无法从基础转换为派生 [英] Unable to cast from base to derived

问题描述

我正在阅读一本关于NHibernate的书，并在单元测试中遇到了一个演员问题。这是问题所在：

I'm going through a book on NHibernate and have run into a casting issue in a unit test. Here's the issue:

        public class Base
        {
        }

        public class Derived : Base
        {
            public string property1 { get; set; }
        }

        public class Derived1 : Base
        {
            public string property1 { get; set; }
            public string property2 { get; set; }
        }

        public class Derived2 : Base
        {
            public string property1 { get; set; }
            public string property2 { get; set; }
            public string property3 { get; set; }
        }

        public void TestDerived( )
        {
            object id = 0;
            using( var transaction = session.BeginTransaction( ) )
            {
                id = session.Save( new OtherClassWithCollectionOfBase
                {
                    Number = "123456789",
                    CollectionOfBase = new HashSet<Base>
                    {
                        new Derived
                        {
                            property1 = "value"
                        },
                        new Derived1
                        {
                            property1 = "value",
                            property2 = "value2"
                        },
                        new Derived2
                        {
                            property1 = "value",
                            property2 = "value2",
                            property3 = "value3",
                        }
                    }
                } );
                transaction.Commit( );
            }

            session.Clear( );

            using( var transaction = session.BeginTransaction( ) )
            {
                var otherclass = session.Get<OtherClassWithCollectionOfBase>( id );
                Assert.That( otherclass.CollectionOfBase.Count, Is.EqualTo( 3 ) );

                var derived = otherclass.CollectionOfBase.OfType<Derived>( ).FirstOrDefault( );
                Assert.That( derived, Is.Not.Null );

                transaction.Commit( );
            }

此断言与  derived == null。我也试过

This asserts with derived == null. I've also tried

var derived = otherclass as Derived;

和

var derived= otherclass.CollectionOfBase.FirstOrDefault(b => b.GetType() == typeof (Derived));

我已经尝试对集合的每个成员进行硬转换，并获得预期的结果NullReferenceException。这可能没有完成，或者我在代码中做错了什么？

I've tried a hard cast on each member of the collection with the expected result NullReferenceException. Can this just not be done or have I done something wrong in the code?

推荐答案

您是否调查了存储对象的详细信息？例如，使用Debugger检查这些变量：

var obj = otherclass.CollectionOfBase.First（）;


var type = obj.GetType（）;

var typeName = type.FullName;

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