MongoDB Atlas: C# driver connection with MongoClientSettings

TL;DR

csharp using MongoDB.Driver; var settings = MongoClientSettings.FromConnectionString( Environment.GetEnvironmentVariable("MONGODB\_URI")); settings.MaxConnectionPoolSize = 20; settings.ServerSelectionTimeout = TimeSpan.FromSeconds(5); var client = new MongoClient(settings); // prove it at startup await client.GetDatabase("admin").RunCommandAsyncBsonDocument);

using MongoDB.Driver;

var settings = MongoClientSettings.FromConnectionString(
    Environment.GetEnvironmentVariable("MONGODB_URI"));
settings.MaxConnectionPoolSize = 20;
settings.ServerSelectionTimeout = TimeSpan.FromSeconds(5);

var client = new MongoClient(settings);
// prove it at startup
await client.GetDatabase("admin").RunCommandAsync[BsonDocument](
    new BsonDocument("ping", 1));

Steps

1. ```csharp

using MongoDB.Driver;

var settings = MongoClientSettings.FromConnectionString( Environment.GetEnvironmentVariable("MONGODB_URI")); settings.MaxConnectionPoolSize = 20; settings.ServerSelectionTimeout = TimeSpan.FromSeconds(5);

var client = new MongoClient(settings); // prove it at startup await client.GetDatabase("admin").RunCommandAsync[BsonDocument]( new BsonDocument("ping", 1));

2. - `MongoClientSettings.FromConnectionString` parses the Atlas SRV string, then you layer pool and timeout settings on top. Do not concatenate query params by hand.
- One `MongoClient` per app lifetime. It is thread-safe; the driver manages the pool internally.
- `ServerSelectionTimeout` of a few seconds turns network misconfig into a fast error with a useful message.
- Register the client as a singleton in your DI container. Scoped or transient registration recreates the pool constantly.
- On shutdown, dispose is handled by the container for singletons; for manual lifetimes call nothing special, just let the singleton die with the host.

## When to use

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## When not to use

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## Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

C# driver MongoClientSettings pattern and Atlas connection prerequisites verified against https://www.mongodb.com/docs/manual/reference/connection-string/ and https://www.mongodb.com/docs/atlas/troubleshoot-connection/ on 2026-09-26.