EF 6 新特性六

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正文

介绍

接下来我将给大家重点介绍一下.Net 6 之后的一些新的变更，文章都是来自于外国大佬的文章，我这边进行一个翻译，并加上一些自己的理解和解释。

源作者链接：https://blog.okyrylchuk.dev/entity-framework-core-6-features-part-2

正文

没有依赖注入的 DbContext 池化

在 EF Core 6.0 中，您可以使用DbContext池而无需依赖注入。PooledDbContextFactory类型已公开。该池是使用DbContextOptions的实例创建的，该实例将用于创建上下文实例。

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)
    {
    }
}

命令源枚举

在 EF Core 6.0 中，新的枚举CommandSource已添加到CommandEventData类型，提供给诊断源和拦截器。枚举值指示 EF 的哪个部分创建命令。

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;
    }
}

数据库上下文：

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; }
}

程序：

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
*/

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。当您有一个具有未知值的枚举时，它很有用，并且它在表中表示为一个可为空的字符串列。

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，它有陷阱。详情按链接

显式设置临时值

在 EF Core 6.0 中，您可以在跟踪实体之前显式设置临时值。当值被标记为临时值时，EF 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");
}