sending null parameter from combox to sp via linq

[speranza]

cant pass null value to sp via linq

this is my sp code
IF @Param1 IS NOT NULL
BEGIN
SET @Param1=@Param1
END
ELSE
BEGIN
SET @Param1=' IS NULL'
END

and this my cs code


mmsDataContext mms = new mmsDataContext( );
RadGrid1.DataSo urce = mms.SON(Country .SelectedItem.V alue);
RadGrid1.DataBi nd();