ASP.NET 4.8升级至ASP.NET Core 6.0遇NullReferenceException问题

将项目从ASP.NET 4.8升级到ASP.NET Core 6.0后，原有代码在ASP.NET环境运行正常，但访问资源地址http://localhost/odata/Users时触发System.NullReferenceException错误，调试器指向var lsret = new List<User>();行，尝试多种方法未解决。

相关代码 #

UserService.cs #

namespace pal_data.Business
{
    public class UserService
    {
        public List<User> GetData()
        {
            try
            {
                var lsret = new List<User>();   // 错误提示：System.NullReferenceException: 对象引用未设置到对象的实例。

                using (SqlConnection con = new(System.Configuration.ConfigurationManager.ConnectionStrings["MyConnPort"].ConnectionString))
                {
                    var strSQL = "select a.*, b.RoleName from AppUser a inner join AccessRole b on a.RoleID = b.AccessRoleID";

                    using (SqlDataAdapter da = new(strSQL, con))
                    {
                        var dt = new DataTable();
                        da.Fill(dt);

                        foreach (DataRow dr in dt.Rows)
                        {
                            var item = new User
                            {
                                UserID = int.Parse(dr["AppUserID"].ToString() ?? ""),
                                RoleID = int.Parse(dr["RoleID"].ToString() ?? ""),
                                Role = dr["RoleName"].ToString() ?? "",
                                UserName = dr["UserName"].ToString() ?? "",
                                Name = dr["Name"].ToString() ?? "",
                                Password = dr["Password"].ToString()
                            };

                            lsret.Add(item);
                        }
                    }
                }

                return lsret;
            }
            catch (SqlException ex)
            {
                // ...
            }
            catch (Exception e)
            {
                // ...
            }
        }
    }
}

UsersController.cs #

namespace pal_data.Controllers
{
    [EnableCors]
    public class UsersController : ODataController
    {
        private readonly UserService m_service = new();

        [EnableQuery]
        public ActionResult<IEnumerable<User>> Get()
        {
            return Ok(m_service.GetData());
        }
    }
}

User.cs #

namespace pal_data.Models
{
    public class User
    {
        public int UserID { get; set; }
        public string? FunctionResult { get; set; }
        public int ReturnCode { get; set; }
        public int RoleID { get; set; }
        public string? Role { get; set; }
        public string? UserName { get; set; }
        public string? Name { get; set; }
        public string? Password { get; set; }
    }
}

可正常运行的UserService版本 #

• 此版本无空引用错误，可正常返回数据

namespace pal_data.Business
{
    public class UserService
    {
        public List<User> GetData()
        {
            var lsret = new List<User>
            {
                new User
                {
                    UserID = 1,
                    RoleID = 1,
                    Role = "asdfsdf",
                    UserName = "sadfsdf",
                    Name = "sdfasdf",
                    Password = "asdfsdf",
                }
            };

            return lsret;
        }
    }
}

问题分析 #

错误提示指向new List<User>()是误导，实际异常根源是：

1. ASP.NET Core中不再使用System.Configuration.ConfigurationManager读取配置，ConnectionStrings["MyConnPort"]会返回null，访问其ConnectionString属性时抛出空引用异常。
2. catch块未做有效处理，既没有返回默认列表也没有重新抛出异常，导致方法最终返回null，上层调用Ok(m_service.GetData())时触发空引用。

解决方案 #

1. 替换配置读取方式为ASP.NET Core原生注入 #

在UserService中通过构造函数注入IConfiguration，替代ConfigurationManager：

namespace pal_data.Business
{
    public class UserService
    {
        private readonly IConfiguration _configuration;

        public UserService(IConfiguration configuration)
        {
            _configuration = configuration;
        }

        public List<User> GetData()
        {
            var lsret = new List<User>();
            try
            {
                string connStr = _configuration.GetConnectionString("MyConnPort") 
                    ?? throw new InvalidOperationException("未找到连接字符串MyConnPort");

                using (SqlConnection con = new(connStr))
                {
                    var strSQL = "select a.*, b.RoleName from AppUser a inner join AccessRole b on a.RoleID = b.AccessRoleID";

                    using (SqlDataAdapter da = new(strSQL, con))
                    {
                        var dt = new DataTable();
                        da.Fill(dt);

                        foreach (DataRow dr in dt.Rows)
                        {
                            var item = new User
                            {
                                UserID = int.Parse(dr["AppUserID"].ToString() ?? ""),
                                RoleID = int.Parse(dr["RoleID"].ToString() ?? ""),
                                Role = dr["RoleName"].ToString() ?? "",
                                UserName = dr["UserName"].ToString() ?? "",
                                Name = dr["Name"].ToString() ?? "",
                                Password = dr["Password"].ToString()
                            };

                            lsret.Add(item);
                        }
                    }
                }
            }
            catch (SqlException ex)
            {
                // 添加错误日志记录
            }
            catch (Exception e)
            {
                // 添加错误日志记录
            }
            return lsret;
        }
    }
}

2. 修改控制器，通过构造函数注入UserService #

ASP.NET Core推荐依赖注入，避免直接实例化服务：

namespace pal_data.Controllers
{
    [EnableCors]
    public class UsersController : ODataController
    {
        private readonly UserService _service;

        public UsersController(UserService service)
        {
            _service = service;
        }

        [EnableQuery]
        public ActionResult<IEnumerable<User>> Get()
        {
            return Ok(_service.GetData());
        }
    }
}

3. 在Program.cs中注册服务 #

var builder = WebApplication.CreateBuilder(args);

// 添加控制器和OData服务
builder.Services.AddControllers().AddOData(options =>
{
    options.Select().Filter().OrderBy();
    options.AddRouteComponents("odata", GetEdmModel());
});

// 注册UserService为依赖注入服务
builder.Services.AddScoped<UserService>();

var app = builder.Build();

// 中间件配置（如CORS、路由等）
app.UseCors();
app.UseRouting();
app.MapControllers();

app.Run();

// 构建EDM模型示例
private static IEdmModel GetEdmModel()
{
    ODataConventionModelBuilder builder = new();
    builder.EntitySet<User>("Users");
    return builder.GetEdmModel();
}

4. 确保appsettings.json中有正确的连接字符串 #

{
  "ConnectionStrings": {
    "MyConnPort": "你的数据库连接字符串"
  }
}

内容的提问来源于stack exchange，提问作者xinthose