How to pass DropDownList selected value from one page to another and set default selected value in other page using C# in ASP.Net

Hi Kibock,

I have created a sample which full fill your requirement

Default4.aspx

<div>
    <asp:DropDownList ID="DDLList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DDLList1_SelectedIndexChanged">
        <asp:ListItem Text="Select" Value="Select"></asp:ListItem>
        <asp:ListItem Text="Apply" Value="Apply"></asp:ListItem>
        <asp:ListItem Text="Test" Value="Test"></asp:ListItem>
    </asp:DropDownList>
</div>

Default4.aspx.cs

protected void DDLList1_SelectedIndexChanged(object sender, EventArgs e)
{
    DropDownList dropdownlist = (sender as DropDownList);
    Response.Redirect("Default5.aspx?SelectedValue=" + dropdownlist.SelectedValue + "");
}

Default5.aspx

<div>
    <asp:DropDownList ID="DDLList2" runat="server" OnSelectedIndexChanged="DDLList2_SelectedIndexChanged">
        <asp:ListItem Text="Select" Value="Select"></asp:ListItem>
        <asp:ListItem Text="Apply" Value="Apply"></asp:ListItem>
        <asp:ListItem Text="Test" Value="Test"></asp:ListItem>
    </asp:DropDownList>
</div>

Default5.aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (Request.QueryString["SelectedValue"] != null)
        {
            DDLList2.Items.FindByValue(Request.QueryString["SelectedValue"].ToString()).Selected = true;
        }
    }
}

protected void DDLList2_SelectedIndexChanged(object sender, EventArgs e)
{
}

ScreenShot