在这篇文章中，你将看到 EF Core 6 中的十个新功能，包括新的特性标注，对时态表、稀疏列的支持，以及其他新功能。

1Unicode 特性

在 EF Core 6.0 中，新的 UnicodeAttribute 允许你将一个字符串属性映射到一个非 Unicode 列，而不需要直接指定数据库类型。当数据库系统只支持 Unicode 类型时，Unicode 特性会被忽略。

public class Book
{public int Id { get; set; }public string Title { get; set; }[Unicode(false)][MaxLength(22)]public string Isbn { get; set; }
}

对应的迁移代码：

protected override void Up(MigrationBuilder migrationBuilder)
{migrationBuilder.CreateTable(name: "Books",columns: table => new{Id = table.Column<int>(type: "int", nullable: false).Annotation("SqlServer:Identity", "1, 1"),Title = table.Column<string>(type: "nvarchar(max)", nullable: true),Isbn = table.Column<string>(type: "varchar(22)", unicode: false, maxLength: 22, nullable: true)},constraints: table =>{table.PrimaryKey("PK_Books", x => x.Id);});
}

2Precision 特性

在 EF Core 6.0 之前，你可以用 Fluent API 配置精度。现在，你也可以用数据标注和一个新的 PrecisionAttribute 来做这件事。

public class Product
{public int Id { get; set; }[Precision(precision: 10, scale: 2)]public decimal Price { get; set; }
}

对应的迁移代码：

protected override void Up(MigrationBuilder migrationBuilder)
{migrationBuilder.CreateTable(name: "Products",columns: table => new{Id = table.Column<int>(type: "int", nullable: false).Annotation("SqlServer:Identity", "1, 1"),Price = table.Column<decimal>(type: "decimal(10,2)", precision: 10, scale: 2, nullable: false)},constraints: table =>{table.PrimaryKey("PK_Products", x => x.Id);});
}

3EntityTypeConfiguration 特性

从 EF Core 6.0 开始，你可以在实体类型上放置一个新的 EntityTypeConfiguration 特性，这样 EF Core 就可以找到并使用适当的配置。在此之前，类的配置必须被实例化并从 OnModelCreating 方法中调用。

public class ProductConfiguration : IEntityTypeConfiguration<Product>
{public void Configure(EntityTypeBuilder<Product> builder){builder.Property(p => p.Name).HasMaxLength(250);builder.Property(p => p.Price).HasPrecision(10, 2);}
}
[EntityTypeConfiguration(typeof(ProductConfiguration))]
public class Product
{public int Id { get; set; }public decimal Price { get; set; }public string Name { get; set; }
}

4Column 特性

当你在模型中使用继承时，你可能不满意创建的表中默认的 EF Core 列顺序。在 EF Core 6.0 中，你可以用 ColumnAttribute 指定列的顺序。

此外，你还可以使用新的 Fluent API--HasColumnOrder() 来实现。

public class EntityBase
{[Column(Order = 1)]public int Id { get; set; }[Column(Order = 99)]public DateTime UpdatedOn { get; set; }[Column(Order = 98)]public DateTime CreatedOn { get; set; }
}
public class Person : EntityBase
{[Column(Order = 2)]public string FirstName { get; set; }[Column(Order = 3)]public string LastName { get; set; }public ContactInfo ContactInfo { get; set; }
}
public class Employee : Person
{[Column(Order = 4)]public string Position { get; set; }[Column(Order = 5)]public string Department { get; set; }
}
[Owned]
public class ContactInfo
{[Column(Order = 10)]public string Email { get; set; }[Column(Order = 11)]public string Phone { get; set; }
}

对应的迁移代码：

protected override void Up(MigrationBuilder migrationBuilder)
{migrationBuilder.CreateTable(name: "Employees",columns: table => new{Id = table.Column<int>(type: "int", nullable: false).Annotation("SqlServer:Identity", "1, 1"),FirstName = table.Column<string>(type: "nvarchar(max)", nullable: true),LastName = table.Column<string>(type: "nvarchar(max)", nullable: true),Position = table.Column<string>(type: "nvarchar(max)", nullable: true),Department = table.Column<string>(type: "nvarchar(max)", nullable: true),ContactInfo_Email = table.Column<string>(type: "nvarchar(max)", nullable: true),ContactInfo_Phone = table.Column<string>(type: "nvarchar(max)", nullable: true),CreatedOn = table.Column<DateTime>(type: "datetime2", nullable: false),UpdatedOn = table.Column<DateTime>(type: "datetime2", nullable: false)},constraints: table =>{table.PrimaryKey("PK_Employees", x => x.Id);});
}

