The C# driver throws an exception when the connection string is not valid. In one case the error message contains the entire connection string, which would include the username/password if present.

See:

https://github.com/mongodb/mongo-csharp-driver/blob/v2.8.1/src/MongoDB.Driver.Core/Core/Configuration/ConnectionString.cs#L671

While this error message does not necessarily mean the username/password is exposed, error messages are often logged, in which case the username/password could be recorded in places where it shouldn't be.