EF Core 6 新功能汇总
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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($"Created DbContext with ID context1.ContextId");
// Output: Created DbContext with ID e49db9b7-a0b0-4b54-8d0d-2cbd6c4cece7:1
using var context2 = factory.CreateDbContext();
Console.WriteLine($"Created DbContext with ID context2.ContextId");
// Output: Created DbContext with ID b5a35bcb-270d-40f1-b668-5f76da1f35ad:1
class 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 EF1001
public 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-nulls10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 = -5
class 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原文：bit.ly/3G6Yt3m
作者：Oleg Kyrylchuk
翻译：精致码农
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