使用.NET 6+CQRS+MediatR搭建分层API时EF Core迁移报错求助

问题重现 #

执行首次EF Core迁移时触发异常：
Unable to create an object of type 'WorldCupApiDbContext'

场景：使用.NET 6结合CQRS与MediatR搭建分层Web API，已尝试指定MigrationsAssembly配置，但问题未解决。

相关代码 #

Program.cs #

using System.Reflection;
using MediatR;
using Microsoft.EntityFrameworkCore;
using WorldCupApi.Application.Handler.QueryHandler;
using WorldCupApi.Repository.Entities;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

builder.Services.AddDbContextPool<WorldCupApiDbContext>(options =>
{
    options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection"));
});

builder.Services.AddMediatR(typeof(GetGroupQueryHandler).GetTypeInfo().Assembly);

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

WorldCupApiDbContext.cs #

using Microsoft.EntityFrameworkCore;

namespace WorldCupApi.Repository.Entities;

public class WorldCupApiDbContext : DbContext
{
    public WorldCupApiDbContext(DbContextOptions<WorldCupApiDbContext> options) : base(options)
    {
        
    }
    
    //entities
    public DbSet<Group> Groups { get; set; }
    public DbSet<Team> Teams { get; set; }
}

已尝试的配置修改 #

builder.Services.AddDbContextPool<WorldCupApiDbContext>(options =>
{
    options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection"), 
        b=> b.MigrationsAssembly("WorldCupApi.Repository"));
});

解决方案 #

1. 显式指定迁移命令的项目参数 #

分层架构下，EF Core工具无法自动识别启动项目和DbContext所在项目，需要在命令行中明确指定：

# 替换为你的实际项目名称
dotnet ef migrations add InitialCreate --project WorldCupApi.Repository --startup-project WorldCupApi.Web

2. 实现IDesignTimeDbContextFactory（官方推荐方案） #

在WorldCupApi.Repository项目中添加工厂类，让迁移工具能正确初始化DbContext：

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

namespace WorldCupApi.Repository.Entities;

public class WorldCupApiDbContextFactory : IDesignTimeDbContextFactory<WorldCupApiDbContext>
{
    public WorldCupApiDbContext CreateDbContext(string[] args)
    {
        // 加载Web项目的配置文件，确保连接字符串一致
        IConfigurationRoot configuration = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("../WorldCupApi.Web/appsettings.json") // 根据实际项目路径调整
            .Build();

        var optionsBuilder = new DbContextOptionsBuilder<WorldCupApiDbContext>();
        optionsBuilder.UseSqlServer(configuration.GetConnectionString("DefaultConnection"),
            b => b.MigrationsAssembly("WorldCupApi.Repository"));

        return new WorldCupApiDbContext(optionsBuilder.Options);
    }
}

3. 验证连接字符串配置 #

确保Web项目的appsettings.json中存在有效连接字符串：

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=WorldCupApiDb;Trusted_Connection=True;MultipleActiveResultSets=true"
  }
}

4. 临时方案：添加无参构造函数（不推荐生产环境使用） #

迁移工具在部分场景下会尝试使用无参构造函数创建实例，可临时添加：

public class WorldCupApiDbContext : DbContext
{
    // 保留原有依赖注入构造函数
    public WorldCupApiDbContext(DbContextOptions<WorldCupApiDbContext> options) : base(options)
    {
    }

    // 仅用于迁移的无参构造函数
    protected WorldCupApiDbContext()
    {
    }

    // ... DbSet定义
}

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