ASP.NET Core中WebApiTestContext服务无法解析问题求助

我是ASP.NET新手，正在开发一个Hello World级别的GET API，目标是从MySQL数据库获取数据并返回给客户端。

我使用以下命令生成DbContext及相关文件：

dotnet ef dbcontext scaffold "Server=localhost;Database=webApiTest;User Id=root;Password=000;" Microsoft.EntityFrameworkCore.SqlServer -o Models/

由于项目默认没有Startup.cs，我手动创建该文件以将自动生成的WebApiTestContext注册为依赖服务：

using Microsoft.EntityFrameworkCore;
using WebApplication3.Models;

namespace WebApplication3;

public class Startup
{
    private readonly IConfiguration _configuration;
    
    public Startup(IConfiguration configuration)
    {
        _configuration = configuration;
    }
    
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<WebApiTestContext>(options =>
            options.UseMySQL(_configuration.GetConnectionString("YourConnectionString")));
    }
}

UserController通过依赖注入使用WebApiTestContext：

using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using WebApplication3.DTO;
using WebApplication3.Models;

namespace WebApplication3.Controllers;
[ApiController]
[Route("api/[controller]")]
public class UserController : ControllerBase
{
    private readonly WebApiTestContext dbContext;
    
    public UserController(WebApiTestContext dbContext)
    {
        this.dbContext = dbContext;
    }

    [HttpGet("GetUsers")]
    public async Task<ActionResult<List<UserDTO>>> Get()
    {
        var List = await dbContext.Users.Select(s=> new UserDTO
        {
            Id = s.Id,
            FirstName = s.FirstName,
            LastName = s.LastName,
            UserName = s.Username,
            Password = s.Password,
            EnrollmentDate = s.EnrollmentDate,
        }).ToListAsync();
        if (List.Count < 0)
        {
            return NotFound();
        }
        return List;
    }
}

自动生成的WebApiTestContext代码如下：

using System;
using System.Collections.Generic;
using Microsoft.EntityFrameworkCore;

namespace WebApplication3.Models;

public partial class WebApiTestContext : DbContext
{
    public WebApiTestContext()
    {
    }

    public WebApiTestContext(DbContextOptions<WebApiTestContext> options)
        : base(options)
    {
    }

    public virtual DbSet<User> Users { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
#warning To protect potentially sensitive information in your connection string, you should move it out of source code. You can avoid scaffolding the connection string by using the Name= syntax to read it from configuration - see https://go.microsoft.com/fwlink/?linkid=2131148. For more guidance on storing connection strings, see http://go.microsoft.com/fwlink/?LinkId=723263.
        => optionsBuilder.UseMySQL("Server=localhost;Database=webApiTest;User Id=root;Password=000;");

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>(entity =>
        {
            entity.HasKey(e => e.Id).HasName("PRIMARY");

            entity.ToTable("user");

            entity.Property(e => e.EnrollmentDate).HasColumnType("datetime");
            entity.Property(e => e.FirstName).HasMaxLength(45);
            entity.Property(e => e.LastName).HasMaxLength(45);
            entity.Property(e => e.Password).HasMaxLength(45);
            entity.Property(e => e.Username).HasMaxLength(45);
        });

        OnModelCreatingPartial(modelBuilder);
    }

    partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
}

运行时出现如下错误：

System.InvalidOperationException: Unable to resolve service for type 'WebApplication3.Models.WebApiTestContext' while attempting to activate 'WebApplication3.Controllers.UserController'.

1. 修正EF Core脚手架命令，重新生成DbContext #

你使用了SQL Server的EF Core Provider（Microsoft.EntityFrameworkCore.SqlServer）连接MySQL，这会导致生成的DbContext不兼容。需要改用MySQL专用的Pomelo EF Core Provider：

首先安装依赖包：

dotnet add package Pomelo.EntityFrameworkCore.MySql
dotnet add package Microsoft.EntityFrameworkCore.Design

然后重新执行脚手架命令：

dotnet ef dbcontext scaffold "Server=localhost;Database=webApiTest;User Id=root;Password=000;" Pomelo.EntityFrameworkCore.MySql -o Models/

2. 移除手动创建的Startup.cs，在Program.cs中注册DbContext #

.NET 6及以后版本默认使用顶级语句，不再需要单独的Startup.cs，手动创建的Startup.cs不会被框架自动加载，这是依赖注入失败的核心原因。

打开Program.cs，添加DbContext注册代码：

var builder = WebApplication.CreateBuilder(args);

// 注册DbContext，替换为你的MySQL版本
builder.Services.AddDbContext<WebApplication3.Models.WebApiTestContext>(options =>
    options.UseMySql(builder.Configuration.GetConnectionString("WebApiTest"), 
        new MySqlServerVersion(new Version(8, 0, 36))));

// 添加控制器服务
builder.Services.AddControllers();

var app = builder.Build();

// 配置HTTP管道
app.UseHttpsRedirection();
app.UseAuthorization();
app.MapControllers();

app.Run();

3. 在appsettings.json中配置连接字符串 #

打开appsettings.json，添加连接字符串节点：

{
  "ConnectionStrings": {
    "WebApiTest": "Server=localhost;Database=webApiTest;User Id=root;Password=000;"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

4. 清理WebApiTestContext中的硬编码连接字符串 #

自动生成的DbContext中，OnConfiguring方法的硬编码连接字符串会覆盖依赖注入的配置，修改为仅在未配置时生效：

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    if (!optionsBuilder.IsConfigured)
    {
#warning 请将连接字符串移出源代码以保护敏感信息
        optionsBuilder.UseMySql("Server=localhost;Database=webApiTest;User Id=root;Password=000;", new MySqlServerVersion(new Version(8, 0, 36)));
    }
}

5. 修正UserController中的逻辑错误 #

List.Count < 0永远为false（集合Count不可能为负数），改为判断集合是否为空：

[HttpGet("GetUsers")]
public async Task<ActionResult<List<UserDTO>>> Get()
{
    var userList = await dbContext.Users.Select(s=> new UserDTO
    {
        Id = s.Id,
        FirstName = s.FirstName,
        LastName = s.LastName,
        UserName = s.Username,
        Password = s.Password,
        EnrollmentDate = s.EnrollmentDate,
    }).ToListAsync();
    
    if (!userList.Any())
    {
        return NotFound();
    }
    
    return userList;
}

内容的提问来源于stack exchange，提问作者S k