObject, (object)"lol"),
                Tuple.Create(CreatedObjectType.AnotherStringObject, (object)"lol"),
                Tuple.Create(CreatedObjectType.AnotherStringObject, (object)"lol"),
                Tuple.Create(CreatedObjectType.AnIntObject, (object)3),
                Tuple.Create(CreatedObjectType.AnIntObject, (object)3),
                Tuple.Create(CreatedObjectType.AnIntObject, (object)4)
            };

            var items = paramsList.Select(x => GetOrCreate(x.Item1, x.Item2, someStringObjects, anotherStringObjects, anIntObjects)).Distinct().ToList();
        }
    }
}