How to: Join by Using Composite Keys (C# Programming Guide)

This example shows how to perform join operations in which you want to use more than one key to define a match. This is accomplished by using a composite key. You create a composite key as an anonymous type or named typed with the values that you want to compare. If the query variable will be passed across method boundaries, use a named type that overrides Equals and GetHashCode for the key. The names of the properties, and the order in which they occur, must be identical in each key.
Example
The following example demonstrates how to use a composite key to join data from three tables:

var query = from o in db.Orders
from p in db.Products
join d in db.OrderDetails
on new {o.OrderID, p.ProductID} equals new {d.OrderID,
d.ProductID} into details
from d in details
select new {o.OrderID, p.ProductID, d.UnitPrice};

Type inference on composite keys depends on the names of the properties in the keys, and the order in which they occur. If the properties in the source sequences do not have the same names, you must assign new names in the keys. For example, if the Orders table and OrderDetails table each used different names for their columns, you could create composite keys by assigning identical names in the anonymous types:

join...on new {Name = o.CustomerName, ID = o.CustID} equals
new {Name = d.CustName, ID = d.CustID }

Composite keys can be also used in a group clause.