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The orderby clause is a Language Integrated Query (LINQ) expression used to sort the elements of a sequence based on one or more key selectors. It dictates the iteration order of the returned IEnumerable<T> or IQueryable<T> by evaluating the specified keys, without mutating the underlying data source.

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Syntax

from rangeVariable in dataSource
orderby keySelector1 [ascending | descending] [, keySelector2 [ascending | descending] ...]
select rangeVariable;
  • keySelector: An expression that extracts the value used for sorting from the range variable. For LINQ to Objects (IEnumerable<T>), the return type typically implements IComparable<T> or IComparable, unless a custom IComparer<T> is injected via method syntax. For IQueryable<T>, the type must simply be translatable by the underlying query provider (e.g., to SQL).
  • ascending / descending: Optional contextual keywords that dictate the sort direction. If omitted, the default is ascending.

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Compiler Translation and Method Syntax

At compile time, the C# compiler translates the orderby query syntax into standard query operator method calls.
  • A single orderby key translates to Enumerable.OrderBy or Enumerable.OrderByDescending (or their Queryable equivalents).
  • Subsequent comma-separated keys translate to Enumerable.ThenBy or Enumerable.ThenByDescending.
Single Key Translation: 
// Query Syntax
var query = from element in sequence
            orderby element.PrimarySortKey descending
            select element;

// Method Syntax Equivalent
var methodQuery = sequence.OrderByDescending(element => element.PrimarySortKey);
Multiple Key Translation: 
// Query Syntax
var query = from element in sequence
            orderby element.PrimarySortKey, element.SecondarySortKey descending
            select element;

// Method Syntax Equivalent
var methodQuery = sequence
    .OrderBy(element => element.PrimarySortKey)
    .ThenByDescending(element => element.SecondarySortKey);

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Technical Characteristics

Deferred Execution The orderby clause utilizes deferred execution. The sorting algorithm is not executed when the query is defined. Instead, the sequence is evaluated and sorted only when the resulting IEnumerable<T> or IQueryable<T> is enumerated (e.g., via a foreach loop or by calling .ToList()). Stable Sorting For LINQ to Objects (IEnumerable<T>), the underlying OrderBy and ThenBy methods perform a stable sort. If two elements yield identical values for all specified key selectors, the sorting algorithm preserves the relative order of those elements as they appeared in the original, unsorted sequence. However, for IQueryable<T> implementations (such as Entity Framework or LINQ to SQL), stable sorting is generally not guaranteed. Relational databases do not guarantee a stable sort unless a unique key is explicitly included in the sort criteria. Type Comparison By default, in LINQ to Objects, the orderby clause relies on Comparer<T>.Default to compare the extracted keys. This comparer first checks if the key type implements the generic IComparable<T> interface. If it does not, it falls back to checking for the non-generic IComparable interface. If the type implements neither interface and no custom comparer is provided, an ArgumentException is thrown at runtime during enumeration. While method syntax allows passing a custom IComparer<T> directly into the OrderBy method (bypassing the interface requirement), the query syntax orderby clause does not support explicit comparer injection. For IQueryable<T>, comparison logic is dictated entirely by the translation engine and the target data store. Memory Allocation Because sorting requires evaluating the entire sequence to determine the correct order of the first yielded element, orderby forces the enumeration of the entire source sequence into memory (or into a temporary buffer) before yielding the first result. This makes it a stateful, blocking operation in LINQ to Objects.
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