Copy checked Rows from one GridView to Another with DropDownList select value using C# in ASP.Net

I binded to dropdownlist I create Void Method. After this method put in drpKategori_SelectedIndexChanged 

 private void Elemans()
        {
            using (Entities medya = new Entities())
            {

                for (int i = 0; i < example.Rows.Count; i++)
                {

                    DropDownList ddl = (DropDownList)example.Rows[i].Controls[5].FindControl("drpEleman");
                    ddl.DataSource = medya.Elemanlar.ToList();
                    ddl.DataTextField = "Pozisyon";
                    ddl.DataValueField = "ID";
                    ddl.DataBind();

                    ddl.Items.Insert(0, new ListItem("Seçiniz", "0"));
                }




            }

        }

protected void drpKategori_SelectedIndexChanged(object sender, EventArgs e)
       {
                 
           Uruns();          
      
           Elemans();
 
       }

Hi AliYilmaz,

Please refer below code which satisfiy your requirement

HTML

<div>
    <asp:DropDownList runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnChange"
        ID="ddlCategory">
        <asp:ListItem Text="Select" />
        <asp:ListItem Text="UrunAd 1" />
        <asp:ListItem Text="UrunAd 2" />
        <asp:ListItem Text="UrunAd 3" />
        <asp:ListItem Text="UrunAd 4" />
    </asp:DropDownList>
    <br />
    <br />
    <asp:GridView ID="example" AutoGenerateColumns="false" DataKeyNames="ID" runat="server"
        ClientIDMode="Static" OnRowDataBound="example_RowDataBound">
        <Columns>
            <asp:TemplateField HeaderText="Ürün Seç" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:CheckBox ID="chcsec" CssClass="form-control" runat="server" AutoPostBack="true"
                        OnCheckedChanged="chcsec_CheckedChanged" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Ürün Ad" ItemStyle-Width="100">
                <ItemTemplate>
                    <asp:Label ID="lblUrun" runat="server" Text='<%# Eval("UrunAd") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Stok" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="lblStok" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("Stok") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Fiyat" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtfiyat" Style="padding: 3px 5px;" CssClass="form-control" runat="server">500</asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Adet" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtAdet" Style="padding: 3px 5px;" CssClass="form-control" runat="server"></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Eleman Seç" ItemStyle-Width="100">
                <ItemTemplate>
                    <asp:DropDownList ID="drpEleman" CssClass="form-control" runat="server">
                    </asp:DropDownList>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Eleman Adet" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtElemanAdet" Style="padding: 3px 5px;" CssClass="form-control" runat="server"></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Eleman Fiyat" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtElemanFiyat" Style="padding: 3px 5px;" CssClass="form-control"
                        runat="server"></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:CommandField ButtonType="Link" ShowEditButton="true" ShowDeleteButton="true"
                ItemStyle-Width="150" />
        </Columns>
    </asp:GridView>
    <br />
    <asp:GridView runat="server" ID="gvSiparis" />
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        BindGrid("");
    }
}

private void BindGrid(string category)
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[] { new DataColumn("ID", typeof(int)), new DataColumn("UrunAd"), new DataColumn("Stok", typeof(int)) });
    dt.Rows.Add(1, "UrunAd 1", 10);
    dt.Rows.Add(2, "UrunAd 2", 15);
    dt.Rows.Add(3, "UrunAd 3", 8);
    dt.Rows.Add(4, "UrunAd 4", 2);
    dt.Rows.Add(5, "UrunAd 3", 6);
    dt.Rows.Add(6, "UrunAd 2", 9);
    dt.Rows.Add(7, "UrunAd 1", 1);
    dt.Rows.Add(8, "UrunAd 4", 18);
    dt.Rows.Add(9, "UrunAd 4", 14);
    if (string.IsNullOrEmpty(category))
    {
        example.DataSource = dt;
    }
    else
    {
        string expression = "UrunAd = '" + category + "'";
        dt.DefaultView.RowFilter = expression;
        example.DataSource = dt;
    }
    example.DataBind();
}

