How to constrain generic type to must have a construtor that takes certain parameters?

I have a wrapper generic class that intended to be used with a set of types. Those types are generated by a utility and are all derived from a base class ClientBase. While ClientBase has only a default constructor, all generated types have default constructor as well as a constructor takes a string as parameter. In the constructor of the wrapper class, I instantiate an instance of the type with the constructor that takes a string. Here is a sample code:

public class ClientBase
{ }

public class GenericProxy<T>
    where T: ClientBase, new()
{
    T _proxy;

    public GenericProxy(string configName)
    {
        _proxy = new T(configName);    
    }
}

This code does not compile because type T is not guaranteed to have a constructor that takes a string. Is there a way to define a constrain on the generic class to enforce that the type T must have a constructor that take a string? If this is not possible, what are good alternatives to handle this kind of situation?

Answers


It's not possible. I'd like to see "static interfaces" to handle this, but don't expect them any time soon...

Alternatives:

  • Specify a delegate to act as a factory for T
  • Specify another interface to act as a factory for T
  • Specify an interface on T itself for initialization (and add a constraint so that T implements the interface)

The first two are really equivalent. Basically you'd change your proxy class to something like this:

public class GenericProxy<T>
    where T: ClientBase, new()
{
    string _configName;
    T _proxy;
    Func<string, T> _factory;

    public GenericProxy(Func<string, T> factory, string configName)
    {
        _configName = configName;
        _factory = factory;
        RefreshProxy();
    }

    void RefreshProxy() // As an example; suppose we need to do this later too
    {
        _proxy = _factory(_configName);
    }
}

(I assume you're going to want to create more instances later - otherwise you might as well pass an instance of T into the constructor.)


Unfortunately what you're trying to do isn't possible.

MSDN article on Type Constraints


This does not answer your actual question, constraining a method, but for completeness here's how you can do what you're asking at run time, using reflection:

private T  Get<T>(string id)
    {
    var  constructor = typeof(T).GetConstructor(new Type[] { typeof(X), typeof(Y) });
    if (constructor == null)  throw new InvalidOperationException("The type submitted, " + typeof(T).Name + ", does not support the expected constructor (X, Y).");

    var  data = GetData(id);
    return (T)constructor.Invoke(new object[] { data.x, data.y });
    }

As Jon notes, there is no inbuilt support for this - but as an aside you can create a typed delegate to the constructor (faster than reflection) using Expression. The code to do this can be found in MiscUtil (in MiscUtil.Linq.Extensions.TypeExt).


Here is a full working example based on @JonSkeet answer:

using System;
using System.Collections.Generic;

namespace GenericProxy
{
    class Program
    {
        static void Main()
        {
            GenericProxy<ClientBase> proxy = new GenericProxy<ClientBase>(ClientBase.Factory, "cream");

            Console.WriteLine(proxy.Proxy.ConfigName); // test to see it working
        }
    }

    public class ClientBase
    {
        static public ClientBase Factory(string configName)
        {
            return new ClientBase(configName);
        }

        // default constructor as required by new() constraint
        public ClientBase() { }

        // constructor that takes arguments
        public ClientBase(string configName) { _configName = configName; }

        // simple method to demonstrate working example
        public string ConfigName
        {
            get { return "ice " + _configName; }
        }

        private string _configName;
    }

    public class GenericProxy<T>
        where T : ClientBase, new()
    {
        public GenericProxy(Func<string, T> factory, string configName)
        {
            Proxy = factory(configName);
        }

        public T Proxy { get; private set; }
    }
}

Expect to see the following output: ice cream


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