Show all image in Folder on Server

Question

Hello everybody, I want to show all image in Folder on Server by using Image control. I using a string array to save all file name and when I click button "show", the image is showed on Image control by order. Here my code.
//String array to save filename
    public string[] nfile=new string[100];
    int i ;
    int j;//to show by order
protected void Button3_Click(object sender, EventArgs e)
    {
        //Load all image to string array
        i = 0;
        btnNext.Enabled = true;
      
        foreach (string file in Directory.GetFiles(Server.MapPath("Sample") + "\\", "*.jpg"))
        {

            string namefile = file.Remove(0, 60);//"d:\DZHosts\LocalUser\toanhoi\www.buiductoan.somee.com\Sample\","");
            namefile.Trim();//namefile="a.jpg"
            nfile[i] = namefile;
            i++;
        }
        j = 0;
    }
//Button show
        if (j <= i)
        {
            ImgView.ImageUrl ="~/Sample" + nfile[j];
            j++;
        }
But If I click show button, nfile array wil return null.I don't understand.Can you help me to fix?