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在使用通用类型和继承时，使用REST API序列化XML时出现错误

c#xmlserialization
4

当我将使用泛型类型的返回对象序列化为XML时，我的REST API出现了错误。但是，当使用JSON时，此错误不会重现。

以下是一个简化的示例，演示了我的问题：

using System;
using System.Net;
using System.Net.Http;
using System.Runtime.Serialization;
using System.Threading.Tasks;
using System.Web.Http;

namespace Areas.API
{
    public abstract class AnimalController<TProperties> : ApiController
        where TProperties : new()
    {
        protected abstract TProperties makeNew( Int32 Id );

        [Route( "{id}" )]
        public AnimalReturn<TProperties> Get( Int32 id )
        {
            TProperties p = makeNew( id );
            return new AnimalReturn<TProperties>()
            {
                Properties = p,
                Request = Request,
                StatusCode = HttpStatusCode.OK
            };
        }
    }


    public class AnimalReturn<TProperties> : AnimalResult
    {
        public TProperties Properties;
    }

    public class AnimalResult : IHttpActionResult
    {
        [IgnoreDataMember]
        public HttpStatusCode StatusCode = HttpStatusCode.OK;

        [IgnoreDataMember]
        public HttpRequestMessage Request = null;

        public Task<HttpResponseMessage> ExecuteAsync( System.Threading.CancellationToken cancellationToken )
        {
            HttpResponseMessage response = null;
            if( null != Request )
            {
                response = Request.CreateResponse( StatusCode, this );
            }
            return Task.FromResult( response );
        } // ExecuteAsync()
    }

    [RoutePrefix( "api/v3/Fish" )]
    public class FishController : AnimalController<FishController.FishProperties>
    {
        public class FishProperties
        {
            public bool IsShark;
        }

        protected override FishProperties makeNew( Int32 Id )
        {
            return new FishProperties()
            {
                IsShark = true
            };
        }
    }

    [RoutePrefix( "api/v3/Dog" )]
    public class DogController : AnimalController<DogController.DogProperties>
    {
        public class DogProperties
        {
            public string Breed;
            public Int32 TagNo;
        }

        protected override DogProperties makeNew( Int32 Id )
        {
            return new DogProperties()
            {
                Breed = "Labrador",
                TagNo = 12345
            };
        }

    }
}

当我运行这个API调用时：
localhost/api/v3/Dog/1

使用标题

Accept: text/xml

我遇到了这个错误：

<Error>
    <Message>An error has occurred.</Message>
    <ExceptionMessage>The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.</ExceptionMessage>
    <ExceptionType>System.InvalidOperationException</ExceptionType>
    <StackTrace />
    <InnerException>
        <Message>An error has occurred.</Message>
        <ExceptionMessage>Type 'Areas.API.AnimalReturn`1[[Areas.API.DogController+DogProperties, NbtWebApp, Version=2012.2.4.1, Culture=neutral, PublicKeyToken=null]]' with data contract name 'AnimalReturnOfDogController.DogPropertiesS2PP9ThI:http://schemas.datacontract.org/2004/07/Areas.API' is not expected. Consider using a DataContractResolver or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.</ExceptionMessage>
        <ExceptionType>System.Runtime.Serialization.SerializationException</ExceptionType>
        <StackTrace>   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiTypeAtTopLevel(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle originalDeclaredTypeHandle, Type graphType)
   at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
   at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph)
   at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)
   at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.WebHost.HttpControllerHandler.&lt;WriteBufferedResponseContentAsync&gt;d__1b.MoveNext()</StackTrace>
    </InnerException>
</Error>

然而，当我移除继承关系（注意AnimalReturn2），问题就消失了：

using System;
using System.Net;
using System.Net.Http;
using System.Runtime.Serialization;
using System.Threading.Tasks;
using System.Web.Http;

namespace Areas.API
{
    public abstract class AnimalController<TProperties> : ApiController
        where TProperties : new()
    {
        protected abstract TProperties makeNew( Int32 Id );

