C# 用于筛选响应的RESTURI参数

我试图根据从RESTURI检索的参数筛选从数据库检索的对象列表。示例URI是

http://127.0.0.1:8080/api/employee?FirstName=Abigail&LastName=Ybarra

它将为我检索具有指定名字和姓氏的对象列表

我在命中此类URI时遇到的错误如下：

<ExceptionMessage>
Object reference not set to an instance of an object
</ExceptionMessage>
<ExceptionType>System.NullReferenceException</ExceptionType>

MyWebService.Controllers.EmployeeApiController+<>c__DisplayClass4_0.<ListEmployees>b__0 (MyWebService.Models.Employee x) [0x00000] in <8bf6371a770245f989f67352a05d8bb6>:0 at System.Linq.Enumerable+WhereListIterator`1[TSource].MoveNext () [0x00037] in /private/tmp/source-mono-2017-02/bockbuild-2017-02/profiles/mono-mac-xamarin/build-root/mono-x86/external/corefx/src/System.Linq/src/System/Linq/Where.cs:369 at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList

EmployeeParametes.cs

    [RoutePrefix("api/employee")]
    public class EmployeeApiController : ApiController
    {

        readonly EmployeePersistence persistence;


        public EmployeeApiController()
        {
            persistence = new EmployeePersistence();
        }

        [HttpGet]
        [Route("{id:long}")]
        public IHttpActionResult GetEmployee(long id) 
        {
            return Json(persistence.GetEmployee(id));
        }

        [HttpGet]
        [Route("")]
        public IHttpActionResult ListEmployees([FromUri] EmployeeParameters parameters)
        {
            return Json(persistence.GetEmployeeList().Where(x => x.FirstName.Equals(parameters.FirstName)));
        }
    }
}

public class EmployeeParameters
{
    /**
     * First name of the employee.
     */
    public string FirstName { get; set; }

    /*
     * Last name of the employee.
     */
    public string LastName { get; set; }

    /**
     * Place of birth of the employee.
     */
    public string BirthPlace { get; set; }

    /**
     * Gender of the employee.
     */
    public int Gender { get; set; }

    /**
     * OIB of the employee.
     */
    public string OIB { get; set; }


    /**
     *  Current place of the employee.
     */
    public string CurrentPlace { get; set; }


    /**
     *  Department code of the employee.
     */
    public string Department { get; set; }

    public EmployeeParameters()
    {
    }
}

    public List<Employee> GetEmployeeList()
        {
            string sqlString = "SELECT * FROM Employee";

            MySqlCommand cmd = new MySqlCommand(sqlString, conn);

            List<Employee> employees = new List<Employee>();

            using (MySqlDataReader reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    employees.Add(ReadFromDatabase(reader));
                }
            }

            return employees;
        }

Employee ReadFromDatabase(MySqlDataReader dataReader)
        {
            if (!dataReader.Read())
            {
                return null;
            }

            string firstName = null;
            string lastName = null;
            string birthPlace = null;
            string currentPlace = null;
            int gender = -1;
            string departmentCode = null;
            string OIB = null;

            try
            {
                firstName = dataReader.GetString(1);
                lastName = dataReader.GetString(2);
                birthPlace = dataReader.GetString(3);
                currentPlace = dataReader.GetString(4);
                gender = dataReader.GetInt32(5);
                departmentCode = dataReader.GetString(6);
                OIB = dataReader.GetString(7);
            }
            catch (SqlNullValueException)
            {
                // log the error
            }

            return new Employee(
                firstName != null ? firstName : "N/A",
                lastName != null ? lastName : "N/A",
                birthPlace != null ? birthPlace : "N/A",
                currentPlace != null ? currentPlace : "N/A",
                gender != -1 ? (gender == 1 ? EmployeeGender.M : EmployeeGender.F) : 
                    EmployeeGender.UNDEFINED,
                departmentCode != null ?
                    DepartmentCodeExtensions.GetEnumValue(DepartmentCode.NONE, departmentCode) :
                    DepartmentCode.NONE,
                OIB != null ? OIB : "N/A"
            );
        }

Persistence.cs

    [RoutePrefix("api/employee")]
    public class EmployeeApiController : ApiController
    {

        readonly EmployeePersistence persistence;


        public EmployeeApiController()
        {
            persistence = new EmployeePersistence();
        }

        [HttpGet]
        [Route("{id:long}")]
        public IHttpActionResult GetEmployee(long id) 
        {
            return Json(persistence.GetEmployee(id));
        }

        [HttpGet]
        [Route("")]
        public IHttpActionResult ListEmployees([FromUri] EmployeeParameters parameters)
        {
            return Json(persistence.GetEmployeeList().Where(x => x.FirstName.Equals(parameters.FirstName)));
        }
    }
}

public class EmployeeParameters
{
    /**
     * First name of the employee.
     */
    public string FirstName { get; set; }

