继上一篇之后，这一篇将给大家带来另外十个 EF Core 6 中的新功能特性，包括值转换器、脚手架和 DbContext 的改进等。

1HasConversion 支持值转换器

在 EF Core 6.0 中，HasConversion 方法的泛型重载方法可以指定内置或自定义的值转换器。

public class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Person>().Property(p => p.Address).HasConversion<AddressConverter>();}
}
public class Person
{public int Id { get; set; }public string FirstName { get; set; }public string LastName { get; set; }public Address Address { get; set; }
}
public class Address
{public string Country { get; set; }public string Street { get; set; }public string ZipCode { get; set; }
}
public class AddressConverter : ValueConverter<Address, string>
{public AddressConverter(): base(v => JsonSerializer.Serialize(v, (JsonSerializerOptions)null),v => JsonSerializer.Deserialize<Address>(v, (JsonSerializerOptions)null)){}
}

2简化多对多关系的配置

从 EF Core 6.0 开始，你可以在多对多的关系中配置一个连接实体，而无需任何额外的配置。另外，你可以配置一个连接实体，而不需要明确指定左右关系。

public class BloggingContext : DbContext
{public DbSet<Post> Posts { get; set; }public DbSet<Tag> Tags { get; set; }public DbSet<PostTag> PostTags { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Post>().HasMany(p => p.Tags).WithMany(t => t.Posts).UsingEntity<PostTag>();}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=EFCore6Many2Many;Trusted_Connection=True;");
}
public class Post
{public int Id { get; set; }public string Name { get; set; }public List<Tag> Tags { get; set; } = new List<Tag>();
}
public class Tag
{public int Id { get; set; }public string Text { get; set; }public List<Post> Posts { get; set; } = new List<Post>();
}
public class PostTag
{public int PostId { get; set; }public int TagId { get; set; }public DateTime AddedDate { get; set; }
}

3脚手架多对多关系的改进

EF Core 6.0 改进了现有数据库的脚手架。它可以检测到连接表并为其生成多对多的映射。

如下示例数据库：

通过 CLI：

dotnet ef dbcontext scaffold "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=EFCore6Many2Many" Microsoft.EntityFrameworkCore.SqlServer --context ExampleContext --output-dir Models

来自生成的 DbContext 的 OnModelCreating：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{modelBuilder.Entity<Post>(entity =>{entity.HasMany(d => d.Tags).WithMany(p => p.Posts).UsingEntity<Dictionary<string, object>>("PostTag",l => l.HasOne<Tag>().WithMany().HasForeignKey("TagId"),r => r.HasOne<Post>().WithMany().HasForeignKey("PostId"),j =>{j.HasKey("PostId", "TagId")j.ToTable("PostTags")j.HasIndex(new[] { "TagId" }, "IX_PostTags_TagId");});});OnModelCreatingPartial(modelBuilder);
}

4脚手架生成可空引用类型

EF Core 6.0 改进了现有数据库的脚手架。当项目中启用了可空引用类型（NRT），EF Core 会自动用 NRT 构建 DbContext 和实体类型。

如实例表：

CREATE TABLE [Posts] ([Id] int NOT NULL IDENTITY,[Name] nvarchar(max) NOT NULL,[Description] nvarchar(max) NULL,CONSTRAINT [PK_Posts] PRIMARY KEY ([Id])
)

会生成模型：

public partial class Post
{public int Id { get; set; }public string Name { get; set; } = null!;public string? Desciption { get; set; }
}

5脚手架生成数据库注释

EF Core 6.0 将数据库注释与代码注释关联起来。

数据库例子：

CREATE TABLE [Posts] ([Id] int NOT NULL IDENTITY,[Name] nvarchar(max) NOT NULL,[Description] nvarchar(max) NULL,CONSTRAINT [PK_Posts] PRIMARY KEY ([Id]));
EXEC sp_addextendedproperty@name = N'MS_Description', @value = 'The post table',@level0type = N'Schema', @level0name = dbo,@level1type = N'Table',  @level1name = Posts
EXEC sp_addextendedproperty@name = N'MS_Description', @value = 'The post identifier',@level0type = N'Schema', @level0name = dbo,@level1type = N'Table',  @level1name = Posts,@level2type = N'Column', @level2name = [Id];
EXEC sp_addextendedproperty@name = N'MS_Description', @value = 'The post name',@level0type = N'Schema', @level0name = dbo,@level1type = N'Table',  @level1name = Posts,@level2type = N'Column', @level2name = [Name];
EXEC sp_addextendedproperty@name = N'MS_Description', @value = 'The description name',@level0type = N'Schema', @level0name = dbo,@level1type = N'Table',  @level1name = Posts,@level2type = N'Column', @level2name = [Description];

生成的模型：

/// <summary>
/// The post table
/// </summary>
public partial class Post
{/// <summary>/// The post identifier/// </summary>public int Id { get; set; }/// <summary>/// The post name/// </summary>public string Name { get; set; }/// <summary>/// The description name/// </summary>public string Description { get; set; }
}

6AddDbContextFactory 注册 DbContext

在 EF Core 5.0 中，你可以注册一个工厂来手动创建 DbContext 实例。从 EF Core 6.0 开始，可用 AddDbContextFactory 注册 DbContext。所以你可以根据你的需要同时注入工厂和 DbContext。

var serviceProvider = new ServiceCollection().AddDbContextFactory<ExampleContext>(builder =>builder.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database = EFCore6Playground")).BuildServiceProvider();var factory = serviceProvider.GetService<IDbContextFactory<ExampleContext>>();
using (var context = factory.CreateDbContext())
{// Contexts obtained from the factory must be explicitly disposed
}using (var scope = serviceProvider.CreateScope())
{var context = scope.ServiceProvider.GetService<ExampleContext>();// Context is disposed when the scope is disposed
}
class ExampleContext : DbContext
{ }

