How to Perform Multi-Field Joins in a Single LINQ Query?

LINQ multi-field connection: efficiently associate multiple entities

In LINQ, connecting multiple entities usually uses the join keyword. Although single-field joins are more common, multi-field joins can be easily implemented through simple syntax adjustments.

Multi-field join syntax

The syntax for joining multiple fields in a single LINQ query is as follows:

from x in entity
join y in entity2 
   on new { x.field1, x.field2 } equals new { y.field1, y.field2 }

The

new keyword creates an anonymous type that combines the fields from two entities used for the join. The anonymous type on the left side of the equal sign represents the connection condition of the first entity, and the right side represents the connection condition of the second entity.

Example

Here is an example:

var result = from x in entity
   join y in entity2 
   on new { x.field1, x.field2 } equals new { y.field1, y.field2 }

This query will join field1 and field2 based on the entity and entity2 fields, returning a combined sequence containing records matching the specified fields in both entities.

Other notes

It should be noted that the data types of the connection fields must be compatible. In addition, the join keyword performs an inner join, which means that only records with matching field values ​​will be returned.

If you need to perform a left outer join, you can use the GroupJoin operator. For example:

var result = from x in entity
   join y in entity2 
   on new { x.field1, x.field2 } equals new { y.field1, y.field2 } into yGroup
   from y in yGroup.DefaultIfEmpty()

This query will perform a left outer join, ensuring that all records in the entity table are returned, even if there are no matching records in the entity2 table.

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