The language designers have added powerful new features to C# that enable you to write SQL-like queries on any arbitrary sequence. Simply put, if you can foreach it, you can query it.

The Language Integrated Query (LINQ) project facilitates a common scheme for accessing data across different sources, such as XML documents, relational databases, and in-memory data structures. This ...

You have a method that contains a LINQ query. If you use the right interface, other developers can piggyback on your method with their LINQ queries without losing efficiency.

You don't ever have to write a complicated LINQ/Entity Framework query, but you do need to know when to call the ToList method on your query. Here's a tip on both.

Because I almost never write long linq queries in Linq to Objects without just converting to the pipeline-esque (or fluent if you prefer) style via extensions methods.