To add a new form to your application, you don't need to create a new Windows Form Application. Typically, your application's default form is Form1.
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Then right click and do this
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Now, add form by selecting Windows Form
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Set the event for the button in Form1 to something like this
private void button1_Click(object sender, EventArgs e)
{
var form2 = new Form2();
form2.Show();
}
If you want to interact with both forms, use form2.Show(); otherwise, use form2. ShowDialog() is used to stall the caller form until it is closed.