This will not work, as Show is an instance method, not a static mathod:
register form = new register();
register.Show();
You probably meant:
register form = new register();
form.Show();
Note:
Your naming is non standard – types in .NET are normally in PascalCase – to be consistent, your should name the class Register. Additionally, using the variable form is not very descriptive – registerForm would be better.
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solved A type or namespace name ‘register’ could not be found