1 = 1两个不同的表连接

var query = from post in database.Posts
            join meta in database.Post_Metas on 1 equals 1
            where post.ID == id
            select new { Post = post, Meta = meta };

我做过类似的事情;

var certificationClass = _db.INDIVIDUALLICENSEs
    .Join(_db.INDLICENSECLAsses,
        IL => IL.LICENSE_CLASS,
        ILC => ILC.NAME,
        (IL, ILC) => new { INDIVIDUALLICENSE = IL, INDLICENSECLAsse = ILC })
    .Where(o => 
        o.INDIVIDUALLICENSE.GLOBALENTITYID == "ABC" &&
        o.INDIVIDUALLICENSE.LICENSE_TYPE == "ABC")
    .Select(t => new
        {
            value = t.PSP_INDLICENSECLAsse.ID,
            name = t.PSP_INDIVIDUALLICENSE.LICENSE_CLASS,                
        })
    .OrderBy(x => x.name);

我发现如果你熟悉SQL语法，使用LINQ查询语法会更清晰、更自然，并且更容易发现错误:

var id = 1;
var query =
   from post in database.Posts
   join meta in database.Post_Metas on post.ID equals meta.Post_ID
   where post.ID == id
   select new { Post = post, Meta = meta };

如果您真的坚持使用lambdas，那么您的语法就有很大的问题。下面是相同的查询，使用LINQ扩展方法:

var id = 1;
var query = database.Posts    // your starting point - table in the "from" statement
   .Join(database.Post_Metas, // the source table of the inner join
      post => post.ID,        // Select the primary key (the first part of the "on" clause in an sql "join" statement)
      meta => meta.Post_ID,   // Select the foreign key (the second part of the "on" clause)
      (post, meta) => new { Post = post, Meta = meta }) // selection
   .Where(postAndMeta => postAndMeta.Post.ID == id);    // where statement

LINQ连接的查询语法

var productOrderQuery = from product in Product.Setup()//outer sequence
                        join order in OrderDetails.Setup()//inner sequence
                        on product.Id equals order.ProductId //key selector
                        select new//result selector
                        {
                            OrderId = order.Id,
                            ProductId = product.Id,
                            PurchaseDate = order.PurchaseDate,
                            ProductName = product.Name,
                            ProductPrice = product.Price
                        };

LINQ连接的方法语法

var productOrderMethod = Product.Setup().//outer sequence
    Join(OrderDetails.Setup(), //inner sequence
    product => product.Id//key selector
    ,order=> order.ProductId //key selector
    ,(product,order)=> //projection result
        new
        {
            OrderId = order.Id,
            ProductId = product.Id,
            PurchaseDate = order.PurchaseDate,
            ProductName = product.Name,
            ProductPrice = product.Price
        }
    );

Product.cs供参考

class Product
{
    public int Id { get; set; }
    public string Name { get; set; }

    public decimal Price { get; set; }
    public static IEnumerable<Product> Setup()
    {
        return new List<Product>()
        {
            new Product(){Id=1, Name="Bike", Price=30.33M },
            new Product(){Id=2, Name="Car", Price=50.33M },
            new Product(){Id=3, Name="Bus", Price=60.33M }
        };
    }
}

cs类，以供参考

class OrderDetails
{
    public int Id { get; set; }
    public virtual int ProductId { get; set; }

    public DateTime PurchaseDate { get; set; }
    public static IEnumerable<OrderDetails> Setup()
    {
        return new List<OrderDetails>()
        {
            new OrderDetails(){Id=1, ProductId=1, PurchaseDate= DateTime.Now },
            new OrderDetails(){Id=2, ProductId=1, PurchaseDate=DateTime.Now.AddDays(-1) },
            new OrderDetails(){Id=3, ProductId=2, PurchaseDate=DateTime.Now.AddDays(-2) }
        };
    }

}

1 = 1两个不同的表连接

var query = from post in database.Posts
            join meta in database.Post_Metas on 1 equals 1
            where post.ID == id
            select new { Post = post, Meta = meta };

因为当我开始LINQ + EntityFramework时，我盯着这些例子看了一天。

如果你正在使用EntityFramework，并且你在Post模型对象上设置了一个名为Meta的导航属性，这非常简单。如果你用的是实体，没有导航属性，还在等什么?

database
  .Posts
  .Where(post => post.ID == id)
  .Select(post => new { post, post.Meta });

如果你先做代码，你可以这样设置属性:

class Post {
  [Key]
  public int ID {get; set}
  public int MetaID { get; set; }
  public virtual Meta Meta {get; set;}
}