        [Route( "{id}" )]
        public AnimalReturn2<TProperties> Get( Int32 id )
        {
            TProperties p = makeNew( id );
            return new AnimalReturn2<TProperties>()
            {
                Properties = p,
                Request = Request,
                StatusCode = HttpStatusCode.OK
            };
        }
    }

    public class AnimalReturn2<TProperties> : IHttpActionResult
    {
        public TProperties Properties;

        [IgnoreDataMember]
        public HttpStatusCode StatusCode = HttpStatusCode.OK;

        [IgnoreDataMember]
        public HttpRequestMessage Request = null;

        public Task<HttpResponseMessage> ExecuteAsync( System.Threading.CancellationToken cancellationToken )
        {
            HttpResponseMessage response = null;
            if( null != Request )
            {
                response = Request.CreateResponse( StatusCode, this );
            }
            return Task.FromResult( response );
        } // ExecuteAsync()
    }

    [RoutePrefix( "api/v3/Fish" )]
    public class FishController : AnimalController<FishController.FishProperties>
    {
        public class FishProperties
        {
            public bool IsShark;
        }

        protected override FishProperties makeNew( Int32 Id )
        {
            return new FishProperties()
            {
                IsShark = true
            };
        }
    }

    [RoutePrefix( "api/v3/Dog" )]
    public class DogController : AnimalController<DogController.DogProperties>
    {
        public class DogProperties
        {
            public string Breed;
            public Int32 TagNo;
        }

        protected override DogProperties makeNew( Int32 Id )
        {
            return new DogProperties()
            {
                Breed = "Labrador",
                TagNo = 12345
            };
        }

    }
}

现在当我发送同样的请求时，我得到的结果是：
<AnimalReturn2OfDogController.DogPropertiesS2PP9ThI 
    xmlns:i="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns="http://schemas.datacontract.org/2004/07/Areas.API">
    <Properties>
        <Breed>Labrador</Breed>
        <TagNo>12345</TagNo>
    </Properties>
</AnimalReturn2OfDogController.DogPropertiesS2PP9ThI>

问题是：是否有一种方法可以解决XML序列化问题，同时仍然使用继承？
我尝试过在不同的地方和不同的组合中使用KnownTypes（如此处建议：Unexpected Type - Serialization Exception 和这里："Type not expected", using DataContractSerializer - but it's just a simple class, no funny stuff?），虽然我仍然怀疑那里有一个简单的答案。
我也试图禁用代理类，但似乎没有帮助。
谢谢。
我尝试在不同的地方和不同的组合中使用KnownTypes - 你是在哪里/如何尝试使用它们的？ - dbc
我尝试将所有这些添加到AnimalReturn中：[KnownType("AnimalResult")],[KnownType("DogController")], [KnownType("DogController.DogProperties")]。虽然我在摆弄它时肯定尝试了其他东西，但我无法记住所有的组合。 - ssalter
使其成为已知类型（typeof(AnimalResult)）。 - ssalter
3个回答
1
序列化是在基类方法AnimalResult.ExecuteAsync()内进行的，该方法调用 HttpRequestMessageExtensions.CreateResponse<T> Method (HttpRequestMessage, HttpStatusCode, T)。
            response = Request.CreateResponse( StatusCode, this );

由于this被静态类型化为AnimalResult，因此该类型成为传递给扩展方法的泛型参数T。 因此，请尝试将从中派生的类型作为KnownType属性添加到AnimalResult中：

[KnownType(typeof(AnimalReturn<DogController.DogProperties>))]
[KnownType(typeof(AnimalReturn<FishController.FishProperties>))]
public class AnimalResult : IHttpActionResult
{
}

