Entity Framework (EF) Core 是轻量化、可扩展、开源和跨平台版的常用 Entity Framework 数据访问技术。

EF Core 可用作对象关系映射程序 (O/RM)

• 通过依赖关系

 public void ConfigureServices(IServiceCollection services) {     services.AddControllers();     services.AddDbContext<ApplicationDbContext>(         options => options.UseSqlServer("name=ConnectionStrings:DefaultConnection")); } public class ApplicationDbContext : DbContext {     public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)         : base(options)     {     } } public class MyController {     private readonly ApplicationDbContext _context;     public MyController(ApplicationDbContext context)     {         _context = context;     } }

• 通过关键字 “new”

 public class ApplicationDbContext : DbContext {     private readonly string _connectionString;     public ApplicationDbContext(string connectionString)     {         _connectionString = connectionString;     }     protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)     {         optionsBuilder.UseSqlServer(_connectionString);     } } //或者 public class ApplicationDbContext : DbContext {     public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)         : base(options)     {     } } var contextOptions = new DbContextOptionsBuilder<ApplicationDbContext>()     .UseSqlServer(@"Server=(localdb)mssqllocaldb;Database=Test")     .Options;  using var context = new ApplicationDbContext(contextOptions);

• 在 AddDbContext 和相关方法中
• 在 OnConfiguring 中
• 使用 new 显式构造

 public sealed class ApplicationDbContext1 : ApplicationDbContextBase {     public ApplicationDbContext1(DbContextOptions<ApplicationDbContext1> contextOptions)         : base(contextOptions)     {     } } public sealed class ApplicationDbContext2 : ApplicationDbContextBase {     public ApplicationDbContext2(DbContextOptions<ApplicationDbContext2> contextOptions)         : base(contextOptions)     {     } }    protected ApplicationDbContext(DbContextOptions contextOptions)         : base(contextOptions)     {     }

DBContext的配置入口

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)     {         //配置数据库提供程序，还有其他数据库配置如下图：         optionsBuilder.UseSqlServer(@"Server=(localdb)mssqllocaldb;Database=Test");         //日志记录配置，通过在控制台输出         optionsBuilder.LogTo(Console.WriteLine, LogLevel.Information).EnableSensitiveDataLogging();         //关于其他DBContext配置，参看下图     }     protected override void OnModelCreating(ModelBuilder modelBuilder)     {         base.OnModelCreating(modelBuilder, typeof(BaseEntity));     }

实体的配置入口方法

      protected override void OnModelCreating(ModelBuilder modelBuilder)     {          modelBuilder.Entity<AuditEntry>();     }

 modelBuilder.Entity<Blog>()             .Property(b => b.Url)             .IsRequired();  //代替修改实体 还有这种等等             [NotMapped] public class BlogMetadata protected override void OnModelCreating(ModelBuilder modelBuilder) {     modelBuilder.Ignore<BlogMetadata>(); }

     public class BloggingContext : DbContext {     public DbSet<Blog> Blogs { get; set; }     public DbSet<Post> Posts { get; set; }      protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)     {         optionsBuilder.UseSqlServer(             @"Server=(localdb)mssqllocaldb;Database=Blogging;Trusted_Connection=True");     } }  public class Blog {     public int BlogId { get; set; }     public string Url { get; set; }     public int Rating { get; set; }     public List<Post> Posts { get; set; } }

• 在上下文的 DbSet 属性中公开。
• 通过实体 Blog.Posts 的导航属性发现的。
• 在 OnModelCreating 中指定的。

 internal class MyContext : DbContext {      public DbSet<Blog> Blogs { get; set; }      protected override void OnModelCreating(ModelBuilder modelBuilder)     {          modelBuilder.Entity<AuditEntry>();     } }  public class Blog {     public string Url { get; set; }      public List<Post> Posts { get; set; } }  public class Post {      public string Content { get; set; }  }

  _mIPODataDBContext.Entry(t_TaskOrderFiles).Property(x => x.Url).IsModified = true;  _mIPODataDBContext.SaveChanges();

 //跟踪 var blog = context.Blogs.SingleOrDefault(b => b.BlogId == 1); blog.Rating = 5; context.SaveChanges(); //不跟踪 var blogs = context.Blogs     .AsNoTracking()     .ToList(); //在上下文实例级别取消跟踪 context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;

   //同级Include，子级，更深级别的关联 ThenInclude   var blogs = context.Blogs         .Include(blog => blog.Posts)         .ThenInclude(post => post.Author)         .ToList();

 var query = from photo in context.Set<PersonPhoto>()             join person in context.Set<Person>()                 on new { Id = (int?)photo.PersonPhotoId, photo.Caption }                 equals new { Id = person.PhotoId, Caption = "SN" }             where b.ProjectOrderId == Guid.Parse(proIdParam.Value) && p.IsDel == false             select new { person, photo };              SELECT 
.[PersonId], 
.[Name], 
.[PhotoId], [p0].[PersonPhotoId], [p0].[Caption], [p0].[Photo] FROM [PersonPhoto] AS [p0] INNER JOIN [Person] AS 
 ON ([p0].[PersonPhotoId] = 
.[PhotoId] AND ([p0].[Caption] = N'SN'))

 var query = from b in context.Set<Blog>()             from p in context.Set<Post>().Where(p => b.BlogId == p.BlogId)             select new { b, p };              SELECT [b].[BlogId], [b].[OwnerId], [b].[Rating], [b].[Url], 
.[PostId], 
.[AuthorId], 
.[BlogId], 
.[Content], 
.[Rating], 
.[Title] FROM [Blogs] AS [b] INNER JOIN [Posts] AS 
 ON [b].[BlogId] = 
.[BlogId]

 var query = from b in context.Set<Blog>()             join p in context.Set<Post>()                 on b.BlogId equals p.BlogId into grouping             from p in grouping.DefaultIfEmpty()             select new { b, p };

 var query2 = from b in context.Set<Blog>()              from p in context.Set<Post>().Where(p => b.BlogId == p.BlogId).DefaultIfEmpty()              select new { b, p };  SELECT [b].[BlogId], [b].[OwnerId], [b].[Rating], [b].[Url], 
.[PostId], 
.[AuthorId], 
.[BlogId], 
.[Content], 
.[Rating], 
.[Title] FROM [Blogs] AS [b] LEFT JOIN [Posts] AS 
 ON [b].[BlogId] = 
.[BlogId]

 var query = from b in context.Set<Blog>()             join p in context.Set<Post>()                 on b.BlogId equals p.BlogId into grouping             select new { b, Posts = grouping.Where(p => p.Content.Contains("EF")).ToList() };

 var query = from p in context.Set<Post>()             group p by p.AuthorId             into g             where g.Count() > 0             orderby g.Key             select new { g.Key, Count = g.Count() };              SELECT 
.[AuthorId] AS [Key], COUNT(*) AS [Count] FROM [Posts] AS 
 GROUP BY 
.[AuthorId] HAVING COUNT(*) > 0 ORDER BY 
.[AuthorId]    

  var rowsModified = context.Database.ExecuteSql($"UPDATE [Blogs] SET [Url] = NULL");    var overAverageIds = context.Database     .SqlQuery<int>($"SELECT [BlogId] AS [Value] FROM [Blogs]")     .Include(b => b.Posts)     .Where(id => id > context.Blogs.Average(b => b.BlogId))     .ToList();