InMemoryEmulator.MongoDB

A high-fidelity, in-process fake for the MongoDB .NET/C# Driver — zero Docker, zero network, instant startup.

Why?

• Instant — ~0ms startup (vs 2-15s for Docker/Mongo2Go)
• Portable — No Docker, no mongod binaries, no ports
• Reliable — No flaky timeouts, port conflicts, or container failures
• High-fidelity — Real filter, update, and aggregation semantics (not mocks)

Quick Start

dotnet add package InMemoryEmulator.MongoDB          # for MongoDB.Driver 3.x
dotnet add package InMemoryEmulator.MongoDB.V2Driver  # for MongoDB.Driver 2.x

Unit Tests

var result = InMemoryMongo.Create<Order>("orders");
var collection = result.Collection;

await collection.InsertOneAsync(new Order { Total = 99.99m });
var found = await collection.Find(o => o.Total > 50).FirstOrDefaultAsync();

Integration Tests (DI)

services.UseInMemoryMongoDB(options =>
{
    options.DatabaseName = "testdb";
    options.AddCollection<Order>("orders");
});

What's Supported

Feature	Status
Full CRUD (Insert, Find, Update, Replace, Delete, BulkWrite)	✅
Aggregation Pipeline (34 stages, 100+ expression operators)	✅
Indexes (Unique, Compound, Sparse, Partial Filter, TTL)	✅
Change Streams	✅
Transactions (snapshot isolation)	✅
GridFS	✅
Schema Validation ($jsonSchema)	✅
Capped Collections + Tailable Cursors	✅
Views	✅
LINQ (AsQueryable())	✅
Fault Injection	✅
State Persistence (Export/Import)	✅
DI Integration (2 extension methods)	✅

NuGet Packages

Package	Description	NuGet
InMemoryEmulator.MongoDB	Core library for MongoDB.Driver 3.x
InMemoryEmulator.MongoDB.V2Driver	Core library for MongoDB.Driver 2.x
InMemoryEmulator.MongoDB.JsTriggers	$function, $accumulator, $where via Jint

Both packages share the same namespace (InMemoryEmulator.MongoDB) and feature set. Pick the one matching your driver version — the API is identical.

Documentation

See the Wiki for full documentation:

• Getting Started
• Setup Guide (5 DI patterns)
• Features
• Filter & Update Operators
• Aggregation Pipeline (34 stages)
• Migration Guide (from Mongo2Go / Testcontainers)
• Known Limitations

Requirements

• .NET 8.0+
• InMemoryEmulator.MongoDB requires MongoDB.Driver 3.x (tested up to 3.8.1)
• InMemoryEmulator.MongoDB.V2Driver requires MongoDB.Driver 2.x (tested up to 2.30.0)

License

MIT