你还可以使用 MakeGenericMethod() 调用 HttpRequestMessageExtensions.CreateResponse<T> 方法，并将 this.GetType() 作为类型参数，以使其正常工作。请参见如何调用静态泛型类方法，如果直到运行时才知道泛型类型参数？。 更新 以下是一个示例，演示如何在运行时调用静态泛型方法，使用返回的 AnimalResult 的实际类型，从而避免需要定义所有可能的派生类作为已知类型：
public class AnimalResult : IHttpActionResult
{
    [IgnoreDataMember]
    public HttpStatusCode StatusCode = HttpStatusCode.OK;

    [IgnoreDataMember]
    public HttpRequestMessage Request = null;

    private static HttpResponseMessage StaticCreateResponse<TAnimalResult>(TAnimalResult animalResult) where TAnimalResult : AnimalResult
    {
        if (animalResult == null)
            throw new ArgumentNullException(); // Should have been checked outside.
        Debug.Assert(typeof(TAnimalResult) == animalResult.GetType());
        return animalResult.Request.CreateResponse(animalResult.StatusCode, animalResult);
    }

    HttpResponseMessage CreateResponse()
    {
        if (Request == null)
            return null;
        else
        {
            var method = typeof(AnimalResult).GetMethod("StaticCreateResponse", BindingFlags.NonPublic | BindingFlags.Static);
            var genericMethod = method.MakeGenericMethod(new[] { GetType() });
            return (HttpResponseMessage)genericMethod.Invoke(null, new object[] { this });
        }
    }

    public Task<HttpResponseMessage> ExecuteAsync( CancellationToken cancellationToken )
    {
        var response = CreateResponse();
        return Task.FromResult( response );
    } // ExecuteAsync()
}
这的确解决了问题。不幸的是，我的实际代码有大约40个派生类型，并且未来可能会更多，这似乎不是一个好的解决方案。我对您将类型传递到CreateResponse中的想法很感兴趣，但我无法让它工作。 - ssalter
@ssalter - 你尝试将类型传递给CreateResponse了吗？你能否更新你的问题，展示一下你所做的和发生了什么？ - dbc
我找不到通过反射获取CreateResponse的方法。我遇到了这个问题：http://stackoverflow.com/questions/28977541/cant-find-getrequestcontext-in-webapi - ssalter
@ssalter - 对于那个问题我也不太清楚发生了什么，所以我更新了我的答案，使得AnimalResult在自身内部动态调用一个私有静态方法。 - dbc
有趣。虽然我认为我发布的内容也可以令人满意地解决问题，但我会将您的标记为解决方案。唯一的缺点是反射的成本。 - ssalter
1
尽管@dbc的答案也解决了问题，但为新类管理KnownType()语句的激增，仅通过运行时错误来发现问题并不适用于我的应用程序。
我找到了另一种方法，既可以为类提供类型，又可以在编译时强制执行它。在我的上面的示例中，它应该是这样的：
    public class AnimalReturn<TProperties> : AnimalResult<AnimalReturn<TProperties>>
    {
        public TProperties Properties;
    }

    public class AnimalResult<T> : IHttpActionResult
        where T : class
    {
        [IgnoreDataMember]
        public HttpStatusCode StatusCode = HttpStatusCode.OK;

        [IgnoreDataMember]
        public HttpRequestMessage Request = null;

        public Task<HttpResponseMessage> ExecuteAsync( System.Threading.CancellationToken cancellationToken )
        {
            HttpResponseMessage response = null;
            if( null != Request )
            {
                response = Request.CreateResponse( StatusCode, this as T );
            }
            return Task.FromResult( response );
        } // ExecuteAsync()
    }

你需要将你尝试返回的类型作为泛型类型传递给AnimalResult，然后在将其作为参数提供给CreateResponse时将其强制转换为该类型。
0

使用XML序列化器，一切都将正常工作。 若要更改序列化器，请在类WebApiConfig的Register方法中使用config.Formatters.XmlFormatter.UseXmlSerializer = true;

我尝试将那行代码添加到WebApiConfig中，并继续重现问题。 - ssalter
我尝试了你在这里发布的相同代码，它可以正常工作。请分享其他可能有助于深入解决问题的细节。 - Kumar Waghmode
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