Web API错误：'ObjectContent`1'类型无法序列化

Question

我尝试了所有可能的解决方案，但仍无法得到正确的答案。以下是我的代码：

WebApiConfig.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.Http;
using System.Net.Http.Headers;

namespace WebAPI
{
    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services

            // Web API routes
            config.MapHttpAttributeRoutes();
            //test
            var json = config.Formatters.JsonFormatter;
            json.SerializerSettings.PreserveReferencesHandling = Newtonsoft.Json.PreserveReferencesHandling.Objects;
            config.Formatters.Remove(config.Formatters.XmlFormatter);

            config.Routes.MapHttpRoute(
                name: "DefaultApi",
               // routeTemplate: "api/{controller}/{id}",
               routeTemplate: "institute/{controller}/{id}",
               defaults: new { id = RouteParameter.Optional }           
            );

            //change xml to JSON output
            // config.Formatters.JsonFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/html"));

        }
    }
}

StudentController.cs

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using System.Web.Http.Description;
using WebAPI;

namespace WebAPI.Controllers
{
    public class StudentsController : ApiController
    {
        private InstituteEntities db = new InstituteEntities();

        // GET: api/Students
        public IQueryable<Student> GetStudents()
        {
            return db.Students;
        }

        // GET: api/Students/5
        [ResponseType(typeof(Student))]
        public IHttpActionResult GetStudent(int id)
        {
            Student student = db.Students.Find(id);
            if (student == null)
            {
                return NotFound();
            }

            return Ok(student);
        }

        // PUT: api/Students/5
        [ResponseType(typeof(void))]
        public IHttpActionResult PutStudent(int id, Student student)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != student.Id)
            {
                return BadRequest();
            }

            db.Entry(student).State = EntityState.Modified;

            try
            {
                db.SaveChanges();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!StudentExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return StatusCode(HttpStatusCode.NoContent);
        }

        // POST: api/Students
        [ResponseType(typeof(Student))]
        public IHttpActionResult PostStudent(Student student)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            db.Students.Add(student);

            try
            {
                db.SaveChanges();
            }
            catch (DbUpdateException)
            {
                if (StudentExists(student.Id))
                {
                    return Conflict();
                }
                else
                {
                    throw;
                }
            }

            return CreatedAtRoute("DefaultApi", new { id = student.Id }, student);
        }

        // DELETE: api/Students/5
        [ResponseType(typeof(Student))]
        public IHttpActionResult DeleteStudent(int id)
        {
            Student student = db.Students.Find(id);
            if (student == null)
            {
                return NotFound();
            }

            db.Students.Remove(student);
            db.SaveChanges();

            return Ok(student);
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                db.Dispose();
            }
            base.Dispose(disposing);
        }

        private bool StudentExists(int id)
        {
            return db.Students.Count(e => e.Id == id) > 0;
        }
    }
}

的Web.config

<?xml version="1.0" encoding="utf-8"?>
<!-- For more information on how to configure your ASP.NET application, please visit http://go.microsoft.com/fwlink/?LinkId=301879 -->
<configuration>
  <configSections>
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
  </configSections>
  <appSettings></appSettings>
  <system.web>
    <compilation debug="true" targetFramework="4.5.2" />
    <httpRuntime targetFramework="4.5.2" />
  </system.web>
  <system.webServer>
    <modules runAllManagedModulesForAllRequests="true">
      <remove name="WebDAVModule"/>
      <!-- ADD THIS -->
    </modules>
    <handlers>
      <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
      <remove name="OPTIONSVerbHandler" />
      <remove name="TRACEVerbHandler" />
      <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
    </handlers>
  </system.webServer>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Helpers" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-5.2.3.0" newVersion="5.2.3.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
    </compilers>
  </system.codedom>
  <connectionStrings>
    <add name="InstituteEntities" connectionString="metadata=res://*/Institute.csdl|res://*/Institute.ssdl|res://*/Institute.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=BPWSVR01;initial catalog=Institute;persist security info=True;user id=sa;password=sa;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />
  </connectionStrings>
  <entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
      <parameters>
        <parameter value="mssqllocaldb" />
      </parameters>
    </defaultConnectionFactory>
    <providers>
      <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
    </providers>
  </entityFramework>
</configuration>

我的错误如下：

{
    "$id": "1",
    "Message": "An error has occurred.",
    "ExceptionMessage": "The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.",
    "ExceptionType": "System.InvalidOperationException",
    "StackTrace": null,
    "InnerException": {
        "$id": "2",
        "Message": "An error has occurred.",
        "ExceptionMessage": "The ADO.NET provider with invariant name 'System.Data.SqlClient' is either not registered in the machine or application config file, or could not be loaded. See the inner exception for details.",
        "ExceptionType": "System.ArgumentException",
        "StackTrace":"   at System.Data.Entity.Infrastructure.DependencyResolution.DefaultProviderFactoryResolver.<GetService>b__0(ArgumentException e, String n)
            at System.Data.Entity.Infrastructure.DependencyResolution.DefaultProviderFactoryResolver.GetService(Type type, Object key, Func`3 handleFailedLookup)
            at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
            at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
            at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
            at System.Data.Entity.Infrastructure.DependencyResolution.RootDependencyResolver.GetService(Type type, Object key)
            at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
            at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
            at System.Data.Entity.Infrastructure.DependencyResolution.CompositeResolver`2.GetService(Type type, Object key)
            at System.Data.Entity.Infrastructure.DependencyResolution.DbDependencyResolverExtensions.GetService[T](IDbDependencyResolver resolver, Object key)
            at System.Data.Entity.Core.EntityClient.EntityConnection.ChangeConnectionString(String newConnectionString)
            at System.Data.Entity.Internal.LazyInternalConnection.InitializeFromConnectionStringSetting(ConnectionStringSettings appConfigConnection)
            at System.Data.Entity.Internal.LazyInternalConnection.TryInitializeFromAppConfig(String name, AppConfig config)
            at System.Data.Entity.Internal.LazyInternalConnection.Initialize()"
    }
}

Answer 1

查看内部异常：

  

The ADO.NET provider with invariant name 'System.Data.SqlClient' is either not registered in the machine or application config file

很明显，您的问题与实体框架有关：您没有正确地将System.Data.SqlClient程序集部署到您的服务器。

这些问题可以帮助您找到解决此问题的方法：

• Entity Framework ADO.NET Sql.Data.Client provider not found
• ADO.NET provider with invariant name 'System.Data.SqlClient;' cannot be found (Entity Framework MVC)