读取异步 - 错误： "Type is an interface or abstract class and cannot be instantiated."

Question

我有下面的代码，它从web api服务中获取一个对象。

在以下代码行

response.Content.ReadAsAsync<CMLandingPage>().Result;

我得到以下异常：

InnerException = {"Could not create an instance of type MLSReports.Models.IMetaData. Type is an interface or abstract class and cannot be instantiated. Path 'BaBrInfo.Series[0].name', line 1, position 262."}

我们非常感谢任何指点。

 CMLandingPage lpInfo = new CMLandingPage();
            try
            {
                using (HttpClient client = new HttpClient())
                {
                    // Add an Accept header for JSON format
                    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
                    response = client.PostAsJsonAsync(LPChartinfoapiURL, criteria.Mlsnums).Result;
                }
                // Throw exception if not a success code.
                response.EnsureSuccessStatusCode();
                // Parse the response body.
                lpInfo = response.Content.ReadAsAsync<CMLandingPage>().Result;
                }

CMLanding页面：

namespace MLSReports.Models
{
    public class CMLandingPage
    {
        public CMLandingPage()  {  }
        public CMColumn BaBrInfo { get; set; }
     }
  public class CMColumnItem<T> : IMetaData
    {
        #region Constructors and Methods
        public CMColumnItem()   {   }
        #endregion
        #region Properties and Fields
        public string name { get; set; }
        public List<T> data { get; set; }
        public string color { get; set; }
        #endregion
    }
    public class CMColumn
    {
        #region Constructor and Method
        public CMColumn()
        {
           Series = new List<IMetaData>();
        }
        #endregion
        #region Properties and Fields
        public string ChartType { get; set; }
        public string ChartTitle { get; set; }
        public List<IMetaData> Series { get; set; }
        #endregion    
    }
}

Answer 1

您的CmColumn类具有以下属性：

    public List<IMetaData> Series { get; set; }

WebApi控制器显然试图根据您发送的参数值来构造对象。当它获得名称为"BaBrInfo.Series[0].name"的值时，它知道应该创建一个新的IMetaData对象，以便设置其name并将其添加到Series属性中，但IMetaData只是一个接口：它不知道要构造什么类型的对象。

尝试将IMetaData更改为实现该接口的某种具体类型。

Answer 2

您需要让序列化程序将类型包含在json负载中，让反序列化程序使用包含的类型：

//Serialization
var config = new HttpConfiguration();
config.Formatters.JsonFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.Auto;
return Request.CreateResponse(HttpStatusCode.OK, dto, config);
//Deserialization
var formatter = new JsonMediaTypeFormatter
{
    SerializerSettings = { TypeNameHandling = TypeNameHandling.Auto }
};
response.Content.ReadAsAsync<CMLandingPage>(new [] { formatter }).Result;

HTTPCONTENT 中使用NewtonJSON.NET的多态串行化