LINQ Support

LINQ Support

Overview

InMemoryEmulator.MongoDB supports LINQ queries via GetItemLinqQueryable<T>() / AsQueryable(). The MongoDB .NET Driver translates LINQ expressions into aggregation pipeline operations, which the emulator then evaluates.

LINQ2 vs LINQ3

The MongoDB .NET Driver supports two LINQ providers:

Feature	LINQ2	LINQ3
Default in	Driver < 2.19	Driver ≥ 2.19
Translation target	$match / $project	Full aggregation pipeline
StringComparison support	Limited	Full
DateTimeOffset support	Limited	Full

The InMemoryEmulator evaluates the rendered aggregation pipeline, so both LINQ2 and LINQ3 output work as long as the rendered operators are supported.

Supported Operations

Basic Queries

var queryable = collection.AsQueryable();

// Where
var results = queryable.Where(x => x.Status == "active").ToList();

// OrderBy
var sorted = queryable.OrderBy(x => x.Name).ToList();

// First / Single
var first = queryable.First(x => x.Id == someId);

// Count
var count = queryable.Count(x => x.IsActive);

Projections

var names = queryable.Select(x => new { x.Name, x.Email }).ToList();

Aggregation

var grouped = queryable
    .GroupBy(x => x.Category)
    .Select(g => new { Category = g.Key, Count = g.Count(), Total = g.Sum(x => x.Price) })
    .ToList();

Skip / Take

var page = queryable.OrderBy(x => x.Name).Skip(20).Take(10).ToList();

Known Differences

Area	Behavior
StringComparison	LINQ2 may not translate some StringComparison overloads; prefer LINQ3
Nullable<T> comparisons	Edge cases in null propagation may differ between LINQ providers
Complex nested projections	Very deep projections may evaluate differently
IQueryable extensions	Custom extension methods not recognized by the driver won't translate

Tips

1. Use LINQ3 (the default in driver ≥ 2.19) for the broadest operator support
2. Use AsQueryable() instead of GetItemLinqQueryable() for simpler syntax
3. For complex queries, prefer the fluent builder API (Builders<T>.Filter / .Update) over LINQ — it maps directly to MongoDB operators without translation

See Also

• Filter & Update Operators — MongoDB operator reference
• Aggregation Pipeline — Pipeline stages and expressions