Unable to connect to MongoDB (MongoLabs) via C# client

If I had to hazard a guess, the issue is most likely a firewall issue. You should check the following

• nslookup of the host (ds048719.mlab.com) from the C# Application Host
• ping of the host (ds048719.mlab.com) from the C# Application Host (might fail, depending on mLab's settings)
• That your IP address is whitelisted
• Test the connection using the Mongo Shell from the same host where the C# Application is running. mLab has docs here.
• Test the connection with a raw telnet, eg telnet ds048719.mlab.com 48719
• Ensure you are using the correct authenticationDatabase (in your example, this is specified by the /db), this is usually admin but could be your database name if you are on a shared instance.

You can find the docs on connecting with the C# driver in the MongoDB C# Driver Docs. It is important to note the following:

The Database Component

The database component is optional and is used to indicate which database to authenticate against. When the database component is not provided, the “admin” database is used.

mongodb://host:27017/mydb

Above, the database by the name of “mydb” is where the credentials are stored for the application.

NOTE:

Some drivers utilize the database component to indicate which database to work with by default. The .NET driver, while it parses the database component, does not use the database component for anything other than authentication.

Finally, I would suggest in the future, obfuscate the hostname and port when posting to SO. While security through obscurity alone is a bad policy, it certainly adds a layer of defense for your MongoDB deployment.