I developed Desktop application in C# .Net for client. client purchased software and installed it one than more system without my permission. How to restict him to not installed software on Multiple systems?
You can incorporate a check at instal and/or startup that accesses a licence server that you control.
Send a formal letter explaining the problem and offering a discounted price for the second install. if the client is honest they will respond suitably and you can negotiate. If they won’t cooperate then you have a problem. You can get lawyers involved but then your relationship with the client is destroyed.
Like I said, you can incorporate a check at install and/or startup time that accesses a licence server that you control. In your licence server you can check which copy of the code it is, how many copies are running etc, and permit or refuse the app to continue running.
Alternatively you can require the client machine's MachineGuid/MAC etc and burn that into the version you ship to them. Your code can check that it's running on the machine that it was licensed for.
But the first thing to consider is your relationship with the client. If you don't mind antagonising them then just implement aomething in your sofyware and shut down their unlicenced copy/copies. But if you want to keep then as happy clients (and a good reference for future new clients) you need to resolve this by discussion and negotiation.
OK, that's good.
In my experience, licencing per-machine for small companies is full of problems. The client has a hardware crash and needs to run the app on another machine immediately; the app is on a couple of machines and an employee copies it to another machine not realising he's making an unlicenced copy; they only need to run one copy of the app at any one time, but it could be on either of 2 machines; the list goes on.
Given that you have, and want to keep, good relations then maybe you should look at licensing the client company based on their expected number of simultaneous users, with annual reviews. Burn the company name into the code you ship to them, and relax.