How to constrain generic type to must have a const

问题:

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?

回答1:

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.)

回答2:

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

MSDN article on Type Constraints

回答3:

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 });
    }

回答4:

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).

回答5:

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