Front	Back
What does LINQ stand for?	Language Integrated Query.
What are the two syntax styles in LINQ?	Query syntax (`from x in collection select x;`) and method syntax (`collection.Select(x => x)`).
What is the difference between `Where()` and `Select()` in LINQ?	`Where()` filters elements, while `Select()` transforms them.
What does `FirstOrDefault()` do in LINQ?	Returns the first matching element or `default` if no match is found.
What is the difference between `First()` and `FirstOrDefault()`?	`First()` throws an exception if no elements match, while `FirstOrDefault()` returns `default`.
What does `SingleOrDefault()` do in LINQ?	Returns the only matching element or `default` if no match is found; throws if multiple matches exist.
What is the difference between `Single()` and `SingleOrDefault()`?	`Single()` throws an exception if there is more than one match, while `SingleOrDefault()` allows zero results.
How do you order elements in LINQ?	Using `OrderBy()` and `OrderByDescending()`.
How do you sort by multiple criteria in LINQ?	Using `ThenBy()` and `ThenByDescending()`.
How do you group elements in LINQ?	Using `GroupBy()`: `var grouped = people.GroupBy(p => p.Age);`
What is `SelectMany()` used for?	Flattens nested collections into a single collection.
What is the difference between `Select()` and `SelectMany()`?	`Select()` returns a collection of collections, while `SelectMany()` flattens the result.
What is the difference between `ToList()` and `ToArray()`?	`ToList()` returns a `List<T>`, while `ToArray()` returns an array.
What does `Distinct()` do in LINQ?	Removes duplicate elements from a collection.
What does `Skip()` do in LINQ?	Skips a specified number of elements from the beginning of the collection.
What does `Take()` do in LINQ?	Takes a specified number of elements from the beginning of the collection.
How do you retrieve the last N elements of a collection using LINQ?	Using `.Skip(collection.Count - N)`.
What does `Any()` do in LINQ?	Returns `true` if any elements match a condition.
What does `All()` do in LINQ?	Returns `true` if all elements match a condition.
What is the difference between `Any()` and `All()`?	`Any()` checks if at least one element matches, `All()` checks if every element matches.
What does `Aggregate()` do in LINQ?	Performs a cumulative operation on elements, like summing or concatenation.
What does `Sum()` do in LINQ?	Calculates the sum of numeric values in a collection.
What does `Average()` do in LINQ?	Calculates the average of numeric values in a collection.
What does `Min()` do in LINQ?	Finds the smallest value in a collection.
What does `Max()` do in LINQ?	Finds the largest value in a collection.
What is the difference between `Count()` and `LongCount()`?	`Count()` returns an `int`, while `LongCount()` returns a `long` for large datasets.
What is the difference between `Join()` and `GroupJoin()` in LINQ?	`Join()` performs an inner join, while `GroupJoin()` groups the results.
How do you perform a left join in LINQ?	Using `GroupJoin()` with `DefaultIfEmpty()`.
What does `Except()` do in LINQ?	Returns elements that exist in one collection but not another.
What does `Intersect()` do in LINQ?	Returns elements that exist in both collections.
What does `Union()` do in LINQ?	Combines two collections and removes duplicates.
What is the difference between `Union()` and `Concat()`?	`Union()` removes duplicates, while `Concat()` simply merges collections without filtering duplicates.
How do you check if two collections contain the same elements?	Using `SequenceEqual()`.
What does `DefaultIfEmpty()` do in LINQ?	Returns a default value if a collection is empty.
What is the purpose of `ElementAt()` in LINQ?	Retrieves an element at a specific index.
How do you reverse a collection in LINQ?	Using `Reverse()`.
What is Parallel LINQ (PLINQ)?	A feature that enables parallel execution of LINQ queries for performance optimization.
How do you enable PLINQ?	By using `.AsParallel()`: `collection.AsParallel().Where(x => x > 10).ToList();`
What is `AsEnumerable()` used for in LINQ?	It converts a queryable collection to an `IEnumerable<T>` to execute further operations in memory.
How do you improve LINQ query performance?	Using `AsNoTracking()`, caching results, and limiting in-memory operations.
What is deferred execution in LINQ?	Query execution is delayed until the results are actually iterated.
What is immediate execution in LINQ?	When methods like `ToList()`, `ToArray()`, or `Count()` force execution immediately.
What are expression trees in LINQ?	A way to represent code as data, used in `IQueryable<T>` queries.
What is the difference between `IEnumerable<T>` and `IQueryable<T>`?	`IEnumerable<T>` executes queries in memory, while `IQueryable<T>` allows translation into database queries.
How does `Chunk()` work in C# 12 LINQ?	It splits a collection into fixed-size chunks: `var chunks = collection.Chunk(3);`
How do you efficiently remove duplicates from a large dataset?	Using `Distinct()` or a `HashSet<T>`.
How do you efficiently paginate data using LINQ?	Using `Skip((page - 1) * pageSize).Take(pageSize)`.
What does `Range()` do in LINQ?	Generates a sequence of numbers: `Enumerable.Range(1, 10);`
What does `Repeat()` do in LINQ?	Creates a sequence with repeated values: `Enumerable.Repeat("Hello", 5);`
What does `Empty<T>()` do in LINQ?	Returns an empty sequence of type `T`.