C#: Generic types that have a constructor?

Question

I have the following C# test code:

  class MyItem
  {
    MyItem( int a ) {}
  }

  class MyContainer< T >
    where T : MyItem, new()
  {
    public void          


        

Answer 1

IMO, the best approach here is an initialize method, i.e.

interface ISomeInterface {
    void Init(int i);
}
class Foo : ISomeInterface {
    void ISomeInterface.Init(int i) { /* ... */ }
}
static class Program {
    static T Create(int i) where T : class, ISomeInterface, new() {
        T t = new T();
        t.Init(i);
        return t;
    }
    static void Main() {
        Foo foo = Create(123);
    }
}

However, you can do what you want with Expression (but without compile-time support):

using System;
using System.Linq.Expressions;
class Foo {
    public Foo(int i) { /* ... */ }
}
static class Program {
    static T Create(int i) {
        return CtorCache.Create(i);
    }
    static class CtorCache {
        static Func ctor;
        public static T Create(int i) {
            if (ctor == null) ctor = CreateCtor();
            return ctor(i);
        }
        static Func CreateCtor() {
            var param = Expression.Parameter(typeof(int), "i");
            var ci = typeof(T).GetConstructor(new[] {typeof(int)});
            if(ci == null) throw new InvalidOperationException("No such ctor");
            var body = Expression.New(ci, param);
            return Expression.Lambda>(body, param).Compile();
        }
    }
    static void Main() {
        Foo foo = Create(123);
    }
}

Note that this caches and reuses the delegate for performance.