7无依赖性注入的 DbContext 池

在 EF Core 6.0 中，你可以使用没有依赖注入的 DbContext 池。PooledDbContextFactory 类型已经定义为 public 了。池是用 DbContextOptions 创建的，它将被用来创建 DbContext 实例。

var options = new DbContextOptionsBuilder<ExampleContext>().UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6Playground").Options;var factory = new PooledDbContextFactory<ExampleContext>(options);using var context1 = factory.CreateDbContext();
Console.WriteLine($"Created DbContext with ID {context1.ContextId}");
// Output: Created DbContext with ID e49db9b7-a0b0-4b54-8d0d-2cbd6c4cece7:1using var context2 = factory.CreateDbContext();
Console.WriteLine($"Created DbContext with ID {context2.ContextId}");
// Output: Created DbContext with ID b5a35bcb-270d-40f1-b668-5f76da1f35ad:1class ExampleContext : DbContext
{public ExampleContext(DbContextOptions<ExampleContext> options): base(options){}
}

8CommandSource 枚举

在 EF Core 6.0 中，新的枚举 CommandSource 已经被添加到 CommandEventData 类型中，提供给诊断源和拦截器。这个枚举值表明了 EF 的哪个部分创建了这个命令。

在 Db 命令拦截器中使用 CommandSource：

class ExampleInterceptor : DbCommandInterceptor
{public override InterceptionResult<DbDataReader> ReaderExecuting(DbCommand command,CommandEventData eventData, InterceptionResult<DbDataReader> result){if (eventData.CommandSource == CommandSource.SaveChanges){Console.WriteLine($"Saving changes for {eventData.Context.GetType().Name}:");Console.WriteLine();Console.WriteLine(command.CommandText);}if (eventData.CommandSource == CommandSource.FromSqlQuery){Console.WriteLine($"From Sql query for {eventData.Context.GetType().Name}:");Console.WriteLine();Console.WriteLine(command.CommandText);}return result;}
}

DbContext:

class ExampleContext : DbContext
{public DbSet<Product> Products { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6CommandSource").AddInterceptors(new ExampleInterceptor());
}
class Product
{public int Id { get; set; }public string Name { get; set; }public decimal Price { get; set; }
}

Program:

using var context = new ExampleContext();context.Products.Add(new Product { Name = "Laptop", Price = 1000 });
context.SaveChanges();var product = context.Products.FromSqlRaw("SELECT * FROM dbo.Products").ToList();/* Output:
Saving changes for ExampleContext:SET NOCOUNT ON;
INSERT INTO[Products] ([Name], [Price])
VALUES(@p0, @p1);
SELECT[Id]
FROM[Products]
WHERE @@ROWCOUNT = 1 AND[Id] = scope_identity();From Sql query for ExampleContext:SELECT* FROM dbo.Products
*/

9值转换器允许转换空值

在 EF Core 6.0 中，值转换器允许转换空值。当你有一个未知值的枚举，并且它在表中表示一个可空的字符串列时，这很有用。

public class ExampleContext : DbContext
{public DbSet<Dog> Dogs { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Dog>().Property(c => c.Breed).HasConversion<BreedConverter>();}protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder){optionsBuilder.EnableSensitiveDataLogging().LogTo(Console.WriteLine).UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=EFCore6ValueConverterAllowsNulls;");}
}
public enum Breed
{Unknown,Beagle,Bulldog
}
public class Dog
{public int Id { get; set; }public string Name { get; set; }public Breed? Breed { get; set; }
}
public class BreedConverter : ValueConverter<Breed, string>
{
#pragma warning disable EF1001public BreedConverter(): base(v => v == Breed.Unknown ? null : v.ToString(),v => v == null ? Breed.Unknown : Enum.Parse<Breed>(v),convertsNulls: true){}
#pragma warning restore EF1001
}

但要注意，这里面有陷阱。详情请见链接：

https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-6.0/whatsnew#allow-value-converters-to-convert-nulls

10明确设置临时值

在 EF Core 6.0 中，你可以在实体被追踪之前显式地给它们设置临时值。当值被标记为临时值时，EF 将不会像以前那样重置它。

using var context = new ExampleContext();Blog blog = new Blog { Id = -5 };
context.Add(blog).Property(p => p.Id).IsTemporary = true;var post1 = new Post { Id = -1 };
var post1IdEntry = context.Add(post1).Property(e => e.Id).IsTemporary = true;
post1.BlogId = blog.Id;var post2 = new Post();
var post2IdEntry = context.Add(post2).Property(e => e.Id).IsTemporary = true;
post2.BlogId = blog.Id;Console.WriteLine($"Blog explicitly set temporary ID = {blog.Id}");
Console.WriteLine($"Post 1 explicitly set temporary ID = {post1.Id} and FK to Blog = {post1.BlogId}");
Console.WriteLine($"Post 2 generated temporary ID = {post2.Id} and FK to Blog = {post2.BlogId}");// Output:
// Blog explicitly set temporary ID = -5
// Post 1 explicitly set temporary ID = -1 and FK to Blog = -5
// Post 2 generated temporary ID = -2147482647 and FK to Blog = -5class Blog
{public int Id { get; set; }
}
class Post
{public int Id { get; set; }public int BlogId { get; set; }
}
class ExampleContext : DbContext
{public DbSet<Blog> Blogs { get; set; }public DbSet<Post> Posts { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFCore6TempValues");
}

11结尾

本文所有代码示例都可以在我的 GitHub 中找到：

https://github.com/okyrylchuk/dotnet6_features/tree/main/EF%20Core%206#miscellaneous-enhancements