    /*
     * Last name of the employee.
     */
    public string LastName { get; set; }

    /**
     * Place of birth of the employee.
     */
    public string BirthPlace { get; set; }

    /**
     * Gender of the employee.
     */
    public int Gender { get; set; }

    /**
     * OIB of the employee.
     */
    public string OIB { get; set; }


    /**
     *  Current place of the employee.
     */
    public string CurrentPlace { get; set; }


    /**
     *  Department code of the employee.
     */
    public string Department { get; set; }

    public EmployeeParameters()
    {
    }
}

    public List<Employee> GetEmployeeList()
        {
            string sqlString = "SELECT * FROM Employee";

            MySqlCommand cmd = new MySqlCommand(sqlString, conn);

            List<Employee> employees = new List<Employee>();

            using (MySqlDataReader reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    employees.Add(ReadFromDatabase(reader));
                }
            }

            return employees;
        }

Employee ReadFromDatabase(MySqlDataReader dataReader)
        {
            if (!dataReader.Read())
            {
                return null;
            }

            string firstName = null;
            string lastName = null;
            string birthPlace = null;
            string currentPlace = null;
            int gender = -1;
            string departmentCode = null;
            string OIB = null;

            try
            {
                firstName = dataReader.GetString(1);
                lastName = dataReader.GetString(2);
                birthPlace = dataReader.GetString(3);
                currentPlace = dataReader.GetString(4);
                gender = dataReader.GetInt32(5);
                departmentCode = dataReader.GetString(6);
                OIB = dataReader.GetString(7);
            }
            catch (SqlNullValueException)
            {
                // log the error
            }

            return new Employee(
                firstName != null ? firstName : "N/A",
                lastName != null ? lastName : "N/A",
                birthPlace != null ? birthPlace : "N/A",
                currentPlace != null ? currentPlace : "N/A",
                gender != -1 ? (gender == 1 ? EmployeeGender.M : EmployeeGender.F) : 
                    EmployeeGender.UNDEFINED,
                departmentCode != null ?
                    DepartmentCodeExtensions.GetEnumValue(DepartmentCode.NONE, departmentCode) :
                    DepartmentCode.NONE,
                OIB != null ? OIB : "N/A"
            );
        }

公共列表GetEmployeeList（） { string sqlString=“选择*来自员工”； MySqlCommand cmd=新的MySqlCommand（sqlString，conn）； 列出员工=新列表（）； 使用（MySqlDataReader=cmd.ExecuteReader（）） { while（reader.Read（）） { 添加（ReadFromDatabase（reader））； } } 返回员工； } 员工ReadFromDatabase（MySqlDataReader） { 如果（！dataReader.Read（）） { 返回null； } 字符串firstName=null； 字符串lastName=null； 字符串出生地=null； 字符串currentPlace=null； int性别=-1； 字符串departmentCode=null； 字符串OIB=null； 尝试 { firstName=dataReader.GetString（1）； lastName=dataReader.GetString（2）； 出生地=dataReader.GetString（3）； currentPlace=dataReader.GetString（4）； 性别=dataReader.GetInt32（5）； departmentCode=dataReader.GetString（6）； OIB=dataReader.GetString（7）； } 捕获（SqlNullValueException） { //记录错误 } 返回新员工( firstName！=null？firstName:“不适用”， lastName！=null？lastName:“不适用”， 出生地！=空？出生地：“不适用”， currentPlace！=null？currentPlace:“不适用”， 性别！=-1？（性别==1？EmployeeGender.M:EmployeeGender.F）： EmployeeGender.UNDEFINED， 部门代码！=空？ DepartmentCodeExtensions.GetEnumValue（DepartmentCode.NONE，DepartmentCode）： 部门代码。无， OIB！=空？OIB:“不适用” ); }

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我猜问题出在路线上，但不确定到底是什么或如何解决。非常感谢您的帮助。

我认为出现异常是因为在

while

-循环和方法

ReadFromDatabase

中调用

.Read（）

。您应该从后者删除

.Read（）

调用

正如您现在的代码一样，在从读卡器提取数据之前，将在读卡器中执行两次调用

.Read（）

。

---

另一个问题是，如果读者无法阅读更多内容，您将

null

放入员工列表中，从而保证在尝试使用此元素的

FirstName

属性时出现

NullPointerException

。

请提供异常的完整堆栈跟踪，与三个.cs文件的完整内容一起-否则，很难判断错误在哪里。@J.N.已按要求更新。您是否已运行调试器并验证是Parameters对象为null，而不是Persistence类返回的

Employees

集合？