我有一个使用ASP.NET Core，Angular 5和ADO.NET的应用程序
在我决定更改代码以使用从我的appssettings.json文件中获取的数据库连接字符串设置会话变量之前，它一直运行良好。
我以此为参考：https://docs.microsoft.com/en-us/aspnet/core/fundamentals/app-state?view=aspnetcore-2.2
但是，当我尝试设置会话变量时，我在SetSessionVariable（）方法中获得了一个Null对象引用。
错误是：
启动应用程序时发生错误。 NullReferenceException：对象引用未设置为的实例 宾语。 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");
}
});
}
}
----根据以下建议，我将代码包含在此处：
// Middleware extension method.
using Microsoft.AspNetCore.Builder;
namespace Angular5NetcoreAdo
{
public static class RequestConnectionMiddlewareExtensions
{
public static IApplicationBuilder UseRequestConnection(this
IApplicationBuilder builder)
{
return builder.UseMiddleware<RequestConnectionMiddleware>();
}
}
}
using Microsoft.AspNetCore.Http;
using System.Threading.Tasks;
using System;
using Microsoft.Extensions.Configuration;
namespace Angular5NetcoreAdo
{
public class RequestConnectionMiddleware
{
public IConfiguration Configuration { get; }
public HttpContext HttpContext { get; }
private readonly RequestDelegate _next;
public RequestConnectionMiddleware(RequestDelegate next,
IConfiguration configuration)
{
_next = next;
Configuration = configuration;
}
public async Task InvokeAsync(HttpContext context)
{
// Set the session variable with the database connection string from appsettings.json.
HttpContext.Session.SetString("ConnectionString", Convert.ToString(Configuration.GetConnectionString("DBAngular5NetcoreAdoDatabase")));
await _next(context);
}
}
}
现在在Startup.cs中，我在app.UseSession（）;之后调用新的中间件消息方法。
app.UseSession();
// Call the middlware now to set the session variable with the database
// connection string from appsettings.json.
app.UseRequestConnection();
------------------现在添加，因为新错误引用了此类。
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;
}
}
}
}
答案 0 :(得分：1)
是因为文档说
HttpContext.Session在UseSession被访问之前无法访问 叫。
，然后在ConfigurationServices方法中调用它，其中HttpContext.Session还不存在。
因此您将需要create your own middleware并在Configure方法中的UseSession()方法之后调用它。