c# - 与 MySQL 连接的 NullReferenceException

Question

这会让我发疯。

我构建了一个用于 Intranet 的简单网络应用程序，它在本地测试期间运行良好。当我将它部署到 IIS 服务器上时，它给我带来了问题，我们认为这与错误的 web.config 文件有关。

我完全按照 MySQL 文档所述仔细检查并配置了 web.config 文件，现在该应用程序也无法在本地运行。这显然是相关的，但我无法弄清楚是什么原因造成的。

使用 MySQL 设置，我得到一个 NullReferenceException 和这个堆栈跟踪:

[NullReferenceException: Object reference not set to an instance of an object.]
MySql.Data.MySqlClient.MySqlProviderServices.GetDbProviderManifestToken(DbConnection connection) +56
System.Data.Entity.Core.Common.DbProviderServices.GetProviderManifestToken(DbConnection connection) +234
MySql.Data.Entity.MySqlManifestTokenResolver.ResolveManifestToken(DbConnection connection) +36
System.Data.Entity.Utilities.DbConnectionExtensions.GetProviderInfo(DbConnection connection, DbProviderManifest& providerManifest) +55
System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection) +56
System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext) +79
System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input) +112
System.Data.Entity.Internal.LazyInternalContext.InitializeContext() +635
System.Data.Entity.Internal.InternalContext.Initialize() +28
System.Data.Entity.Internal.InternalContext.GetEntitySetAndBaseTypeForType(Type entityType) +27
System.Data.Entity.Internal.Linq.InternalSet`1.Initialize() +79
System.Data.Entity.Internal.Linq.InternalSet`1.GetEnumerator() +28
System.Data.Entity.Infrastructure.DbQuery`1.System.Collections.IEnumerable.GetEnumerator() +64
System.Web.UI.WebControls.ListViewPagedDataSource.GetEnumerator() +305
System.Web.UI.WebControls.ListView.CreateItemsWithoutGroups(ListViewPagedDataSource dataSource, Boolean dataBinding, InsertItemPosition insertPosition, ArrayList keyArray) +215
System.Web.UI.WebControls.ListView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding) +952
System.Web.UI.WebControls.ListView.PerformDataBinding(IEnumerable data) +63
System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data) +128
System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) +34
System.Web.UI.WebControls.ModelDataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) +80
System.Web.UI.WebControls.DataBoundControl.PerformSelect() +143
System.Web.UI.WebControls.ListView.PerformSelect() +114
System.Web.UI.WebControls.BaseDataBoundControl.DataBind() +74
System.Web.UI.WebControls.BaseDataBoundControl.EnsureDataBound() +114
System.Web.UI.WebControls.ListView.CreateChildControls() +77
System.Web.UI.Control.EnsureChildControls() +92
System.Web.UI.Control.PreRenderRecursiveInternal() +42
System.Web.UI.Control.PreRenderRecursiveInternal() +160
System.Web.UI.Control.PreRenderRecursiveInternal() +160
System.Web.UI.Control.PreRenderRecursiveInternal() +160
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +883

尝试逐步执行代码只会导致它在第一项运行时立即失败，似乎没有链接到任何特定行。这是我的 web.config 文件中的内容，现在我已经更改了它。

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
  </configSections>
  <appSettings>
    <add key="PrinterTemplate" value="--redacted--" />
    <add key="PrinterQueue" value="--redacted--" />
    <add key="FileVault" value="--redacted--" />
    <add key="AdminGroup" value="--redacted--" />
  </appSettings>
  <location path="Modify.aspx">
    <system.web>
      <authorization>
        <allow roles="--redacted--" />
        <deny users="*,?" />
      </authorization>
    </system.web>
  </location>
  <location path="AccessDenied.aspx">
    <system.web>
      <authorization>
        <allow users="*" />
      </authorization>
    </system.web>
  </location>
  <connectionStrings>
    <add name="DefaultConnection" connectionString="DRIVER={MySQL ODBC 3.51 Driver};Server=servername;port=3306;Database=usermanuals;UId=username;password=password" providerName="MySql.Data.MySqlClient" />
  </connectionStrings>
  <system.web>
    <compilation debug="true" targetFramework="4.5.2" />
    <httpRuntime targetFramework="4.5.2" />
    <authentication mode="Windows" />
    <authorization>
      <deny users="?" />
    </authorization>
    <pages>
      <namespaces>
        <add namespace="System.Web.Optimization" />
      </namespaces>
      <controls>
        <add assembly="Microsoft.AspNet.Web.Optimization.WebForms" namespace="Microsoft.AspNet.Web.Optimization.WebForms" tagPrefix="webopt" />
      </controls>
    </pages>
  </system.web>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" culture="neutral" publicKeyToken="30ad4fe6b2a6aeed" />
        <bindingRedirect oldVersion="0.0.0.0-6.0.0.0" newVersion="6.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="WebGrease" culture="neutral" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-1.5.2.14234" newVersion="1.5.2.14234" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
    </compilers>
  </system.codedom>
  <system.data>
    <DbProviderFactories>
      <remove invariant="MySql.Data.MySqlClient" />
      <add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" description=".Net Framework Data Provider for MySQL" type="MySql.Data.MySqlClient.MySqlClientFactory, MySql.Data" />
    </DbProviderFactories>
  </system.data>
  <entityFramework codeConfigurationType="MySql.Data.Entity.MySqlEFConfiguration, MySql.Data.Entity.EF6">
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework"/>
    <providers>
      <provider invariantName="MySql.Data.MySqlClient"
          type="MySql.Data.MySqlClient.MySqlProviderServices, MySql.Data.Entity.EF6"/>
      <provider invariantName="System.Data.SqlClient"
          type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer"/>
    </providers>
  </entityFramework>
</configuration>

我看到很多人说这与他们对连接字符串的实际使用有关，但我的并不是在实际调用数据库时触发，而是在网络应用程序运行时触发。无论如何，这就是我在代码中实际使用连接字符串的方式。我的所有用法都具有相同的结构，只是命令不同。

using (OdbcConnection connection = new OdbcConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
{
    connection.Open();
    using (OdbcCommand command = new OdbcCommand("SELECT id, title FROM manuals", connection))
    using (OdbcDataReader dr = command.ExecuteReader())
    {
       //databindings and stuff.
    }
}

Answer 1

ASP.NET 项目默认使用 Entity Framework 来管理用户。如果您不使用此功能，则可以从 App_Start 文件夹中的 Startup.Auth.cs 文件中注释掉以下三行:

 public void ConfigureAuth(IAppBuilder app)
    {
        // Configure the db context, user manager and signin manager to use a single instance per request
        //app.CreatePerOwinContext(ApplicationDbContext.Create);
        //app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
        //app.CreatePerOwinContext<ApplicationSignInManager>(ApplicationSignInManager.Create);
        ...
    }