LC022: Nested Collection Materialization Inside Projection

What it flags

Flags nested collection materialization inside projections because it can be expensive, provider-version sensitive, or better expressed with direct projection/split-query shaping.

Why it matters

LinqContraband reports this rule as an advisory performance signal. Modern EF Core can translate some correlated collection projections, so the diagnostic should prompt review rather than automatic removal.

Typical fix

Keep the projection provider-friendly, flatten the shape, use split queries where appropriate, or keep the nested materializer when a DTO contract requires a concrete collection.

The code fix is intentionally conservative. It only removes ToList() when the receiver type already matches the materialized type, such as a List<T> navigation projected as navigation.ToList(). It does not rewrite ToArray(), dictionary/set materializers, anonymous/object initializer members, or type-changing shapes such as stringValue.ToList().

Samples

See samples/LinqContraband.Sample/Samples/LC022_ToListInSelectProjection/ for a focused example.

The crime

var query = db.Customers
    .Select(c => new
    {
        c.Id,
        OrderIds = c.Orders.Select(o => o.Id).ToList()
    });

A better shape

var query = db.Customers
    .Select(c => new
    {
        c.Id,
        OrderIds = c.Orders.Select(o => o.Id)
    });

var results = await query.ToListAsync();