I have recently installed the EF Core 3.1.6 on a home project using postgresql.

我最近在一个使用PostgreSQL的家庭项目上安装了EF Core 3.1.6。

I have the connection string, and can add migrations easily with the Package Manager Console. If I run my application the Database.Migrate() will migrate the migrations easily. However if I attempt to update the database via the Package Manager Console with an 'update-database' I receive this error:

我有连接字符串，可以使用Package Manager控制台轻松添加迁移。如果我运行我的应用程序，那么Database.Migrate()将很容易地迁移迁移。但是，如果我尝试通过Package Manager控制台使用‘UPDATE-DATABASE’来更新数据库，我会收到这个错误：

System.ArgumentException: Host can't be null
   at Npgsql.NpgsqlConnector.Open(NpgsqlTimeout timeout, Boolean async, CancellationToken cancellationToken)
   at Npgsql.NpgsqlConnection.<>c__DisplayClass32_0.<<Open>g__OpenLong|0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Npgsql.NpgsqlConnection.Open()
   at Npgsql.EntityFrameworkCore.PostgreSQL.Storage.Internal.NpgsqlDatabaseCreator.Exists()
   at Microsoft.EntityFrameworkCore.Migrations.HistoryRepository.Exists()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.Migrate(String targetMigration)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.UpdateDatabase(String targetMigration, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabaseImpl(String targetMigration, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabase.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Host can't be null

I do not know what is happening, if I am missing a nuget package, etc. Any possible help and explanation for what is happening would be greatly appreciated.

我不知道发生了什么，如果我丢失了一个Nuget包，等等。任何可能的帮助和解释正在发生的事情将非常感激。

You should set ConnectionStrings in program.cs|startup.cs.

您应该在Program.cs|Startup.cs中设置ConnectionStrings。

Add this into the program.cs (.net7)

将此代码添加到Program.cs(.net7)中

builder.Services.Configure<ConnectionStrings>(
    builder.Configuration.GetSection(nameof(ConnectionStrings))
);

or startup.cs (.net6):

或者Startup.cs(.net6)：

services.Configure<ConnectionStrings>(Configuration.GetSection("ConnectionStrings"));

add bellow lines in Program.cs file

在Program.cs文件中添加以下行

builder.Services.AddDbContext<ApplicationDBContext>(options =>
options.UseNpgsql(builder.Configuration.GetConnectionString("DefaultConnection")));
builder.Services.AddScoped<IDataAccessProvider, DataAccessProvider>();

and bellow connection string in appsettings.json

和下面appsettings.json中的连接字符串

  "ConnectionStrings": {
    "DefaultConnection": "User ID=postgres;Password=******;Host=127.0.0.1;Port=****;Database=at;"
  },   "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"

I Solved it. It had to do with the way I was building the connection string on runtime and not buildtime.

我解决了这个问题。这与我在运行时而不是构建时构建连接字符串的方式有关。

You should export your env variables first from terminal like this:

您应该首先从终端导出您的环境变量，如下所示：

export $(grep -v '^#' ./dev.env | xargs)

{dev.env} -> change it to your env file

Check your connection string in your startup class you may have different connection string name with the config of the database in the startup when you are using appsetting.json

检查启动类中的连接字符串当您使用appsetting.json时，启动时数据库的配置可能具有不同的连接字符串名称

Startup configuration

启动配置

appsetting.json

Appsetting.json

Ok this may sounds stupid. But our team accidentally ignore the relevant appsettings.json file for our contract test.

好吧，这可能听起来很愚蠢。但是我们的团队意外地忽略了合同测试的相关appsettings.json文件。

CopyToPublishDirectory was set to Never.

CopyToPublishDirectory设置为Never。

Thus the connection string was never set in the first place. When docker runs, the connection string was empty.

因此，从一开始就没有设置连接字符串。当docker运行时，连接字符串为空。

I have this code snippet that read env file and set it to process environment variables

我有这个代码片段，它读取环境文件并将其设置为处理环境变量

      using System.Runtime.CompilerServices;

      namespace Bug_tracker.Utils
      {
        public static class Env
        {
         [ModuleInitializer]
         public static void ReadEnvFile()
         {
         if (!File.Exists("./.env")) return;

         foreach (var line in File.ReadAllLines("./.env"))
         {
            if (string.IsNullOrWhiteSpace(line) || line.StartsWith("#")) continue;

            var parts = line.Split('=', 2);

            if (parts.Length == 2)
            {
                var name = parts[0].Trim();
                var value = parts[1].Trim();
                value = value.Substring(1, value.Length - 2);
                Console.WriteLine(name + "=" + value);
                Environment.SetEnvironmentVariable(name, value);
             };
         };

     }

  }
 }

Using attribute [ModuleInitializer] to mark a method that will run when a module is loaded

使用属性[ModuleInitializer]标记将在加载模块时运行的方法

This solution fixes my problem

这个解决方案解决了我的问题

I solved this problem by putting the --startup-project flag in my migration script like this:

我通过在迁移脚本中放置--startup-project标志解决了这个问题，如下所示：

dotnet ef database update --startup-project "startup project path" --project "DbContext path" --verbose

DotNet ef数据库更新--启动-项目“启动项目路径”--项目“DbContext路径”--详细