通用静态缓存和协方差

Question

I was trying to build a static generic cache which is going to hold method info of several methods of the form Apply(Event e). A similar code is used in RedirectToWhen, from CQRS Lokad framework.

The problem is that the generic cache doesn't consider derived classes if any. This is a simple test that shows the undesired behavior:

  [TestMethod]
    public void TestGenericsInheritance()
    {
        var sut = new DerivedFromAbstractType();
        Utils.UsefulMethod<DerivedFromAbstractType>(sut);
        Assert.AreEqual(10, sut.Value);
    }

    public abstract class AbstractType
{
    public int Value { get; set; }
}

public class DerivedFromAbstractType : AbstractType
{
    public void AnyOtherMethod()
    {
        Value = 10;
    }
}

public static class Utils
{
    public static void UsefulMethod<T>(T instance)
    {
        MethodInfo info = typeof(T)
            .GetMethods(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance)
            .Where(m => m.Name == "AnyOtherMethod")
            .Where(m => m.GetParameters().Length == 0).FirstOrDefault();
        info.Invoke(instance,null);
     }
}

typeof(T) returns AbstractType, so I can't use it to build a generic static cache. How could I get a generic cache for methods awared of derived types?

Answer 1

use

instance.GetType()

if instance is not null and typeof(T) os instance is null.

Answer 2

I know this is necro-ing a bit, but I had the same problem, with typeof(T) returning the base type. This is useful when wanting to create a BaseAggregate which implements an Emit/Apply pattern. I solved it like this :

 private static class Cache
        {
            private static readonly IDictionary<Type, IDictionary<Type, MethodInfo>> _dict =
                new Dictionary<Type, IDictionary<Type, MethodInfo>>();

            public static IDictionary<Type, MethodInfo> GetDictionaryForType(Type type)
            {
                if (_dict.ContainsKey(type))
                {
                    return _dict[type];
                }
                var dict = type.GetMethods(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance)
                    .Where(m => m.Name == "When")
                    .Where(m => m.GetParameters().Length == 1)
                    .ToDictionary(m => m.GetParameters().First().ParameterType, m => m);
                _dict.Add(type, dict);
                return dict;
            }
        }

This still caches nicely, and object.GetType() returns the correct type to be fed into this method.