C# – .Net C# Remote Method Invocation (RMI) for Client/Server Communication

First of all, this sounds like something that you shouldn't be trying to do.

For example, if you have some application on A that calls B which in turn calls C, then it shouldn't be the responsibility of A to figure out why isn't B working. Simply call B and if it doesn't work, then it doesn't work.

Now, if you really want to do this, you would need some service on B that forwards the request to C (or something like that). If you don't want to install anything on B, then the only way is to (ab)use some service that's already installed on it. But I can't think of any common service that would allow you to do this.

Yes you can.

You can set up a webservices API that connects to your database, thus keeping the database connection secure and wholly within your network. This is instead of the web server that would normally serve the web site to the users.

The desktop clients then connect to the webservices to communicate with the database. You could have the standard CRUD methods for your objects or you could wrap them up in more intelligent layers that do validation and verification of the data.

You just need to publish the desktop client installer on the web somewhere where everyone (or where at least those you want access) can see it. You can prevent unauthorised access quite easily - you either generate the user accounts and mail out the details or you vet the requests for access as they come in.

You can even make the application self updating so that the user is always on the latest version.