What is C# equivalent of geography sql server datatype in .net framework 4.0?

If you are using entity-framework then the datatype you search is DbGeography.

Here is some more information about DbGeography object -> Details.

It may sound obvious, but why not use the same data type that has been installed as a UDT in SQL Server - SqlGeography?

The following works fine against a SQL Server 2012 instance. I'm unable to test against SQL Server 2008 but I'd assume it should work the same:

using System;
using Microsoft.SqlServer.Types;
using System.Data.SqlClient;
using System.Data;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main()
        {
            var geom1 = SqlGeography.STGeomFromText(
                        new System.Data.SqlTypes.SqlChars(
                        "LINESTRING(-122.360 47.656, -122.343 47.656)"), 4326);
            var geom2 = SqlGeography.STGeomFromText(
                        new System.Data.SqlTypes.SqlChars(
                        "LINESTRING(-100.0 45.0, -1420 49.0)"), 4326);
            using(var conn = new SqlConnection(
                  @"Server=Server;Database=master;Integrated Security=SSPI;"))
            {
                using (var cmd = new SqlCommand(
                    "select @parm1.STIntersects(@parm2)", conn))
                {
                    var p1 = cmd.Parameters.Add("@parm1", SqlDbType.Udt);
                    p1.UdtTypeName = "geography";
                    p1.Value = geom1;
                    var p2 = cmd.Parameters.Add("@parm2", SqlDbType.Udt);
                    p2.UdtTypeName = "geography";
                    p2.Value = geom2;
                    conn.Open();
                    Console.WriteLine(cmd.ExecuteScalar());
                }
            }
            Console.ReadLine();
        }
    }

}