protected void chcsec_CheckedChanged(object sender, EventArgs e)
{
    if ((sender as CheckBox).Checked)
    {
        if (ViewState["CheckedRow"] == null)
        {
            DataTable dt = CheckedData();
            ViewState["CheckedRow"] = dt;
        }
        else
        {
            DataTable previousCheckedRows = ViewState["CheckedRow"] as DataTable;
            DataTable dtCurrentCheckedRows = CheckedData();
            dtCurrentCheckedRows.Merge(previousCheckedRows);
            ViewState["CheckedRow"] = dtCurrentCheckedRows;
        }
    }
    else
    {
        GridViewRow row = ((sender as CheckBox).NamingContainer as GridViewRow);
        Label lblUrun = (row.FindControl("lblUrun") as Label);
        Label lblStok = (row.FindControl("lblStok") as Label);

        DataTable deleteRow = (ViewState["CheckedRow"] as DataTable).DefaultView.ToTable(true);
        DataRow[] dr = deleteRow.Select("UrunAd = '" + lblUrun.Text + "' AND Stok = '" + lblStok.Text + "'");
        foreach (DataRow d in dr)
        {
            deleteRow.Rows.Remove(d);
        }
        ViewState["CheckedRow"] = deleteRow;
    }
    gvSiparis.DataSource = (ViewState["CheckedRow"] as DataTable).DefaultView.ToTable(true);
    gvSiparis.DataBind();
}

private DataTable CheckedData()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[] { new DataColumn("UrunAd"), new DataColumn("Stok", typeof(int)), new DataColumn("Pozisyon") });
    foreach (GridViewRow rw in example.Rows)
    {
        if (rw.RowType == DataControlRowType.DataRow)
        {
            CheckBox chk = (CheckBox)rw.Cells[0].FindControl("chcsec");
            Label lb = (Label)rw.Cells[1].FindControl("lblUrun");
            Label lb1 = (Label)rw.Cells[2].FindControl("lblStok");
            DropDownList drpEleman = (DropDownList)rw.Cells[5].FindControl("drpEleman");

            if (chk.Checked)
            {
                dt.Rows.Add(lb.Text, lb1.Text, drpEleman.SelectedItem.Value);
            }
        }
    }
    return dt;
}

protected void OnChange(object sender, EventArgs e)
{
    BindGrid(ddlCategory.SelectedItem.Text.Trim());
}

protected void example_RowDataBound(object sender, GridViewRowEventArgs e)
{
    //Your DataBase Table Bind to DataTable here.
    DataTable dropdownBindTable = new DataTable();
    dropdownBindTable.Columns.AddRange(new DataColumn[] { new DataColumn("Pozisyon"), new DataColumn("ID") });
    dropdownBindTable.Rows.Add("Pozisyon1", 1);
    dropdownBindTable.Rows.Add("Pozisyon2", 2);
    dropdownBindTable.Rows.Add("Pozisyon3", 3);
    dropdownBindTable.Rows.Add("Pozisyon4", 4);
    dropdownBindTable.Rows.Add("Pozisyon5", 5);
    if (e.Row.RowType == DataControlRowType.DataRow)
    {  
        //You need to bind dropdownlist once and it will apply to all rows dropdownlist.
        DropDownList drpEleman = (e.Row.FindControl("drpEleman") as DropDownList);
        drpEleman.DataSource = dropdownBindTable;
        drpEleman.DataTextField = "Pozisyon";
        drpEleman.DataValueField = "ID";
        drpEleman.DataBind();
        drpEleman.Items.Insert(0, new ListItem("Please Select", ""));
    }
}

ScreenShot

Thank you. But Can we do something like this? The first gridview's data is the second gridview's textbox, dropdown etc. Can we fill it. Because at the same time we can change the data in the second GridView.