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The let clause in C# is a Language Integrated Query (LINQ) component used within query expressions to compute a value and bind it to a new range variable. This newly introduced identifier remains in scope for the remainder of the query, allowing the evaluated result to be referenced in subsequent clauses without re-evaluating the underlying expression.

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Syntax

The let clause requires a valid identifier and an expression that yields a value. The compiler infers the type of the new range variable from the expression. 
from rangeVariable in dataSource
let newVariable = expression
// newVariable and rangeVariable are both in scope here
select newVariable;

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Compiler Translation and Mechanics

Under the hood, the C# compiler translates query expressions containing a let clause into standard method-based LINQ calls. The let clause does not have a direct equivalent extension method (like Where or OrderBy). Instead, the compiler implements it by injecting a Select projection that creates an anonymous type. This anonymous type encapsulates both the original range variable(s) and the newly introduced let variable. The compiler then uses a concept called a transparent identifier to pass this composite object down the query pipeline.

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Query Syntax Example

IEnumerable<int> query = 
    from x in sequence
    let y = x * 2
    where y > 10
    select x + y;

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Method Syntax Translation

The compiler translates the query syntax above into the following method syntax equivalent before ultimately compiling it down to Intermediate Language (IL) bytecode: 
IEnumerable<int> query = sequence
    // The 'let' clause forces a projection into an anonymous type
    .Select(x => new { x, y = x * 2 })
    // The transparent identifier (here named 't') carries both variables forward
    .Where(t => t.y > 10)
    .Select(t => t.x + t.y);

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Technical Characteristics

  • Immutability: The range variable introduced by the let clause is strictly read-only. It cannot be reassigned or mutated within the query expression.
  • Scope: The variable is scoped to the query expression. It becomes available immediately after the let declaration and remains in scope until the query is terminated by a select or group clause.
  • Memory Allocation: Because the compiler translates let into a Select projection that instantiates an anonymous type, each let clause introduces a new object allocation per element in the sequence. In performance-critical paths with large datasets, this hidden allocation overhead is a necessary architectural consideration.
  • Type Inference: Explicit typing is not permitted in a let clause. The compiler strictly enforces var-like type inference based on the right-hand expression. Multiple let clauses can be chained sequentially, with each subsequent clause able to reference the range variables created by preceding ones.
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