Onselectedindexchanged event of dropdownlist is not firing in ASP.NET C#

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C#

I actually added one dropdownlist in my project and then just fired event onselectedindexchanged and i tried a lot to fire it,
at last i tied its really not working please help me.. and one more thing i have added one template also in my project. is there any problem regarding that template?

What I have tried:

here is my code:
<asp:DropDownList ID="drl1" runat="server" OnSelectedIndexChanged="drl1_SelectedIndexChanged" >

CODE:
if (!IsPostBack)
{
ShowDDL();
}

private void ShowDDL()
{
var size = from PS in drop.Product_Size
select new { PS.Size, PS.Size_Id };
drl1.DataValueField = "Size_Id";
drl1.DataTextField = "Size";
drl1.DataSource = size.ToList();
drl1.DataBind();
drl1.Items.Insert(0, "-Select Size-");
}


try
{
string size = drl1.SelectedValue;
var products = (from PM in drop.PRODUCT_DBM
join PS in drop.Product_Size on PM.product_size_id equals PS.Size_Id
where PS.Size == size
select new
{
PS.Size
});
}
catch (Exception ex)
{
}