.NET Framework迁移至.NET 8后配置读取空引用异常求助

问题背景 #

将.NET Framework应用迁移至.NET 8，微软迁移工具执行无报错，但本地运行触发空引用异常，异常发生在Program.cs读取App.config配置项的代码处，具体是System.Configuration.ConfigurationManager.AppSettings获取配置值返回null，导致赋值给config对象属性时引发异常。

相关代码与配置 #

Program.cs代码片段 #

using System;
using System.Collections.Generic;

namespace GenericEmail
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length > 0)
            { 
                ConfigSettings config = new ConfigSettings();

                // 从参数获取值
                config.reportType = args[0];
                bool isAdminReport = false;
                bool isTest = true;
                
                if (args.Length > 1)
                {
                    isAdminReport = Convert.ToBoolean(args[1]);
                }

                if (args.Length > 2)
                {
                    isTest = Convert.ToBoolean(args[2]);
                }

                config.isAdminReport = isAdminReport;
                config.isTest = isTest;

                // 从配置文件读取值 - 异常触发处
                config.emailSubject = System.Configuration.ConfigurationManager.AppSettings["EmailSubject-" + config.reportType];
                config.adminEmailAddresses = System.Configuration.ConfigurationManager.AppSettings["Admin-" + config.reportType];
                config.noErrorMessage = System.Configuration.ConfigurationManager.AppSettings["NoError-" + config.reportType];
            }
        }
    }
}

App.config配置文件 #

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <!-- For more information on Entity Framework configuration -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=mytoken" requirePermission="false"/>
  </configSections>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.8"/>
  </startup>
  <appSettings>
    <add key="TestEmailAddress" value="me@email.com"/>
    <add key="EmailServer" value="smtp.myserver"/>
    <add key="EnableSSL" value="false"/>
    <add key="Port" value="25"/>
    
    <add key="EmailSubject-Validation" value="Account # Errors and Unreconciled Jobs"/>
    <add key="Admin-Validation" value="1@email.com; 2@email.com;  3@email.com; 4@email.com"/>
    <add key="NoError-Validation" value="There are no validation errors to report at this time."/>

    <add key="EmailSubject-Blackout" value="Important: AESOP time entered on blackout date"/>
    <add key="Admin-Blackout" value="me@email.com; x@email.com"/>
    <add key="NoError-Blackout" value="There are no absences on blackout dates to report at this time."/>

    <add key="EmailSubject-FFCRA" value="Authorization required for your absence entry"/>
    <add key="Admin-FFCRA" value="me@email.com; x@email.com"/>
    <add key="NoError-FFCRA" value="There are no FFCRA absences to report at this time."/>
    <add key="Sender-FFCRA" value=" y@email.com"/>

    <add key="EmailSubject-FFCRAReport" value="AESOP FFCRA Absence Report"/>
    <add key="Admin-FFCRAReport" value="me@email.com; y@email.com"/>
    <add key="NoError-FFCRAReport" value="There are no FFCRA absences to report at this time."/>
   
    <add key="ClientSettingsProvider.ServiceUri" value=""/>
  </appSettings>
  <connectionStrings>
    <add name="mu_report_dataEntities" connectionString="metadata=res://*/mu_report_data.csdl|res://*/mu_report_data.ssdl|res://*/mu_report_data.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=myserver;initial catalog=mytable;integrated security=True;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>
  <system.web>
    <membership defaultProvider="ClientAuthenticationMembershipProvider">
      <providers>
        <add name="ClientAuthenticationMembershipProvider" type="System.Web.ClientServices.Providers.ClientFormsAuthenticationMembershipProvider, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=mytoken" serviceUri=""/>
      </providers>
    </membership>
    <roleManager defaultProvider="ClientRoleProvider" enabled="true">
      <providers>
        <add name="ClientRoleProvider" type="System.Web.ClientServices.Providers.ClientRoleProvider, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=mytoken" serviceUri="" cacheTimeout="86400"/>
      </providers>
    </roleManager>
  </system.web>
</configuration>

解决步骤 #

1. 安装ConfigurationManager NuGet包 #

.NET 8不再默认包含System.Configuration.ConfigurationManager组件，需手动安装：

Install-Package System.Configuration.ConfigurationManager

2. 修正配置文件属性与内容 #

• 右键App.config文件→属性，将复制到输出目录设置为「始终复制」，确保配置文件能被程序读取到。
• 修改startup节点，移除.NET Framework相关配置，替换为.NET 8的支持：

<startup>
  <supportedRuntime version="v8.0" />
</startup>

3. 验证配置键名匹配 #

检查传入的args[0]值（即config.reportType）是否与App.config中的键名后缀完全一致（区分大小写）。比如传入"validation"（小写）会匹配不到"EmailSubject-Validation"（大写V），导致返回null。可添加调试输出确认实际拼接后的键名：

var subjectKey = "EmailSubject-" + config.reportType;
Console.WriteLine("Attempting to read key: " + subjectKey);
config.emailSubject = ConfigurationManager.AppSettings[subjectKey];

4. （推荐）改用.NET 8原生配置系统 #

若要适配.NET 8最佳实践，建议切换到Microsoft.Extensions.Configuration：

1. 安装必要NuGet包：

Install-Package Microsoft.Extensions.Configuration.Json
Install-Package Microsoft.Extensions.Configuration.FileExtensions

2. 创建appsettings.json文件，将App.config的appSettings转换为JSON格式：

{
  "TestEmailAddress": "me@email.com",
  "EmailServer": "smtp.myserver",
  "EnableSSL": "false",
  "Port": "25",
  "EmailSubject-Validation": "Account # Errors and Unreconciled Jobs",
  "Admin-Validation": "1@email.com; 2@email.com; 3@email.com; 4@email.com",
  "NoError-Validation": "There are no validation errors to report at this time.",
  // 其他配置项...
}

3. 修改Program.cs的配置读取逻辑：

using Microsoft.Extensions.Configuration;
using System.IO;

// 构建配置对象
var configBuilder = new ConfigurationBuilder()
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", optional: false);
IConfiguration configuration = configBuilder.Build();

// 读取配置
config.emailSubject = configuration["EmailSubject-" + config.reportType];
config.adminEmailAddresses = configuration["Admin-" + config.reportType];
config.noErrorMessage = configuration["NoError-" + config.reportType];

5. 检查ConfigSettings类定义 #

确保ConfigSettings类的对应属性为可赋值状态（如public字段或带setter的属性），避免因属性只读导致赋值异常。

内容的提问来源于stack exchange，提问作者Kimber Warden