Asp.net core ASP.Net核心-设置和获取会话变量

我有一个使用ASP.NET内核、Angular 5和ADO.NET的应用程序

在我决定更改代码以使用从appssettings.json文件获取的数据库连接字符串设置会话变量之前，它工作得很好

我将此作为参考：

但是，当我尝试设置会话变量时，我在SetSessionVariable（）方法中得到一个空对象引用

错误是：

启动应用程序时出错。 NullReferenceException:对象引用未设置为 对象Startup.cs中的Angular5NetcoreAdo.Startup.SetSessionVariable（）， 第82行

我的代码是：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.SpaServices.AngularCli;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Angular5NetcoreAdo.Models;

// Added these.
using Microsoft.AspNetCore.Http;
using System;

namespace Angular5NetcoreAdo
{
public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // Added this.
    public HttpContext HttpContext { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

        // In production, the Angular files will be served from this directory.
        services.AddSpaStaticFiles(configuration =>
        {
            configuration.RootPath = "ClientApp/dist";
        });

        services.AddScoped<EmployeeDataAccessLayer>();

        // Added this.
        services.AddSession(options =>
        {
            options.Cookie.Name = ".ConnectionString";
        });

        // Added this.
        SetSessionVariable();
    }

    // Added this.
    public void SetSessionVariable()
    {
         HttpContext.Session.SetString("ConnectionString", Convert.ToString(Configuration.GetConnectionString("DBAngular5NetcoreAdoDatabase")));
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/Error");
        }

        app.UseStaticFiles();
        app.UseSpaStaticFiles();

        // Added this.
        app.UseSession();

        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller}/{action=Index}/{id?}");
        });

        app.UseSpa(spa =>
        {
            spa.Options.SourcePath = "ClientApp";

            if (env.IsDevelopment())
            {
                spa.UseAngularCliServer(npmScript: "start");
            }
        });
    }
}

------------------现在添加为引用此类的新错误

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using Microsoft.Extensions.Configuration;
using Microsoft.AspNetCore.Http;

namespace Angular5NetcoreAdo.Models
{
    public class EmployeeDataAccessLayer
    {
       public HttpContext HttpContext { get; }
       public string connectionString;

       public EmployeeDataAccessLayer(HttpContext httpContext)
       {
           // Set the property.
           HttpContext = httpContext;

           // Get the connection string session variable.
           connectionString = HttpContext.Session.GetString("ConnectionString");
       }

       public IEnumerable<Employee> GetAllEmployees()
       {
          try
           {
               List<Employee> lstemployee = new List<Employee>();

               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("SelectEmployees", con);
                   cmd.CommandType = CommandType.StoredProcedure;

                   con.Open();

                   SqlDataReader rdr = cmd.ExecuteReader();

                   while (rdr.Read())
                   {
                       Employee employee = new Employee();

                       employee.ID = Convert.ToInt32(rdr["EmployeeID"]);
                       employee.Name = rdr["Name"].ToString();
                       employee.Gender = rdr["Gender"].ToString();
                       employee.Department = rdr["Department"].ToString();
                       employee.City = rdr["City"].ToString();

                       lstemployee.Add(employee);
                   }

                   rdr.Close();
                   con.Close();
               }

               return lstemployee;
           }
           catch
           {
               throw;
           }
       }

       public Employee GetEmployeeData(int id)
       {
           try
           {
               Employee employee = new Employee();

               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("SelectEmployeeById", con);
                   cmd.CommandType = CommandType.StoredProcedure; 
                   cmd.Parameters.AddWithValue("@EmpId", id);

                   con.Open();

                   SqlDataReader rdr = cmd.ExecuteReader();

                   while (rdr.Read())
                   {
                       employee.ID = Convert.ToInt32(rdr["EmployeeID"]);
                       employee.Name = rdr["Name"].ToString();
                       employee.Gender = rdr["Gender"].ToString();
                       employee.Department = rdr["Department"].ToString();
                       employee.City = rdr["City"].ToString();
                   }

                   rdr.Close();
                   con.Close();
               }

               return employee;
           }
           catch
           {
               throw;
           }
       }

       public int AddEmployee(Employee employee)
       {
           try
           {
               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("InsertEmployee", con);
                   cmd.CommandType = CommandType.StoredProcedure;

                   cmd.Parameters.AddWithValue("@Name", employee.Name);
                   cmd.Parameters.AddWithValue("@City", employee.City);
                   cmd.Parameters.AddWithValue("@Department", employee.Department);
                   cmd.Parameters.AddWithValue("@Gender", employee.Gender);

                   con.Open();

                   cmd.ExecuteNonQuery();

                   con.Close();
               }

               return 1;
           }
           catch
           {
               throw;
           }
       }

