How to inject code through C#.NET in a Exe created in C#.net

I have created a form in C#.net which ask for key and check validity (expiry date). i place this form as first running form in my application. first time it takes key and every next time loading it check for expiration policy which saved through key in registry. this form is w开发者_高级运维orking fine. but now i want to create a another application which browse any exe created in C# or other language and generate new exe (with my validation form as startup form). means i want to create software which place my form as startup form to any Exe. so that it can work as anti piracy for any application (exe).

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You can analyse and modify .NET applications with the Cecil Library.
http://www.mono-project.com/Cecil

You need to add your form class. Then remember the class that gets loaded in the original application. Load that instead from your form and change the startup to your form.