.NET6 DbContext未配置数据库提供程序（无需OnConfiguring）

问题描述 #

执行EF Core迁移时遇到两类错误：

错误1：未添加DbContext空构造函数时 #

Unable to create an object of type 'ApplicationDbContext'. For the different patterns supported at design time...

对应的DbContext代码：

public class ApplicationDbContext : IdentityDbContext<Pessoas>
{
    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options): base(options) { }
    
    public DbSet<Pessoas> Pessoas { get; set; }
    public DbSet<Creditos> Creditos { get; set; }
    public DbSet<Debitos> Debitos { get; set; }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Ignore<Notification>();
        builder.Entity<Pessoas>().ToTable(nameof(Pessoas)).HasKey(p => p.Id);
        base.OnModelCreating(builder);
    }

    protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
    {
        configurationBuilder.Properties<String>()
            .HaveMaxLength(100);
        configurationBuilder.Properties<int>()
            .HaveMaxLength(4);
        base.ConfigureConventions(configurationBuilder);
    }
}

错误2：添加空构造函数后 #

No database provider has been configured for this DbContext. A provider can be configured by overriding the 'DbContext.OnConfiguring' method or by using 'AddDbContext' on the application service provider. If 'AddDbContext' is used, then also ensure that your DbContext type accepts a DbContextOptions object in its constructor and passes it to the base constructor for DbContext.

对应的DbContext代码：

public class ApplicationDbContext: IdentityDbContext<Pessoas>
{
    public ApplicationDbContext()
    {
    }

    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options): base(options) { }
    
    public DbSet<Pessoas> Pessoas { get; set; }
    public DbSet<Creditos> Creditos { get; set; }
    public DbSet<Debitos> Debitos { get; set; }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Ignore<Notification>();
        builder.Entity<Pessoas>().ToTable(nameof(Pessoas)).HasKey(p => p.Id);
        base.OnModelCreating(builder);
    }

    protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
    {
        configurationBuilder.Properties<String>()
            .HaveMaxLength(100);
        configurationBuilder.Properties<int>()
            .HaveMaxLength(4);
        base.ConfigureConventions(configurationBuilder);
    }
}

当前Program.cs配置 #

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddDbContext<ApplicationDbContext>(
    options => options.UseSqlServer(builder.Configuration["ConnectionString:Connection"])
);
builder.Services.AddIdentity<Pessoas, IdentityRole>(options =>
{
    options.Password.RequireNonAlphanumeric = false;
    options.Password.RequireDigit = false;
    options.Password.RequireUppercase = false;
    options.Password.RequireLowercase = false;
    options.Password.RequiredLength = 3;
}).AddEntityFrameworkStores<ApplicationDbContext>()
    .AddDefaultTokenProviders();

builder.Services.AddScoped<IPessoas, PessoasRepository>();
builder.Services.AddScoped<IPessoasApplication, PessoasService>();

builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

目前只有重写DbContext.OnConfiguring方法并显式指定连接字符串才能正常执行迁移，但希望找到无需重写该方法的解决方案——教程中仅通过Program.cs配置即可正常迁移，克隆的示例项目自己执行迁移时出现相同错误，但朋友执行却成功。

解决方案 #

1. 移除DbContext的空构造函数 #

EF Core设计时工具会优先尝试使用无参构造函数，导致无法读取Program.cs中配置的DbContextOptions。保留带DbContextOptions<ApplicationDbContext>参数的构造函数即可，无需空构造。

2. 执行迁移时指定启动项目（如果DbContext在类库） #

如果你的DbContext类在单独的类库项目中，而Program.cs在Web项目里，执行迁移命令时必须指定启动项目，让EF Core能读取到配置：

使用.NET CLI：

dotnet ef migrations add InitialCreate --project YourDbContextLibraryName --startup-project YourWebProjectName

使用Package Manager Console：

Add-Migration InitialCreate -Project YourDbContextLibraryName -StartupProject YourWebProjectName

3. 验证连接字符串配置 #

确保appsettings.json中的键名与Program.cs中读取的ConnectionString:Connection完全匹配，示例：

{
  "ConnectionString": {
    "Connection": "Server=.;Database=YourDatabaseName;Trusted_Connection=True;TrustServerCertificate=True;"
  }
}

4. 统一EF Core包版本 #

检查项目中所有EF Core相关包（Microsoft.EntityFrameworkCore、Microsoft.EntityFrameworkCore.SqlServer、Microsoft.EntityFrameworkCore.Tools）的版本是否一致，且与当前使用的.NET SDK版本兼容。版本不匹配会导致设计时工具无法正确加载配置。

5. 清理项目缓存并重新生成 #

清理解决方案的bin和obj目录，然后重新生成整个解决方案，再尝试执行迁移命令，避免缓存导致的配置读取异常。

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