Automatic button click but it clicks before my form is shown

Is this a standard WinForm app ?

I can't observe the behavior you describe in a WinForms app with a Button and an OpenFileDialog:

namespace testbuttonload
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Shown(object sender, EventArgs e)
        {
            button1.PerformClick();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            DialogResult result = openFileDialog1.ShowDialog();

            Console.WriteLine(result);
        }
    }
}

Something else in your code is causing the Button to be clicked before the Shown event ... check:

1) your Form Load handler is not calling PerformClick

2) Form click handler, or other events, are not calling PerformClick