       public int UpdateEmployee(Employee employee)
       {
           try
           {
               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("UpdateEmployee", con);
                   cmd.CommandType = CommandType.StoredProcedure;

                   cmd.Parameters.AddWithValue("@EmpId", employee.ID);
                   cmd.Parameters.AddWithValue("@Name", employee.Name);
                   cmd.Parameters.AddWithValue("@City", employee.City);
                   cmd.Parameters.AddWithValue("@Department", employee.Department);
                   cmd.Parameters.AddWithValue("@Gender", employee.Gender);

                   con.Open();

                   cmd.ExecuteNonQuery();

                    con.Close();
               }

               return 1;
           }
           catch
           {
               throw;
           }
       }

       public int DeleteEmployee(int id)
       {
           try
           {
            using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("DeleteEmployee", con);
                   cmd.CommandType = CommandType.StoredProcedure;
                   cmd.Parameters.AddWithValue("@EmpId", id);

                   con.Open();

                   cmd.ExecuteNonQuery();

                    con.Close();
              }

               return 1;
          }
        catch
           {
               throw;
           }
       }
   }
}

使用系统；
使用System.Collections.Generic；
使用系统数据；
使用System.Data.SqlClient；
使用Microsoft.Extensions.Configuration；
使用Microsoft.AspNetCore.Http；
命名空间Angular5NetcoreAdo.Models
{
公共类EmployeeDataAccessLayer
{
公共HttpContext HttpContext{get；}
公共字符串连接字符串；
公共EmployeeDataAccessLayer（HttpContext HttpContext）
{
//设置属性。
HttpContext=HttpContext；
//获取连接字符串会话变量。
connectionString=HttpContext.Session.GetString（“connectionString”）；
}
公共IEnumerable GetAllEmployees（）
{
尝试
{
List lstemployee=新列表（）；
使用（SqlConnection con=newsqlconnection（connectionString））
{
SqlCommand cmd=newsqlcommand（“SelectEmployees”，con）；
cmd.CommandType=CommandType.storedProcess；
con.Open（）；
SqlDataReader rdr=cmd.ExecuteReader（）；
while（rdr.Read（））
{
员工=新员工（）；
employee.ID=Convert.ToInt32（rdr[“EmployeeID]”）；
employee.Name=rdr[“Name”].ToString（）；
employee.Gender=rdr[“Gender”].ToString（）；
employee.Department=rdr[“Department”].ToString（）；
employee.City=rdr[“City”].ToString（）；
lstmemployee.Add（employee）；
}
rdr.Close（）；
con.Close（）；
}
返回员工；
}
抓住
{
投掷；
}
}
公共雇员GetEmployeeData（内部id）
{
尝试
{
员工=新员工（）；
使用（SqlConnection con=newsqlconnection（connectionString））
{
SqlCommand cmd=newsqlcommand（“SelectEmployeeById”，con）；
cmd.CommandType=CommandType.storedProcess；
cmd.Parameters.AddWithValue（“@EmpId”，id）；
con.Open（）；
SqlDataReader rdr=cmd.ExecuteReader（）；
while（rdr.Read（））
{
employee.ID=Convert.ToInt32（rdr[“EmployeeID]”）；
employee.Name=rdr[“Name”].ToString（）；
employee.Gender=rdr[“Gender”].ToString（）；
employee.Department=rdr[“Department”].ToString（）；
employee.City=rdr[“City”].ToString（）；
}
rdr.Close（）；
con.Close（）；
}
返回员工；
}
抓住
{
投掷；
}
}
公共整数加法雇员（雇员）
{
尝试
{
使用（SqlConnection con=newsqlconnection（connectionString））
{
SqlCommand cmd=newsqlcommand（“InsertEmployee”，con）；
cmd.CommandType=CommandType.storedProcess；
cmd.Parameters.AddWithValue（“@Name”，employee.Name）；
cmd.Parameters.AddWithValue（“@City”，employee.City）；
cmd.Parameters.AddWithValue（“@Department”，employee.Department）；
cmd.Parameters.AddWithValue（“@Gender”，employee.Gender）；
con.Open（）；
cmd.ExecuteNonQuery（）；
con.Close（）；
}
返回1；
}
抓住
{
投掷；
}
}
public int UpdateEmployee（员工）
{
尝试
{
使用（SqlConnection con=newsqlconnection（connectionString））
{
SqlCommand cmd=newsqlcommand（“UpdateEmployee”，con）；
cmd.CommandType=CommandType.storedProcess；
cmd.Parameters.AddWithValue（“@EmpId”，employee.ID）；
cmd.Parameters.AddWithValue（“@Name”，employee.Name）；
cmd.Parameters.AddWithValue（“@City”，employee.City）；
cmd.Parameters.AddWithValue（“@Department”，employee.Department）；
cmd.Parameters.AddWithValue（“@Gender”，employee.Gender）；
con.Open（）；
cmd.ExecuteNonQuery（）；
con.Close（）；
}
返回1；
}
抓住
{
投掷；
}
}
公共内部删除员工（内部id）
{
尝试
{
使用（SqlConnection con=newsqlconnection（connectionString））
{
SqlCommand cmd=新的SqlCommand（“deletemployee”，con）；
cmd.CommandType=CommandType.storedProcess；
cmd.Parameters.AddWithValue（“@EmpId”，id）；
con.Open（）；
cmd.ExecuteNonQuery（）；
app.UseSession();

