Common Linq / Standard Query Operator mistakes/mis-steps?

Question

For programmers that do not come from a functional programming background, are there an mistakes to avoid?

Answer 1

The biggest mistake people tend to make is to misunderstand the laziness and evaluation rules for a LINQ query:

Queries are lazy: they are not executed until you iterate over them:

// This does nothing! No query executed!
var matches = results.Where(i => i.Foo == 42);

// Iterating them will actually do the query.
foreach (var match in matches) { ... }

Also, results are not cached. They are computed each time you iterate over them:

var matches = results.Where(i => i.ExpensiveOperation() == true);

// This will perform ExpensiveOperation on each element.
foreach (var match in matches) { ... }

// This will perform ExpensiveOperation on each element again!
foreach (var match in matches) { ... }

Bottom line: know when your queries get executed.