C# – Linq to sql truncating string returned by Stored Procedure

I have asked this question before. but i was not able to get any answer. may be i wasnt very clear. let me give some more details.

I have a SP which returns a long string. here is dbml file code

[Function(Name="dbo.spX")]
public ISingleResult<spXResult> spX([Parameter(DbType="VarChar(8000)")] string str)
{
    IExecuteResult result = this.ExecuteMethodCall(this, ((MethodInfo)(MethodInfo.GetCurrentMethod())), str);
    return ((ISingleResult<spXResult>)(result.ReturnValue));
}

and here is spXResult class

public partial class spXResult
{
    private string _XML_F52E2B61_18A1_11d1_B105_00805F49916B;

    public spXResult()
    {  }

    [Column(Name="[XML_F52E2B61-18A1-11d1-B105-00805F49916B]", 
     Storage="_XML_F52E2B61_18A1_11d1_B105_00805F49916B", 
     DbType="NText", UpdateCheck=UpdateCheck.Never)]
    public string XML_F52E2B61_18A1_11d1_B105_00805F49916B
    {
        get
        {
            return this._XML_F52E2B61_18A1_11d1_B105_00805F49916B;
        }
        set
        {
            if ((this._XML_F52E2B61_18A1_11d1_B105_00805F49916B != value))
            {
                this._XML_F52E2B61_18A1_11d1_B105_00805F49916B = value;
            }
         }
     }
}

and here is my code

ISingleResult<spXResult> result = ctx.spX("1234");

string returnStr = result.First().XML_F52E2B61_18A1_11d1_B105_00805F49916B;

everything is fine, when the result is not a long string, but as soon as the sp returns a very long string, it truncates the result. i have no clue why.. can someone please help.

thanks