how to get dropdownlist selected value on controller action

Question

//Model

public class ObjectModel

{

[Required(ErrorMessage = "Please Select District")]

[Display(Name = "Select District")]

public IList<SelectListItem> DistrictNamesModel { get; set; }

public int DistrictId { get; set; }

}

//View

@model MVCtrendsetters.Models.ObjectModel

@{

ViewBag.Title = "Insert";

}

<h2>Insert</h2>

@using (Html.BeginForm("Insert", "AppForm", FormMethod.Post, new { @enctype = "multipart/form-data" }))

{

@Html.AntiForgeryToken()

@Html.ValidationSummary(true)

<fieldset>

<legend>ObjectModel</legend>

<div>

@Html.DropDownListFor(x => x.DistrictNamesModel, Model.DistrictNamesModel , "--Select--", new { @id = "ddlDistrict"});

</div>

<p>

<input type="submit" value="Create" />

</p>

</fieldset>

}

//Controller

ObjectModel OM = new ObjectModel();

DB dd = new DB();

DataSet ds = new DataSet();

ds = dd.getAllDistrict();

List<SelectListItem> DistrictNames = new List<SelectListItem>();

foreach (DataRow item in ds.Tables[0].Rows)

{

DistrictNames.Add(new SelectListItem { Text = item["CityName"].ToString(), Value = (item["Id"].ToString()) });

}

OM.DistrictNamesModel = DistrictNames;

return View(OM);

[HttpPost]

[ValidateAntiForgeryToken]

public ActionResult Insert(ObjectModel OM , string DistrictId)

{

return View(OM);

}

Myy question Is How will i get DistrictId in [Httpost] Insert method.. I used a dropdown which is filled correctly. My table has Column Name City adn Id....PLEASE HELP....I will be really thankful