The File Upload problem?

Normally, I place a FileUpload1 control in webpage, then select a file from dialog using FileUpload1, finish click Button1 to

operate the uploaded file.

I hope to click a button to open file dialog, then select a file from dialog, finsish operate the file , how can I do?

 

 

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (FileUpload1.HasFile)
        {
            FileUpload1.FileContent;
            ...
        }
    }< ...