I'm pretty new to LINQ to SQL and have come across a problem with dynamic Where clauses and Joins. For exampe, if I have 2 parameters, one of them optional, I need to find a way to include/exclude the optional parameter from my query. In SQL I would build up statement as a string and then execute it. I have read an example by ScottGU - http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx - which handles a single table but I can't find a way to do this with multiple joins.

Here is the code I am using

Public Function GetCompByFacilityID(ByVal intFacilityID As Integer, ByVal strUsername As String) As Object
        Dim dc As New DCMainDataContext
        Dim query1 = Nothing

        Try
            query1 = (From cmp In dc.CMP_Comp _
                         Join fac In dc.Facilities On cmp.FacilityID Equals fac.FacilityID _
                         Join usrfac In dc.UserFacilities On fac.FacilityID Equals usrfac.FacilityID _
                         Join cmpfrm In dc.LU_FormOfComp On cmp.FormComplimentID Equals cmpfrm.FormComplimentID _
                         Where usrfac.Username = strUsername And fac.FacilityID = intFacilityID _
                         Select cmp.FacilityID, fac.FacilityName, cmp.CompDate, cmpfrm.FormComp, cmp.ReceivedFrom _
                         Order By FacilityName).ToList
        Catch ex As Exception
            Throw ex
        Finally
            dc = Nothing
        End Try
        Return query1
    End Function

If the value of intFacility ID is -1 I want to exclude this from the statement altogether. Does anyone have an example of how to do this?