c#-具有泛型返回类型的泛型类和方法的委托

将委托与泛型类一起使用时,会遇到问题.类是泛型的,但方法不是.但是,方法返回类型是泛型类型.

public abstract class BaseEntity {
    public DateTime CreateDateTime { get; set; } = DateTime.Now;
    public long CreateUserId { get; set; }
}

public class ClassA : BaseEntity {

}

class Program {
    private delegate object MyDelegate(long id);
    private static MyDelegate _myHandler;

    static void Main(string[] args) {
        var genericType = typeof(TestClass<>).MakeGenericType(typeof(ClassA));
        var createMethod = genericType.GetMethod("CreateEntity");
        _myHandler = (MyDelegate)Delegate.CreateDelegate(typeof(MyDelegate), null, createMethod);

        var result = _myHandler(5);
    }
}

class TestClass<T> where T : BaseEntity, new() {
    public T CreateEntity(long userId) {
        return new T() { CreateUserId = userId };
    }
}

此代码引发异常.

更新1：
修复代码以使其易于理解.

例外：

An unhandled exception of type 'System.NullReferenceException' occurred in Unknown Module.
Object reference not set to an instance of an object. occurred

解决方法:

问题在这里：

_myHandler = (MyDelegate)Delegate.CreateDelegate(typeof(MyDelegate), null, createMethod);

overload of .CreateDelegate you’re using的第二个参数采用该方法所属类型的实例.

您应该使您的方法静态,或创建一个genericType的实例：

var instance = Activator.CreateInstance(genericType);
_myHandler = (MyDelegate)Delegate.CreateDelegate(typeof(MyDelegate), instance, createMethod);

标签：delegates,c
来源： https://codeday.me/bug/20191108/2008534.html