Dependent Drop Downlist

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MVC

I have two drop downlist in View As follows

@Html.LabelFor(model => model.CgTnCode , "District")
             @Html.DropDownList("TnDscode",null,new{@id="cmbTnDscode" ,@size="1.2"})
             @Html.LabelFor(model => model.CgTnCode , "Towns")
             @Html.DropDownList("Tncode",null,new{@id="cmbTncode" ,@size="1.2"})

in Controller I have Following list:

ViewBag.TnDscode = new SelectList(db.Districts, "DsCode", "DsName");
           ViewBag.Tncode = new SelectList(db.Towns,"TnCode","TnName");

Table Structure:

District: DsCode,DsName
Town: TnCode ,TnName, TnDsCode 
where DsCode=TnDsCode

when I will select District related to that district Town should be populated in Town drop downlist:
can anyone help??