Using IsAssignableFrom with 'open' generic

问题:

Using reflection, I'm attempting to find the set of types which inherit from a given base class. It didn't take long to figure out for simple types, but I'm stumped when it comes to generics.

For this piece of code, the first IsAssignableFrom returns true, but the second returns false. And yet, the final assignment compiles just fine.

class class1 { }
class class2 : class1 { }
class generic1<T> { }
class generic2<T> : generic1<T> { }

class Program
{
    static void Main(string[] args)
    {
        Type c1 = typeof(class1);
        Type c2 = typeof(class2);
        Console.WriteLine("c1.IsAssignableFrom(c2): {0}", c1.IsAssignableFrom(c2));

        Type g1 = typeof(generic1<>);
        Type g2 = typeof(generic2<>);
        Console.WriteLine("g1.IsAssignableFrom(g2): {0}", g1.IsAssignableFrom(g2));

        generic1<class1> cc = new generic2<class1>();
    }
}

So how do I determine at run time whether one generic type definition is derived from another?

回答1:

From the answer to another question:

public static bool IsAssignableToGenericType(Type givenType, Type genericType)
{
    var interfaceTypes = givenType.GetInterfaces();

    foreach (var it in interfaceTypes)
    {
        if (it.IsGenericType && it.GetGenericTypeDefinition() == genericType)
            return true;
    }

    if (givenType.IsGenericType && givenType.GetGenericTypeDefinition() == genericType)
        return true;

    Type baseType = givenType.BaseType;
    if (baseType == null) return false;

    return IsAssignableToGenericType(baseType, genericType);
}

(If you like the answer please upvote the linked answer since the code isn’t mine.)

回答2:

The exact code you posted does not return surprising results.

This says "false":

Type g1 = typeof(generic1<>);
Type g2 = typeof(generic2<>);
Console.WriteLine("g1.IsAssignableFrom(g2): {0}", g1.IsAssignableFrom(g2));

This says "true":

Type g1 = typeof(generic1<class1>);
Type g2 = typeof(generic2<class1>);
Console.WriteLine("g1.IsAssignableFrom(g2): {0}", g1.IsAssignableFrom(g2));

The difference is that open generic types cannot have instances, so one is not "assignable" to the other.

From the docs:

Returns true if c and the current Type represent the same type, or if the current Type is in the inheritance hierarchy of c, or if the current Type is an interface that c implements, or if c is a generic type parameter and the current Type represents one of the constraints of c. false if none of these conditions are true, or if c is null.

In this case, clearly none of these conditions are true. And there's an extra note:

A generic type definition is not assignable from a closed constructed type. That is, you cannot assign the closed constructed type MyGenericList<int> (MyGenericList(Of Integer) in Visual Basic) to a variable of type MyGenericList<T>.

回答3:

In the following case use the method Konrad Rudolph provided could be wrong, like: IsAssignableToGenericType(typeof(A), typeof(A<>));// return false

I think here's a better answer

public static bool IsAssignableFrom(Type extendType, Type baseType)
{
    while (!baseType.IsAssignableFrom(extendType))
    {
        if (extendType.Equals(typeof(object)))
        {
            return false;
        }
        if (extendType.IsGenericType && !extendType.IsGenericTypeDefinition)
        {
            extendType = extendType.GetGenericTypeDefinition();
        }
        else
        {
            extendType = extendType.BaseType;
        }
    }
    return true;
}

the test case, see Using IsAssignableFrom with C# generics for detail

using System;

/**
 * Sam Sha - yCoder.com
 *
 * */
namespace Test2
{
    class MainClass
    {
        public static void Main (string[] args)
        {
            string a = "ycoder";
            Console.WriteLine(a is object);
            A aa = new A();
            //Console.WriteLine(aa is A<>);//con't write code like this
            typeof(A<>).IsAssignableFrom(aa.GetType());//return false

            Trace(typeof(object).IsAssignableFrom(typeof(string)));//true
            Trace(typeof(A<>).IsAssignableFrom(typeof(A)));//false

            AAA aaa = new AAA();
            Trace("Use IsTypeOf:");
            Trace(IsTypeOf(aaa, typeof(A<>)));
            Trace(IsTypeOf(aaa, typeof(AA)));
            Trace(IsTypeOf(aaa, typeof(AAA<>)));

            Trace("Use IsAssignableFrom from stackoverflow - not right:");
            Trace(IsAssignableFrom(typeof(A), typeof(A<>))); // error
            Trace(IsAssignableFrom(typeof(AA), typeof(A<>)));
            Trace(IsAssignableFrom(typeof(AAA), typeof(A<>)));

            Trace("Use IsAssignableToGenericType:");
            Trace(IsAssignableToGenericType(typeof(A), typeof(A<>)));
            Trace(IsAssignableToGenericType(typeof(AA), typeof(A<>)));
            Trace(IsAssignableToGenericType(typeof(AAA), typeof(A<>)));
        }

        static void Trace(object log){
                Console.WriteLine(log);
        }

        public static bool IsTypeOf(Object o, Type baseType)
        {
            if (o == null || baseType == null)
            {
                return false;
            }
            bool result = baseType.IsInstanceOfType(o);
            if (result)
            {
                return result;
            }
            return IsAssignableFrom(o.GetType(), baseType);
        }

        public static bool IsAssignableFrom(Type extendType, Type baseType)
        {
            while (!baseType.IsAssignableFrom(extendType))
            {
                if (extendType.Equals(typeof(object)))
                {
                    return false;
                }
                if (extendType.IsGenericType && !extendType.IsGenericTypeDefinition)
                {
                    extendType = extendType.GetGenericTypeDefinition();
                }
                else
                {
                    extendType = extendType.BaseType;
                }
            }
            return true;
        }

        //from stackoverflow - not good enough
        public static bool IsAssignableToGenericType(Type givenType, Type genericType) {
            var interfaceTypes = givenType.GetInterfaces();

            foreach (var it in interfaceTypes)
                if (it.IsGenericType)
                    if (it.GetGenericTypeDefinition() == genericType) return true;

            Type baseType = givenType.BaseType;
            if (baseType == null) return false;

            return baseType.IsGenericType &&
                baseType.GetGenericTypeDefinition() == genericType ||
                IsAssignableToGenericType(baseType, genericType);
        }
    }

    class A{}
    class AA : A{}
    class AAA : AA{}
}

回答4:

You need to compare the contained type. See: How to get the type of T from a member of a generic class or method?

In other words, I think you need to check whether the type being contained by the generic class is assignable rather than the generic class itself.