Retrieve MongoDB collections as a sequence of `IMongoCollection<T>` in .NET driver - mongodb

Question

IMongoDatabase.ListCollections return a cursor over BsonDocument.
Why doesn't it return a cursor over IMongoCollection<T> instead?
I was trying to write a generic GetCollection method to retrieve the collection given just the document type, something like this:
private IMongoCollection<T> GetCollection<T>()
{
var client = new MongoClient("connectionString");
var db = client.GetDatabase("dbName");
var coll = db.ListCollectionsAsync().Result.ToListAsync().Result
// Find collection of document of type T
// Collection is a BsonDocument instead
.Find(collection => typeof(collection) == typeof(T));
return coll;
}

Answer 1

The driver doesn't know what kind of document is in a collection, which is why it takes a type parameter T. MongoDB itself isn't aware of how the documents in the database map to the types in your application.
It is not possible to take a connection to a "generic" MongoDB deployment and simply discover the collections and types in them. This is code you would need to write, and probably won't work out well as it'll be something akin to trial and error.
If you are simply trying to create a factory type, you will need to build the backing list of collections before calling GetCollection<T>.
You could try using the type name as the collection name. This would make the collection name repeatable (unless the type name is changed). But I've never tested it and it might have some idiosyncrasies in the real world.
public class MyDatabase
{
private readonly IMongoClient _client;
public MyDatabase(string connectionString)
{
_client = new MongoClient(connectionString);
}
public IMongoCollection<T> GetCollection<T>()
{
var db = _client.GetDatabase("dbName");
return db.GetCollection<T>(typeof(T).Name);
}
}
If you prefer collection names to be pluralized, something like Humanizer can help there.
I generally prefer to create a type that has the collections as fields on the class. For example:
public class MyDatabase
{
private readonly IMongoClient _client;
public IMongoCollection<Foo> Foos { get; }
public IMongoCollection<Bar> Bars { get; }
public MyDatabase(string connectionString)
{
_client = new MongoClient(connectionString);
var db = _client.GetDatabase("myDb");
Foos = db.GetCollection<Foo>("foos");
Bars = db.GetCollection<Bar>("bars");
}
}

Answer 2

MongoDB collections have a flexible scheme that allows you to insert any document with any structure into the collection. For example, We can insert the following 3 objects into the test collection and it's valid:
> db.test.insertMany([{one: 1}, {two:2}, {three: 3}])
{
"acknowledged" : true,
"insertedIds" : [
ObjectId("5c87c954ed372bf469367e57"),
ObjectId("5c87c954ed372bf469367e58"),
ObjectId("5c87c954ed372bf469367e59")
]
}
> db.test.find().pretty()
{ "_id" : ObjectId("5c87c954ed372bf469367e57"), "one" : 1 }
{ "_id" : ObjectId("5c87c954ed372bf469367e58"), "two" : 2 }
{ "_id" : ObjectId("5c87c954ed372bf469367e59"), "three" : 3 }
Thus you can't map a MongoDB collection to a .NET Type as the collection has no knowledge of the type.