ASP.NET Web Api System.Runtime.Serialization.SerializationEx

Question

我有一个完全工作的MVC4网站，今天我正在尝试添加一个Web API，但没有成功。

<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 'System.Data.Entity.Infrastructure.DbQuery`1[[LeasingWeb.Models.Car, LeasingWeb, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]' with data contract name 'ArrayOfCar:http://schemas.datacontract.org/2004/07/LeasingWeb.Models' 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.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at WriteCarDBToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract ) at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) 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.<>c__DisplayClass7.<WriteToStreamAsync>b__6() at System.Threading.Tasks.TaskHelpers.RunSynchronously(Action action, CancellationToken token)
</StackTrace>
</InnerException>
</Error>

我的对象是这些：

public class Image
{
    public int ID { get; set; }
    [Required(ErrorMessage="Please select a car")]
    [ForeignKey("Car")]
    public int CarID { get; set; }
    [DisplayName("Picture")]
    [Required]
    [FileExtensions(ErrorMessage = "Please specify a valid image file (.jpg, .jpeg, .gif or .png)", Extensions = ("jpg,png,jpeg"))]
    public string Name { get; set; }

    public virtual Car Car { get; set; }
}
public class Car
{
    public int ID { get; set; }
    [Required]
    [DisplayName("Car Model")]
    public string Name { get; set; }
    [Required]
    public string Company { get; set; }
    [Required]
    [DisplayName("Car Type")]
    [ForeignKey("CarType")]
    public int CarTypeID { get; set; }
    [Required]
    [Range(1,5)]
    [DisplayName("Number Of Doors")]
    public float NumDoors { get; set; }
    [Required]
    [Range(0, Int32.MaxValue)]
    public float Acceleration { get; set; }
    public virtual CarType CarType { get; set; }
}
public class CarType
{
    [Key]
    public int ID { get; set; }
    [Required]
    [DataType(DataType.Text)]
    public string Type { get; set; }
}

还有一个包含它们的对象：

public class CarDB
{
    public IQueryable<Car> Cars { get; set; }
    public IEnumerable<Image> Images { get; set; }
}

API 控制器：

public CarDB Get(int ID = -1)
{
    CarDB car = new CarDB();
    if (ID == -1)
    {
        car = new CarDB { Cars = db.Cars.Include(c => c.CarType), Images = db.Images };
    }
    else
    {
        car = new CarDB { Cars = db.Cars.Where(c => c.ID == ID).Include(c => c.CarType), Images = db.Images.Where(c => c.CarID == ID) };
    }
    return car;
}

感谢任何可以提供帮助的人。

Answer 1

这里的问题是 CarDB 对象上的 Cars 成员。DataContractSerializer 特殊情况集合接口，如 IEnumerable 和 IList，但它没有特殊情况派生接口，如 IQueryable。对于这些接口，它们被视为类型是对象，并且必须将任何实现声明为已知类型。

您可以尝试通过将IQueryable<Car>成员更改为 IEnumerable<Car> 来解决此问题。

Answer 2

我在使用 30 个表和大约 40 个延迟加载外键的遗留代码时遇到了这个问题。我发现与其向所有数据库类添加更多注释，不如使用我希望返回的字段创建一个 ViewModel。如果返回的数据很复杂，那么我建议使用自动映射器。