c# - ReadAsAsync - 抛出异常类型是接口(interface)或抽象类，无法实例化

Question

我有以下代码，尝试从 Web api 服务获取 Appication 对象。我收到以下异常:

InnerException = {"Could not create an instance of type BusinessEntities.WEB.IApplicationField. Type is an interface or abstract class and cannot be instantated. Path '_applicationFormsList[0]._listApplicationPage[0]._listField[0]._applicationFieldID', line 1, position 194."}.

我不明白为什么将 FieldList 更改为 Interface 会导致反序列化对象时出现问题。非常感谢任何指点。

Task<HttpResponseMessage> task = HttpClientDI.GetAsync(someUri);
HttpResponseMessage response = task.Result;

HttpClientHelper.CheckResponseStatusCode(response);

try
{
    Application application = response.Content.ReadAsAsync<ApplicationPage>().Result;
    return application;
}
catch (Exception ex)
{
    throw new ServiceMustReturnApplicationException(response);
}



[Serializable]
public class ApplicationPage
{
    #region Properties
    public int PageOrder { get; set; }
    public string Title { get; set; }
    public string HtmlPage { get; set; }
    public int FormTypeLookupID { get; set; }

    List<IApplicationField> _listField = new List<IApplicationField>(); 
    public List<IApplicationField> FieldList
    {
        get { return _listField; }
        set { _listField = value; }
    }
}

Answer 1

您需要为要反序列化的类的所有接口(interface)指定具体类，以便在反序列化过程中为这些接口(interface)创建实例。

这样做，可以通过为json.net创建自定义转换器来获取:

public class ApplicationFieldConverter : CustomCreationConverter<IApplicationField>
{
    public override IApplicationField Create(Type objectType)
    {
        return new FakeApplicationField();
    }
}

你的代码应该是:

string jsonContent= response.Content.ReadAsStringAsync().Result;
Application application = JsonConvert.DeserializeObject<Application>(jsonContent,
                                 new ApplicationFieldConverter());

注意:方法Content.ReadAsAsync<...>()在 ASP.NET Web API RC 中找不到，您使用的是 beta 版本吗？