Which LINQ Operations Preserve the Order of Elements in a Sequence?

Maintain element order in LINQ

In programming, processing ordered data is a common scenario. When manipulating arrays using LINQ to Objects, it's important to understand which operations preserve the order of elements to avoid accidental modifications.

Operations that maintain order

The following LINQ operations maintain the original order of array elements:

• AsEnumerable: Converts an IEnumerable to a non-generic IEnumerable.
• Cast: Convert elements to a different type without modifying the order.
• Concat: Combines two or more sequences, preserving the order of each sequence.
• Select: Projects each element to a new value but keeps the order.
• ToArray: Materializes an IEnumerable into an array, preserving order.
• ToList: Materializes an IEnumerable into a List, maintaining the original order.

Some operations that maintain order

Some LINQ operations filter or add elements without reordering the remaining elements:

• Distinct: Remove duplicate elements (based on equality comparator). (see description below)
• Except: Returns elements not found in the specified sequence, preserving the order of the original sequence.
• Intersect: Returns elements found in both sequences, preserving the order of the first sequence.
• OfType: Filter elements of a specific type, preserving order.
• Prepend (new in .NET 4.7.1): Adds an element to the beginning of the sequence.
• Skip: Skips the specified number of elements at the beginning of the sequence, preserving the order of the remaining elements.
• SkipWhile: Skip elements until the predicate becomes false, preserving the order of the remaining elements.
• Take: Returns the specified number of elements at the beginning of the sequence, preserving order.
• TakeWhile: Returns elements until the predicate becomes false, preserving order.
• Where: Filters elements based on a predicate, preserving the order of matching elements.
• Zip (new in .NET 4): Pairs elements from two sequences, preserving the order of each sequence.

Operations that destroy the order

The following LINQ operations do not preserve the original order of elements:

• ToDictionary: Converts a sequence into a dictionary. Dictionaries do not maintain order.
• ToLookup: Similar to ToDictionary, the order is not preserved.

Redefine the order of operations

These LINQ operations explicitly change the order of results:

• OrderBy: Sorts elements in ascending order based on the key selector.
• OrderByDescending: Sorts elements in descending order based on the key selector.
• Reverse: Reverse the order of elements.
• ThenBy: Used in conjunction with OrderBy or OrderByDescending to sort by the second key.
• ThenByDescending: Used in conjunction with OrderBy or OrderByDescending to sort by the second key in descending order.

Special circumstances

• GroupBy: The order of grouped objects is based on the order of the first element of each group. The elements within the group are returned in the order in which they appear in the source sequence.
• GroupJoin: Preserves the order of elements in the outer sequence and the order of matching elements in the inner sequence for each outer element.
• Join: Similar to GroupJoin, preserves the order of elements in the outer sequence and matches the inner sequence.
• SelectMany: The order is determined by the order of the source elements and the sequence returned by the selector function.
• Union: The elements from both sequences are produced in their original order, without duplication.

Instructions:

Based on the implementation in .NET, Distinct has been moved to the "preserve order" category, which preserves the order of elements as long as no custom equality comparator is provided.

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