.Net Core Migration System.ArgumentNullException：值不能为空

Posted 2023-04-12

【中文标题】.Net Core Migration System.ArgumentNullException：值不能为空

【英文标题】：.Net Core Migration System.ArgumentNullException: Value cannot be null

【发布时间】：2019-11-01 17:18:35

【问题描述】：

这是我的 DbContext

public class DocumentStudioDbContext : DbContext

    public DocumentStudioDbContext(DbContextOptions<DocumentStudioDbContext> options) : base(options)  
    public DbSet<Documents> Documents  get; set; 

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    
        modelBuilder.HasDefaultSchema(schema: Environment.GetEnvironmentVariable(DSCASGlobals.DS_DatabaseSchema));
        base.OnModelCreating(modelBuilder);
    

    public override int SaveChanges()
    
        Audit();
        return base.SaveChanges();
    

    public async Task<int> SaveChangesAsync()
    
        Audit();
        return await base.SaveChangesAsync();
    

    private void Audit()
    
        var entries = ChangeTracker.Entries().Where(x => x.Entity is Documents && (x.State == EntityState.Added || x.State == EntityState.Modified));
        foreach (var entry in entries)
        
            if (entry.State == EntityState.Added)
            
                ((Documents)entry.Entity).CreatedOn = DateTime.UtcNow;
            
        ((Documents)entry.Entity).UpdatedOn = DateTime.UtcNow;
        
    

当我尝试运行迁移时，我收到此错误：

System.ArgumentNullException：值不能为空。参数名称： 连接字符串在 Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty（字符串值， 字符串参数名称）在 Microsoft.EntityFrameworkCore.SqlServerDbContextOptionsExtensions.UseSqlServer(DbContextOptionsBuilder optionsBuilder，字符串 connectionString，Action1 sqlServerOptionsAction) at DocumentStudio.Startup.<>c.<ConfigureServices>b__1_0(DbContextOptionsBuilder options) in C:\Users\amosa\source\repos\DotNetCore\DocumentStudio\DocumentStudio\Startup.cs:line 48 at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass1_02.b__0(IServiceProvider p, DbContextOptionsBuilder b) 在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.DbContextOptionsFactory[TContext](IServiceProvider applicationServiceProvider, Action2 optionsAction) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass10_01.b__0(IServiceProvider p) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory（FactoryCallSite factoryCallSite、ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.c__DisplayClass1_0.b__0(ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（类型 serviceType, ServiceProviderEngineScope serviceProviderEngineScope) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（类型 服务类型）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider 提供者，类型 serviceType) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider 提供者）在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.c__101.<AddCoreServices>b__10_1(IServiceProvider p) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped（ScopedCallSite scopedCallSite、ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite enumerableCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.c__DisplayClass1_0.b__0(ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（类型 serviceType, ServiceProviderEngineScope serviceProviderEngineScope) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（类型 服务类型）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider 提供者，类型 serviceType) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider 提供者）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetServices[T](IServiceProvider 提供者）在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextTypes() 在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextType（字符串 名称）在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext（字符串 上下文类型）在 Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration（字符串 名称，字符串 outputDir，字符串 contextType）在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl（字符串 名称，字符串 outputDir，字符串 contextType）在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.c__DisplayClass0_1.<.ctor>b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.c__DisplayClass3_0`1.b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute（操作 动作）值不能为空。参数名称：连接字符串

我的 ConnectionString 来自 Visual Studio 中设置的环境变量。我不确定如何执行迁移。

我还需要做什么？

编辑：

Startup.cs 中的 DbContext

services.AddDbContext<DocumentStudioDbContext>(options => options.UseSqlServer(Environment.GetEnvironmentVariable(DSCASGlobals.DS_ConnectionString)));

编辑2：

设法从环境变量中取出变量

【问题讨论】：

在 Statup 类中显示您的 DbContext 注册，并在 appsettings.json 文件中显示连接字符串。

我的连接字符串存储在环境变量中。我不在项目中使用 appsettings.json。

好的！然后展示在DbContext 注册期间如何在 Startup 类中获得ConnectionSring。

Environment.GetEnvironmentVariable(DSCASGlobals.DS_ConnectionString) 是我获取连接字符串的方式。我尝试将它设置为一个变量，它可以工作。

Environment.GetEnvironmentVariable(DSCASGlobals.DS_ConnectionString) 这条线的ConnectionString 为空。请正确检查！将 ConnectionString 存储在环境变量而不是 appsettings.json 文件中的原因是什么？

【参考方案1】：

  <configuration>

          <connectionStrings>
              <add name="DocumentStudioDb"
                connectionString="Server=(localdb)\mssqllocaldb;Database=DocumentStudioDb;Trusted_Connection=True;" />
                   </connectionStrings>
   </configuration>

/////在你的上下文中编写这段代码

 protected override void OnConfiguring(DbContextOptionsBuilder   optionsBuilder)
               
       optionsBuilder.UseSqlServer(ConfigurationManager.ConnectionStrings["DocumentStudioDb"]  .ConnectionString);