IEnumerable的转换 <T> 对于扩展方法问题

Question

I have the following class and extension class (for this example):

public class Person<T>
{
    public T Value { get; set; }
}

public static class PersonExt
{
    public static void Process<TResult>(this Person<IEnumerable<TResult>> p)
    {
        // Do something with .Any().
        Console.WriteLine(p.Value.Any());
    }
}

I was expecting I could write something like the following and it would work, but it doesn't:

var x = new Person<List<String>>();
x.Process();

Since List is lower in the inheritance tree than IEnumerable, shouldn't this be valid? It works if I new up a Person<IEnumerable<String>> of course because that's the direct type.

I'm trying to use an extension method that can be applied to all Person<T> 's as long as T implements IEnumerable<Something> because I need to use the .Any() method.

EDIT: Maybe my understanding of covariance is off? I know IEnumerable<String> should convert to IEnumerable<Object> , but couldn't IList<String> convert to IEnumerable<String> ?

EDIT2: Forgot to mention that I am using .net 4.0.

Answer 1

I know IEnumerable<String> should convert to IEnumerable<Object> , but couldn't IList<String> convert to IEnumerable<String> ?

IList<String> can convert to IEnumerable<String> . The problem is that you're trying to convert Person<List<String>> to Person<IEnumerable<String>> , which is illegal. For example, it's perfectly valid to write:

var x = new Person<IEnumerable<String>>();
x.Value = new string[0];

since Value is of type IEnumerable<String> and a string array is an IEnumerable<String> . However, you cannot write:

var x = new Person<List<String>>();
x.Value = new string[0];

since Value is of type List<String> . Since you can't use a Person<List<String>> in all places where you could use a Person<IEnumerable<String>> , it's not a legal cast.

Note that you can do something similar to what you want if you add a second type parameter to your extension method:

public static void Process<TResult, TList>(this Person<TList> p)
    where TList : IEnumerable<TResult>
{
    Console.WriteLine(p.Value.Any());
}

Unfortunately, the compiler won't be able to infer both type parameters, so you would have to call it like this:

var x = new Person<List<String>>();
x.Process<String, List<String>>();

If you are using C# 4.0 and can use covariance, then you can define a covariant interface for person:

public interface IPerson<out T>
{
    T Value { get; }
}

public class Person<T>
    : IPerson<T>
{
    public T Value { get; set; }
}

And then write your extension method as:

public static void Process<TResult>(this IPerson<IEnumerable<TResult>> p)
{
    // Do something with .Any().
    Console.WriteLine(p.Value.Any());
}

Since IPerson<T>.Value is read-only, a IPerson<List<String>> can be used everywhere that an IPerson<IEnumerable<String>> can be, and the conversion is valid.

Answer 2

I'm not sure you've quite grasped the correct use of generics. In any event ...

The only thing that is incorrect is your declaration of extension method, and the way you are attempting to constrain the extension method.

public static class ThingExtensions
{
    public static void Process<T>(this Thing<T> p)
        where T : IEnumerable<string>
    {
        // Do something with .Any().
        Console.WriteLine(p.Value.Any());
    }
}

All I've really done is rename Person to Thing so that we're not getting hung up on what a Person<List<string>> really is.

public class Thing<T>
{
    public T Value { get; set; }
}

class ListOfString : List<string>
{ }

class Program
{
    static void Main(string[] args)
    {
        var x = new Thing<ListOfString>();
        x.Value = new ListOfString();
        x.Process();

        x.Value.Add("asd");
        x.Process();


        var x2 = new Thing<int>();
        // Error    1   The type 'int' cannot be used as type parameter 'T' 
        // in the generic type or method 
        // 'ThingExtensions.Process<T>(Thing<T>)'. 
        // There is no boxing conversion from 'int' to 
        // 'System.Collections.Generic.IEnumerable<string>'.    
        //x2.Process();

        Console.Read();
    }
}

You could also move the generic constraint to the Thing<T> if that was more applicable.

Answer 3

You mention covariance, but don't actually use it. You have to specify in or out on your generic parameters. Note that co/contravariance doesn't work on class types; they must be applied to interfaces.

So, introducing an interface and making it covariant:

public interface IPerson<out T>
{
  T Value { get; }
}

public class Person<T> : IPerson<T>
{
  public T Value { get; set; }
}

public static class PersonExt
{
  public static void Process<TResult>(this IPerson<IEnumerable<TResult>> p)
  {
    // Do something with .Any(). 
    Console.WriteLine(p.Value.Any());
  }
}

allows this code to compile:

var x = new Person<List<String>>();  
x.Process();