Prevent multiple instances winform [duplicate]

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• How do I prevent the app from terminating when I close the startup form? 4 answers

I've got three forms, Form1, Form2, Form3. I need to naviage between this 3 forms, like

From Form1 i need to open Form2 and close Form1 and I do it like this:

        Form2 myForm = new Form2(user_form1); // Initialize Form2
        this.Hide(); // hide Form1 -> This doesn't hide everytime
        this.Close(); // close Form1 -> This doesn't close everytime
        myForm.ShowDialog(); // open Form2

but sometimes Form1/Form2/Form3 doesn't want to hide and close, next Form is open but previous form is opened too ( but I can't click on it and do anything ).

What Can i do to prevent it? Any ideas?