how to get value from dynamic generated controls after postback ?

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ASP.NET

DynamicMethod

I am having a button which generates 3 controls on click.the controls are Textbox,Dropdownlist and a FileUpload control.
WHen i click Add button first time it works fine for me but when I filled all the three controls and click on Add button, it generates postback and i loos my filled data in controls and controls are regenerated.

Please give a solution for this type of situation.
Here i smy code.:

<td>
<asp:Button ID="btnAddmoreDocs" runat="server" CssClass="Button" Text="Add More"
                                       OnClick="btnAddmoreDocs_Click" /> </td>

Code behind code :

protected void btnAddmoreDocs_Click(object sender, EventArgs e)
    {
        //if (Convert.ToString(ViewState["Generated"]) != "true")
        {
            AddUserControl();
           
        }
    }

private void AddUserControl()
    {
        Table tbldynamic = new Table();
        int i=1;
        if (ViewState["i"] == "0" || ViewState["i"] == "" || ViewState["i"] == null)
        {
            ViewState["i"] = 1;

        }
        else
        {
            i = Convert.ToInt16(ViewState["i"]) + 1;
        }
         for (int j = 1; j <= i; j++)
        {
            TableCell tc = new TableCell();
            TableCell tclabel = new TableCell();
            TableCell tclabelDocName = new TableCell();
            TableCell tcTextDocName = new TableCell();
            TableCell tcFileUpld = new TableCell();
            TableRow tr = new TableRow();
            //CheckBox _checkbox = new CheckBox();
            Label LbelType = new Label();
            LbelType.ID = "lbltype" + j;
            LbelType.Text = "Document Type :";

            Label lblDocName = new Label();
            lblDocName.ID = "lblDocName" + j;// ViewState["i"];
            lblDocName.Text = "Document Name :";

            DropDownList ddltype = new DropDownList();
            ddltype.ID = "ddlDocumntType" + j;// ViewState["i"];
            ddltype.Text = "Document Type";
             
            TextBox txtDocName = new TextBox();
            txtDocName.ID = "txtDocnm" +j;
            Session[txtDocName.ID] = txtDocName;
            FileUpload FileUpd = new FileUpload();
            FileUpd.ID = "filUpldDoc" + j;// ViewState["i"];
            // _checkbox.ID = "chkDynamicCheckBox" + i;
            //_checkbox.Text = "Checkbox" + i;
            tc.Controls.Add(ddltype);
            tclabel.Controls.Add(LbelType);
            tclabelDocName.Controls.Add(lblDocName);
            tcTextDocName.Controls.Add(txtDocName);
            tcFileUpld.Controls.Add(FileUpd);
            tr.Cells.AddAt(0, tclabel);

            tc.Width = 100;
            tr.Cells.AddAt(1, tc);
            tr.Cells.AddAt(2, tclabelDocName);
            string textboxID = "txtDocnm" + j;
            TextBox checkIfTextBox = (TextBox)Session[textboxID];
            if (checkIfTextBox != null)
            {
                txtDocName.Text = checkIfTextBox.Text;
            }
            //if (string.IsNullOrEmpty(Request.Form["txtDocnm" + j.ToString()]))
            //{
            //    txtDocName.Text = Request.Form["txtDocnm" + j.ToString()];
            //}
            tr.Cells.AddAt(3, tcTextDocName);
            tr.Cells.AddAt(4, tcFileUpld);
            tbldynamic.Rows.Add(tr);
            ViewState["i"] = j;// (Convert.ToInt16(ViewState["i"]) + 1).ToString();
        }
        pnlStudDocumnts.Controls.Add(tbldynamic);
    }