执行dotnet ef迁移命令时DbContext实例创建失败求助

跟随GraphQL教程学习，执行命令 dotnet ef migrations add AddPlatformToDb 时触发以下错误：

Unable to create a 'DbContext' of type ''. The exception 'Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContextOptions`1[CommanderGQL.Data.AppDbContext]' while attempting to activate 'CommanderGQL.Data.AppDbContext'.' was thrown while attempting to create an instance. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

项目相关文件内容如下：

CommanderGQL.csproj #

<Project Sdk="Microsoft.NET.Sdk.Web">
<PropertyGroup>
<TargetFramework>net8.0</TargetFramework>
<Nullable>enable</Nullable>
<ImplicitUsings>enable</ImplicitUsings>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="GraphQL.Server.Ui.Voyager" Version="7.7.1" />
<PackageReference Include="HotChocolate.AspNetCore" Version="13.9.0" />
<PackageReference Include="HotChocolate.Data.Entityframework" Version="13.9.0" />
<PackageReference Include="Microsoft.EntityframeworkCore.Design" Version="8.0.2">
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
<PrivateAssets>all</PrivateAssets>
</PackageReference>
<PackageReference Include="Microsoft.EntityframeworkCore.SqlServer" Version="8.0.2" />
</ItemGroup>
</Project>

docker-compose.yaml #

version: '3'
services:
  sqlserver:
    image: "mcr.microsoft.com/mssql/server:2017-latest"
    environment:
      ACCEPT_EULA: "Y"
      SA_PASSWORD: "pa55w0rd!"
      MSSQL_PID: "Express"
    ports:
      - "1433:1433"

appsettings.Development.json #

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "ConnectionStrings": {
    "CommandConStr" : "Server=localhost,1433;Database=CommandsDB;User Id=sa;Password=pa55w0rd!"
  }
}

appsettings.json #

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

Startup.cs #

using CommanderGQL.Data;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace CommanderGQL
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }
        //public IConfiguration Configuration { get; }
        private readonly IConfiguration Configuration;

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<AppDbContext>(opt => opt.UseSqlServer
            (Configuration.GetConnectionString("CommandConStr")));
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                // Configure production environment here
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                // Configure endpoints here
            });
        }
    }
}

Platform.cs #

using System.ComponentModel.DataAnnotations;

namespace CommanderGQL.Models
{
    public class Platform
    {
        [Key]
        public int Id { get; set; }
        public string Name { get; set; }
        public string LicenseKey { get; set; }
    }
}

AppDbContext.cs #

using CommanderGQL.Models;
using Microsoft.EntityFrameworkCore;

namespace CommanderGQL.Data
{
    public class AppDbContext : DbContext
    {
        public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
        {
        }

        public DbSet<Platform> Platforms { get; set; }
    }
}

Program.cs #

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

问题根源是你的.NET 8项目使用了顶级语句模式的Program.cs，但传统的Startup.cs并没有被程序加载，导致EF Core在执行迁移命令时，无法找到AppDbContext的配置服务。

下面提供两种可行的修复方案，任选其一即可：

方案1：直接在Program.cs中配置DbContext #

删除或保留Startup.cs均可，将DbContext的配置逻辑移到Program.cs中，更新后的代码如下：

using CommanderGQL.Data;
using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);

// 配置AppDbContext服务
builder.Services.AddDbContext<AppDbContext>(opt => 
    opt.UseSqlServer(builder.Configuration.GetConnectionString("CommandConStr")));

var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

方案2：让Program.cs启用Startup类 #

如果你想保留Startup.cs的结构，修改Program.cs来加载并使用Startup类：

using CommanderGQL;

var builder = WebApplication.CreateBuilder(args);

// 初始化Startup并配置服务
var startup = new Startup(builder.Configuration);
startup.ConfigureServices(builder.Services);

var app = builder.Build();

// 使用Startup配置请求管道
startup.Configure(app, app.Environment);

app.Run();

完成修改后，重新执行dotnet ef migrations add AddPlatformToDb命令即可正常生成迁移。

内容的提问来源于stack exchange，提问作者kamil