当我将使用泛型类型的返回对象序列化为 XML 时，我的 REST API 出现错误。使用 JSON 时不会重现此错误。

这是一个演示我的问题的简化示例:

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
            };
        }

    }
}

当我运行此 API 调用时:

localhost/api/v3/Dog/1

带标题

Accept: text/xml

我收到这个错误:

<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>

但是，当我删除继承(注意 AnimalReturn2)时，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，当我运行相同的请求时，我得到:

<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:有没有办法在仍然使用继承的情况下解决 XML 序列化问题？

我已经尝试在不同的地方和不同的组合使用 KnownTypes(如此处建议:Unexpected Type - Serialization Exception 和此处:"Type not expected", using DataContractSerializer - but it's just a simple class, no funny stuff?)，但我仍然怀疑那里有一个简单的答案。

我也试过禁用代理类，但这似乎没有帮助。

谢谢。

最佳答案

序列化是在基类方法内部完成的 AnimalResult.ExecuteAsync() ，正在调用 HttpRequestMessageExtensions.CreateResponse<T> Method (HttpRequestMessage, HttpStatusCode, T) :

            response = Request.CreateResponse( StatusCode, this );

自 this静态类型为 AnimalResult ，此类型成为通用参数 T传递给扩展方法。因此，尝试将派生的类型添加为 KnownType AnimalResult 的属性:

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

您还应该能够通过使用 MakeGenericMethod() 使其正常工作调用HttpRequestMessageExtensions.CreateResponse<T>与 this.GetType()作为类型参数。参见 How to invoke static generic class methods if the generic type parameters are unknown until runtime?

更新

这是一个示例，说明如何使用 AnimalResult 的实际类型在运行时调用静态泛型方法。被返回，从而避免需要定义 AnimalResult 的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()
}

关于c# - 使用泛型类型和继承时在 REST API 上序列化 XML 时出错，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/32484810/