When would you replace a LINQ expression with an explicit loop?

Know when a loop is clearer and faster than LINQ

Balances declarative transformations with explicit control flow when allocation, early exit, mutation, diagnostics, or complex state matters.

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I prefer LINQ when it expresses a straightforward data transformation clearly. I use an explicit loop when the operation needs complex state, early exit with detailed failure information, controlled mutation, several outputs, or measured performance that the LINQ form obscures. A loop can also make a one-pass requirement obvious when a fluent chain would enumerate or allocate more than intended.

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Readability, allocations, enumerators, lambdas, intermediate state, early exit, validation, diagnostics, mutation, and performance measurement.

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Why this question matters
LINQ and loops can express the same result while making very different execution details, failure paths, and maintenance costs visible.
I prefer LINQ when it expresses a straightforward data transformation clearly. I use an explicit loop when the operation needs complex state, early exit with detailed failure information, controlled mutation, several outputs, or measured performance that the LINQ form obscures. A loop can also make a one-pass requirement obvious when a fluent chain would enumerate or allocate more than intended.
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A loop can combine filtering, aggregation, validation, and result construction in one pass without intermediate collections. It also gives precise control over break, error context, counters, and capacity. LINQ can still be efficient for simple and chains, and replacing readable code with a manual loop for tiny gains is not automatically better. I look for repeated enumeration, closures, boxing through non-generic APIs, buffering operators, and unnecessary materialization before blaming LINQ as a whole.

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Calling LINQ slow without measuring the path.
Writing a dense loop that changes subtle semantics.
Using several ToList calls to make a fluent chain compile.
Keeping a clever one-liner after it hides validation and failure handling.

When would you replace a LINQ expression with an explicit loop?

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