Question

我正在开发一个Web API，其中GET方法需要返回一个对象，该对象的变量将基于XML文件确定。根据客户端要求，返回的格式必须为XML或JSON。我想将XML文件中的数据以XML格式返回给客户端，并在请求JSON时将其用于JSON。

XML中的节点可能增加或减少，因此 我无法在模型中定义固定的类 。我当前的解决方案是返回一个动态对象，但是下面显示了一个异常。我该怎么做才能避免出现异常情况？

获取Api

[AllowAnonymous]
public class DataController : ApiController
{      
    //GET api/----based on dynamic binding
    public object Get()
    {
        //Read XML
        XDocument xDoc = XDocument.Load(@"D:\data.xml");

        string jsonStr = JsonConvert.SerializeXNode(xDoc);
        dynamic dynamicObject = JsonConvert.DeserializeObject<ExpandoObject>(jsonStr);


        return dynamicObject; //THIS LINE IS THROWING RUNTIME ERROR
    }
}

示例XML文件：

<Data>    
    <Name>abcd</Name>
    <bad>100</bad>
    <status>running</status>    
</Data>

当我尝试访问GET API时，网页上会出现以下错误：

<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 'System.Dynamic.ExpandoObject' with data contract name 'ArrayOfKeyValueOfstringanyType:http://schemas.microsoft.com/2003/10/Serialization/Arrays' is not expected. Consider using a DataContractResolver if you are using DataContractSerializer 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 the serializer.
</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.<WriteBufferedResponseContentAsync>d__1b.MoveNext()
</StackTrace>
</InnerException>
</Error>

Answer 1

出现此错误的原因是，您已经声明了要返回类型object的对象的方法-但实际上已经返回了多态子类型，即ExpandoObject。由于DataContractSerializer（和XmlSerializer）将拒绝序列化意外的多态类型，因此它们会引发您所看到的异常。有关详细信息，请参见

话虽如此，我想提出一种不同的，更简单的方法。首先，定义您的Get()方法以显式返回XElement，如下所示：

public XElement Get()
{
    //Read XML
    XDocument xDoc = XDocument.Load(@"D:\data.xml");
    return xDoc.Root;
}

DataContractSerializer（和XmlSerializer）都可以序列化此类型的对象（因为implements IXmlSerializable），因此您的方法现在将成功返回文件{{1 }}按原样请求XML。

现在，请求JSON时该怎么办？事实证明，Json.NET具有内置的转换器XmlNodeConverter，可以将"D:\data.xml"与JSON进行串行化。 XElement在内部使用它，但是它是公共的，因此可以直接使用。因此，如果您将转换器添加到JsonSerializerSettings.Converters中的转换器的全局Web API列表中，您的方法现在也应该返回适用于JSON的内容。

您未指定所使用的Web API版本。要全局添加转换器，请参见

ASP.NET Web API 2：另请参见 How to set custom JsonSerializerSettings for Json.NET in MVC 4 Web API? 和this answer到 Registering a custom JsonConverter globally in Json.Net 的第二部分。在这种情况下，您的代码将类似于：
```
JsonConvert.SerializeXNode()
```
ASP.NET Core MVC：请参见 JsonSerializerSettings and Asp.Net Core 或 Setting JsonConvert.DefaultSettings asp net core 2.0 not working as expected 。再次在这里，您将访问全局protected void Application_Start() { var config = GlobalConfiguration.Configuration; var settings = config.Formatters.JsonFormatter.SerializerSettings; settings.Converters.Add(new XmlNodeConverter()); }并添加一个JsonSerializerSettings，如上所示。

返回动态对象会引发运行时错误

1 个答案: