C# 8: DialogBox

private void button1_Click(object sender, EventArgs e)
{
openFileDialog1.ShowDialog();
}

Add OpenFileDialog in App Design

For Dialog Message:
MessageBox.Show("The search is complete", "My Application", MessageBoxButtons.OKCancel, MessageBoxIcon.Asterisk);

To set initial directory:
openFileDialog1.initialdirectory = Application.StartupPath;

To set filter:
openFileDialog1.initialdirectory = "Text Files (*.txt)|*.txt"

or you may set this in the open dialog properties