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;}
}

您的键选择器不正确。它们应该接受与所讨论的表类型相同的对象，并返回在连接中使用的键。我想你的意思是:

var query = database.Posts.Join(database.Post_Metas,
                                post => post.ID,
                                meta => meta.Post_ID,
                                (post, meta) => new { Post = post, Meta = meta });

您可以随后应用where子句，而不是作为键选择器的一部分。

这个linq查询应该为您工作。它会得到所有有post meta的帖子。

var query = database.Posts.Join(database.Post_Metas,
                                post => post.postId, // Primary Key
                                meta => meat.postId, // Foreign Key
                                (post, meta) => new { Post = post, Meta = meta });

等价SQL查询

Select * FROM Posts P
INNER JOIN Post_Metas pm ON pm.postId=p.postId

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) }
        };
    }

}

可能是这样的

var myvar = from a in context.MyEntity
            join b in context.MyEntity2 on a.key equals b.key
            select new { prop1 = a.prop1, prop2= b.prop1};

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);