Which LINQ Methods Preserve Array Order, and Which Ones Don't?

LINQ and array order: Detailed explanation of which methods maintain order and which methods do not

When using LINQ to Objects operations on sorted arrays, be careful to avoid operations that destroy the order of the original array. The following analysis will guide you:

Absolute way to maintain order:

• AsEnumerable:
• Cast:
• Concat:
• Select:
• ToArray:
• ToList:

These methods map source elements to result elements while maintaining order.

Usual way to keep order:

• Distinct: Filter out duplicate elements.
• Except: Filters out elements that exist in another sequence.
• Intersect: Filter for common elements in multiple sequences.
• OfType: Filters elements based on type.
• Prepend (new in .NET 4.7.1): Appends the specified value before the element.
• Skip: Skip the specified number of elements.
• SkipWhile: Skips elements until the specified condition is met.
• Take: Returns the specified number of elements.
• TakeWhile: Returns elements until the specified condition is met.
• Where: Filters elements based on a predicate.
• Zip (new in .NET 4): Merge corresponding elements in multiple sequences.

How to destroy the order:

• ToDictionary: Converts elements to a dictionary, resulting in an unordered collection.
• ToLookup: Converts elements into a lookup table, resulting in an unordered collection.

Ways to explicitly redefine the order:

• OrderBy: Sort elements in ascending order.
• OrderByDescending: Sort elements in descending order.
• Reverse: Reverse the order of elements.
• ThenBy: Sort elements by another attribute, keeping the original order.
• ThenByDescending: Sort elements in descending order by another attribute, keeping the original order.

How to redefine the order according to rules:

• GroupBy: Group elements by key, maintaining the order of elements within each group.
• GroupJoin: Joins elements based on keys, maintaining the order of elements from the external source and within each group.
• Join: Joins elements based on keys, maintaining the order of internal and external source elements.
• SelectMany: Generates a sequence of sequences, combining the elements in each sequence in an unpredictable order.
• Union: Combines multiple sequences to generate elements in the order of the provided sequence.

By understanding these nuances, you can maintain the desired order of the array when performing LINQ operations, ensuring its integrity for further processing or display.

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