How solve this problem in MVC3

Hi,
I am new in MVC. I have got problem. I am tiring bind a database to webgrid in MVC3.
I am getting following error. I have not created Entity. But I want programmatically how bind a database to webgrid. On coding line
users = dc.District.ToList();

Error:
System.Data.Edm.EdmEntityType: : EntityType 'District' has no key defined. Define the key for this EntityType.
System.Data.Edm.EdmEntitySet: EntityType: EntitySet ?District? is based on type ?District? that has no keys defined.
Controller Code :
public ViewResult List()
{
var users = new List<District>();
//here MyDatabaseEntities is the dbcontext
using (DistrictDBContext dc = new DistrictDBContext())
{
users = dc.District.ToList();
}
return View(users);
}

Model :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data;
using System.Data.SqlClient;
using System.Data.Entity;
using System.Configuration;

namespace MVCTest.Models
{
public class District
{
public string DISTRICT_ID { get; set; }
public string DISTRICT_NAME { get; set; }
public string YEAR { get; set; }
}

public class DistrictDBContext : DbContext
{
public DistrictDBContext()
: base("name=SqlConnectionString")
{
}
public DbSet<District> District { get; set; }

}


}

List.cshtml
@model List<MVCTest.Models.District>

@{
ViewBag.Title = "List of users";
var grid = new WebGrid(source: Model, canPage: true, rowsPerPage: 10);
grid.Pager(WebGridPagerModes.All);
}
<h2>
List of Users</h2>
<style type="text/css">
/*Here we will add css for style webgrid*/
.webgrid-table
{
font-family: "Trebuchet MS" , Arial, Helvetica, sans-serif;
font-size: 1.2em;
width: 100%;
display: table;
border-collapse: separate;
border: solid 1px #98BF21;
background-color: white;
}
.webgrid-table td, th
{
border: 1px solid #98BF21;
padding: 3px 7px 2px;
}
.webgrid-header
{
background-color: #A7C942;
color: #FFFFFF;
padding-bottom: 4px;
padding-top: 5px;
text-align: left;
}
.webgrid-footer
{
}
.webgrid-row-style
{
padding: 3px 7px 2px;
}
.webgrid-alternating-row
{
background-color: #EAF2D3;
padding: 3px 7px 2px;
}
</style>
<div id="content">
@grid.GetHtml(
tableStyle: "webgrid-table",
headerStyle: "webgrid-header",
footerStyle: "webgrid-footer",
alternatingRowStyle: "webgrid-alternating-row",
rowStyle: "webgrid-row-style",
columns: grid.Columns(
//here i will add column for serial no
grid.Column(header: "Serial No", format: @<text><div>@(item.WebGrid.Rows.IndexOf(item) + 1)</div></text>),
grid.Column(columnName: "DISTRICT_ID", header: "DISTRICT ID"),
grid.Column(columnName: "DISTRICT_NAME", header: "DISTRICT NAME"),
grid.Column(columnName: "YEAR", header: "YEAR")

))
</div>