5时态表

EF Core 6.0 支持 SQL Server 的时态表。一个表可以被配置成一个具有 SQL Server 默认的时间戳和历史表的时态表。

public class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<Person>().ToTable("People", b => b.IsTemporal());}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=TemporalTables;Trusted_Connection=True;");
}
public class Person
{public int Id { get; set; }public string Name { get; set; }
}

对应的迁移代码：

protected override void Up(MigrationBuilder migrationBuilder)
{migrationBuilder.CreateTable(name: "People",columns: table => new{Id = table.Column<int>(type: "int", nullable: false).Annotation("SqlServer:Identity", "1, 1"),Name = table.Column<string>(type: "nvarchar(max)", nullable: true),PeriodEnd = table.Column<DateTime>(type: "datetime2", nullable: false).Annotation("SqlServer:IsTemporal", true).Annotation("SqlServer:TemporalPeriodEndColumnName", "PeriodEnd").Annotation("SqlServer:TemporalPeriodStartColumnName", "PeriodStart"),PeriodStart = table.Column<DateTime>(type: "datetime2", nullable: false).Annotation("SqlServer:IsTemporal", true).Annotation("SqlServer:TemporalPeriodEndColumnName", "PeriodEnd").Annotation("SqlServer:TemporalPeriodStartColumnName", "PeriodStart")},constraints: table =>{table.PrimaryKey("PK_People", x => x.Id);}).Annotation("SqlServer:IsTemporal", true).Annotation("SqlServer:TemporalHistoryTableName", "PersonHistory").Annotation("SqlServer:TemporalHistoryTableSchema", null).Annotation("SqlServer:TemporalPeriodEndColumnName", "PeriodEnd").Annotation("SqlServer:TemporalPeriodStartColumnName", "PeriodStart");
}

你可以用以下方法查询和检索历史数据：

• TemporalAsOf

• TemporalAll

• TemporalFromTo

• TemporalBetween

• TemporalContainedIn

使用时态表：

using ExampleContext context = new();
context.People.Add(new() { Name = "Oleg" });
context.People.Add(new() { Name = "Steve" });
context.People.Add(new() { Name = "John" });
await context.SaveChangesAsync();var people = await context.People.ToListAsync();
foreach (var person in people)
{var personEntry = context.Entry(person);var validFrom = personEntry.Property<DateTime>("PeriodStart").CurrentValue;var validTo = personEntry.Property<DateTime>("PeriodEnd").CurrentValue;Console.WriteLine($"Person {person.Name} valid from {validFrom} to {validTo}");
}
// Output:
// Person Oleg valid from 06-Nov-21 17:50:39 PM to 31-Dec-99 23:59:59 PM
// Person Steve valid from 06-Nov-21 17:50:39 PM to 31-Dec-99 23:59:59 PM
// Person John valid from 06-Nov-21 17:50:39 PM to 31-Dec-99 23:59:59 PM

查询历史数据：

var oleg = await context.People.FirstAsync(x => x.Name == "Oleg");
context.People.Remove(oleg);
await context.SaveChangesAsync();
var history = context.People.TemporalAll().Where(e => e.Name == "Oleg").OrderBy(e => EF.Property<DateTime>(e, "PeriodStart")).Select(p => new{Person = p,PeriodStart = EF.Property<DateTime>(p, "PeriodStart"),PeriodEnd = EF.Property<DateTime>(p, "PeriodEnd")}).ToList();
foreach (var pointInTime in history)
{Console.WriteLine($"Person {pointInTime.Person.Name} existed from {pointInTime.PeriodStart} to {pointInTime.PeriodEnd}");
}// Output:
// Person Oleg existed from 06-Nov-21 17:50:39 PM to 06-Nov-21 18:11:29 PM

检索历史数据：

var removedOleg = await context.People.TemporalAsOf(history.First().PeriodStart).SingleAsync(e => e.Name == "Oleg");Console.WriteLine($"Id = {removedOleg.Id}; Name = {removedOleg.Name}");
// Output:
// Id = 1; Name = Oleg

了解更多关于时态表的信息：

https://devblogs.microsoft.com/dotnet/prime-your-flux-capacitor-sql-server-temporal-tables-in-ef-core-6-0/

