.Net 8添加EF迁移报错：无法创建CoffeeShopContext实例

报错信息 #

Unable to create a 'DbContext' of type 'CoffeeShopContext'. The exception 'Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContextOptions`1[DataLayer.Data.ContextManager.CoffeeShopContext]' while attempting to activate 'DataLayer.Data.ContextManager.CoffeeShopContext'.' was thrown while attempting to create an instance.

相关代码 #

Program.cs #

using CoffeeShopAPI.DbIntializer;
using CoffeeShopAPI.ServiceManager;
using DataLayer.Data.ContextManager;
using Microsoft.EntityFrameworkCore;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

builder.Services.AddDbContext<CoffeeShopContext>(options =>
    options.UseSqlite(builder.Configuration.GetConnectionString("DefaultConnection")));


builder.Services.AddApplicationServices();

var app = builder.Build();

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

app.UseHttpsRedirection();
app.MapControllers();

using (var scope = app.Services.CreateScope())
{
    var db = scope.ServiceProvider.GetRequiredService<CoffeeShopContext>();
    db.Database.Migrate();
    DbInitializer.Initialize(db);
}


app.Run();

Database Context（位于DataLayer > Data > ContextManager类库） #

public class CoffeeShopContext: DbContext
{
    public CoffeeShopContext(DbContextOptions<CoffeeShopContext> options)
         : base(options)
    {

    }

    public DbSet<CoffeeShop> CoffeeShops { get; set; }    
}

解决方案 #

• 指定启动项目执行迁移命令：由于DbContext在单独类库中，EF Core设计时无法自动识别Web项目的服务配置，执行迁移时需明确指定启动项目。示例命令：

  dotnet ef migrations add InitialCreate --project DataLayer --startup-project CoffeeShopAPI
  

  其中--project指定DbContext所在的类库项目，--startup-project指定包含Program.cs的Web API项目。

• 实现IDesignTimeDbContextFactory：在DataLayer类库中添加一个实现IDesignTimeDbContextFactory<CoffeeShopContext>的类，让EF Core在设计时能直接创建DbContext实例：

  using DataLayer.Data.ContextManager;
  using Microsoft.EntityFrameworkCore;
  using Microsoft.EntityFrameworkCore.Design;
  using Microsoft.Extensions.Configuration;
  using System.IO;
  
  public class CoffeeShopContextFactory : IDesignTimeDbContextFactory<CoffeeShopContext>
  {
      public CoffeeShopContext CreateDbContext(string[] args)
      {
          IConfigurationRoot configuration = new ConfigurationBuilder()
              .SetBasePath(Directory.GetCurrentDirectory())
              .AddJsonFile("../CoffeeShopAPI/appsettings.json") // 指向Web项目的配置文件路径
              .Build();
  
          var optionsBuilder = new DbContextOptionsBuilder<CoffeeShopContext>();
          optionsBuilder.UseSqlite(configuration.GetConnectionString("DefaultConnection"));
  
          return new CoffeeShopContext(optionsBuilder.Options);
      }
  }
  

  注意根据项目实际结构调整appsettings.json的路径，确保设计时能读取到连接字符串。

• 检查连接字符串配置：确认Web项目的appsettings.json中存在格式正确的DefaultConnection：

  {
    "ConnectionStrings": {
      "DefaultConnection": "Data Source=CoffeeShop.db"
    }
  }
  

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