LINQ Interview Question #75

How do equality comparers affect joins, grouping, and set operations in LINQ?

Joins, Grouping, Set & Quantifier Operators Mid-Level Intermediate

Quick Interview Answer

Many LINQ operations depend on equality. Join and GroupJoin match keys, GroupBy identifies group keys, and operators such as Distinct, Union, Intersect, Except, and Contains determine whether values are equal.

Detailed Explanation

For custom in-memory comparisons, types can implement appropriate equality semantics or an IEqualityComparer<T> can be supplied where an overload supports it. Database IQueryable providers have their own translation and database comparison semantics, so custom .NET comparers generally cannot simply be translated into SQL.

Code Example

var names = new[]
{
    "admin@example.com",
    "ADMIN@example.com"
};

var unique = names.Distinct(
    StringComparer.OrdinalIgnoreCase);

// One value remains for in-memory LINQ.