6稀疏列

EF Core 6.0 支持 SQL Server 稀疏列。在使用 TPH（table per hierarchy）继承映射时，它可能很有用。

public class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }public DbSet<Employee> Employees { get; set; }public DbSet<User> Users { get; set; }protected override void OnModelCreating(ModelBuilder modelBuilder){modelBuilder.Entity<User>().Property(e => e.Login).IsSparse();modelBuilder.Entity<Employee>().Property(e => e.Position).IsSparse();}protected override void OnConfiguring(DbContextOptionsBuilder options)=> options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=SparseColumns;Trusted_Connection=True;");
}public class Person
{public int Id { get; set; }public string Name { get; set; }
}
public class User : Person
{public string Login { get; set; }
}
public class Employee : Person
{public string Position { get; set; }
}

对应迁移代码：

protected override void Up(MigrationBuilder migrationBuilder)
{migrationBuilder.CreateTable(name: "People",columns: table => new{Id = table.Column<int>(type: "int", nullable: false).Annotation("SqlServer:Identity", "1, 1"),Name = table.Column<string>(type: "nvarchar(max)", nullable: false),Discriminator = table.Column<string>(type: "nvarchar(max)", nullable: false),Position = table.Column<string>(type: "nvarchar(max)", nullable: true).Annotation("SqlServer:Sparse", true),Login = table.Column<string>(type: "nvarchar(max)", nullable: true).Annotation("SqlServer:Sparse", true)},constraints: table =>{table.PrimaryKey("PK_People", x => x.Id);});
}

稀疏列有限制，具体请看文档：

https://docs.microsoft.com/en-us/sql/relational-databases/tables/use-sparse-columns?view=sql-server-ver15

7EF Core 中的最小 API

EF Core 6.0 有它自己的最小 API。新的扩展方法可在同一行代码注册一个 DbContext 类型，并提供一个数据库 Provider 的配置。

const string AccountKey = "[CosmosKey]";var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSqlServer<MyDbContext>(@"Server = (localdb)\mssqllocaldb; Database = MyDatabase");// OR
builder.Services.AddSqlite<MyDbContext>("Data Source=mydatabase.db");// OR
builder.Services.AddCosmos<MyDbContext>($"AccountEndpoint=https://localhost:8081/;AccountKey={AccountKey}", "MyDatabase");var app = builder.Build();
app.Run();class MyDbContext : DbContext
{ }

8迁移包

在 EF Core 6.0 中，有一个新的有利于 DevOps 的功能--迁移包。它允许创建一个包含迁移的小型可执行程序。你可以在 CD 中使用它。不需要复制源代码或安装 .NET SDK（只有运行时）。

CLI：

dotnet ef migrations bundle --project MigrationBundles

Package Manager Console:

Bundle-Migration

更多介绍：

https://devblogs.microsoft.com/dotnet/introducing-devops-friendly-ef-core-migration-bundles/

9预设模型配置

EF Core 6.0 引入了一个预设模型配置。它允许你为一个给定的类型指定一次映射配置。例如，在处理值对象时，它可能很有帮助。

public class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }public DbSet<Product> Products { get; set; }protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder){configurationBuilder.Properties<string>().HaveMaxLength(500);configurationBuilder.Properties<DateTime>().HaveConversion<long>();configurationBuilder.Properties<decimal>().HavePrecision(12, 2);configurationBuilder.Properties<Address>().HaveConversion<AddressConverter>();}
}
public class Product
{public int Id { get; set; }public decimal Price { get; set; }
}
public class Person
{public int Id { get; set; }public string FirstName { get; set; }public string LastName { get; set; }public DateTime BirthDate { 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)){}
}

10已编译模型

在 EF Core 6.0 中，你可以生成已编译的模型（compiled models）。当你有一个大的模型，而你的 EF Core 启动很慢时，这个功能是有意义的。你可以使用 CLI 或包管理器控制台来做。

public class ExampleContext : DbContext
{public DbSet<Person> People { get; set; }protected override void OnConfiguring(DbContextOptionsBuilder options){options.UseModel(CompiledModelsExample.ExampleContextModel.Instance)options.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=SparseColumns;Trusted_Connection=True;");}
}
public class Person
{public int Id { get; set; }public string FirstName { get; set; }public string LastName { get; set; }
}

CLI：

dotnet ef dbcontext optimize -c ExampleContext -o CompliledModels -n CompiledModelsExample

Package Manager Console：

Optimize-DbContext -Context ExampleContext -OutputDir CompiledModels -Namespace CompiledModelsExample

更多关于已编译模型及其限制的介绍：

https://devblogs.microsoft.com/dotnet/announcing-entity-framework-core-6-0-preview-5-compiled-models/
https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-6.0/whatsnew#limitations

11结尾

你可以在我的 GitHub 找到本文所有示例代码：

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