// Call the middlware now to set the session variable with the database
// connection string from appsettings.json. 
app.UseRequestConnection();

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using Microsoft.Extensions.Configuration;
using Microsoft.AspNetCore.Http;

namespace Angular5NetcoreAdo.Models
{
    public class EmployeeDataAccessLayer
    {
       public HttpContext HttpContext { get; }
       public string connectionString;

       public EmployeeDataAccessLayer(HttpContext httpContext)
       {
           // Set the property.
           HttpContext = httpContext;

           // Get the connection string session variable.
           connectionString = HttpContext.Session.GetString("ConnectionString");
       }

       public IEnumerable<Employee> GetAllEmployees()
       {
          try
           {
               List<Employee> lstemployee = new List<Employee>();

               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("SelectEmployees", con);
                   cmd.CommandType = CommandType.StoredProcedure;

                   con.Open();

                   SqlDataReader rdr = cmd.ExecuteReader();

                   while (rdr.Read())
                   {
                       Employee employee = new Employee();

                       employee.ID = Convert.ToInt32(rdr["EmployeeID"]);
                       employee.Name = rdr["Name"].ToString();
                       employee.Gender = rdr["Gender"].ToString();
                       employee.Department = rdr["Department"].ToString();
                       employee.City = rdr["City"].ToString();

                       lstemployee.Add(employee);
                   }

                   rdr.Close();
                   con.Close();
               }

               return lstemployee;
           }
           catch
           {
               throw;
           }
       }

       public Employee GetEmployeeData(int id)
       {
           try
           {
               Employee employee = new Employee();

               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("SelectEmployeeById", con);
                   cmd.CommandType = CommandType.StoredProcedure; 
                   cmd.Parameters.AddWithValue("@EmpId", id);

                   con.Open();

                   SqlDataReader rdr = cmd.ExecuteReader();

                   while (rdr.Read())
                   {
                       employee.ID = Convert.ToInt32(rdr["EmployeeID"]);
                       employee.Name = rdr["Name"].ToString();
                       employee.Gender = rdr["Gender"].ToString();
                       employee.Department = rdr["Department"].ToString();
                       employee.City = rdr["City"].ToString();
                   }

                   rdr.Close();
                   con.Close();
               }

               return employee;
           }
           catch
           {
               throw;
           }
       }

       public int AddEmployee(Employee employee)
       {
           try
           {
               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("InsertEmployee", con);
                   cmd.CommandType = CommandType.StoredProcedure;

                   cmd.Parameters.AddWithValue("@Name", employee.Name);
                   cmd.Parameters.AddWithValue("@City", employee.City);
                   cmd.Parameters.AddWithValue("@Department", employee.Department);
                   cmd.Parameters.AddWithValue("@Gender", employee.Gender);

                   con.Open();

                   cmd.ExecuteNonQuery();

                   con.Close();
               }

               return 1;
           }
           catch
           {
               throw;
           }
       }

       public int UpdateEmployee(Employee employee)
       {
           try
           {
               using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("UpdateEmployee", con);
                   cmd.CommandType = CommandType.StoredProcedure;

                   cmd.Parameters.AddWithValue("@EmpId", employee.ID);
                   cmd.Parameters.AddWithValue("@Name", employee.Name);
                   cmd.Parameters.AddWithValue("@City", employee.City);
                   cmd.Parameters.AddWithValue("@Department", employee.Department);
                   cmd.Parameters.AddWithValue("@Gender", employee.Gender);

                   con.Open();

                   cmd.ExecuteNonQuery();

                    con.Close();
               }

               return 1;
           }
           catch
           {
               throw;
           }
       }

       public int DeleteEmployee(int id)
       {
           try
           {
            using (SqlConnection con = new SqlConnection(connectionString))
               {
                   SqlCommand cmd = new SqlCommand("DeleteEmployee", con);
                   cmd.CommandType = CommandType.StoredProcedure;
                   cmd.Parameters.AddWithValue("@EmpId", id);

                   con.Open();

                   cmd.ExecuteNonQuery();

                    con.Close();
              }

               return 1;
          }
        catch
           {
               throw;
           }
       }
   }
}