Dcount A Parameterised Query Access - database

Question

Beginner at access, so sorry if this is a question that has been asked before (I've searched for ages, so maybe im searching for the wrong thing?). Anyway, in my database I have a form that is used to submit records, and a query that checks for double bookings (it shows all records that overlap with the inputted dates). I have created a 'submit record' button on the form, and all I am trying to do is validate that there are no double bookings by running the double booking query and checking if there are no records.
I have tried using a Dcount function for this, however I cannot seem to find how to run the query as it has parameters (I need to pass the inputs of the form to the query). I am unsure if this is the most efficient way to check for double bookings, but I just need a solution what works at the moment :P. This is my current code, which obviously doesnt work as the sql query requires parameters:
If DCount("*","Double Booked")=0 Then ...
(Where 'Double Booked' is the name of my query in the database)
Sorry if I was unclear, thanks in advance.
EDIT: Here is the double booked query code:
SELECT *
FROM Booking
WHERE (
([Your VanID] = [Booking].[VanID])
AND (
( ([Your Collection Date] >= [Booking].[Collection date]) AND ([Your Collection Date] <= [Booking].[Return date]))
OR ([Your Return Date] >= [Booking].[Collection date]) AND ([Your Return Date] <= [Booking].[Return date])
OR (([Your Collection Date] <= [Booking].[Collection date]) AND ([Your Return Date] >= [Booking].[Return date]))
)
);

Answer 1

If you only want to know if there are double bookings (not how many), DCount is unnecessary. Opening a recordset is enough.
Queries with form fields as parameters run fine when opened directly, or as a RecordSource, but not when opened in VBA.
To be able to do this, I have this function in a public module:
Public Sub EVal_Params(QD As DAO.QueryDef)
On Error GoTo EVal_Params_Err
Dim i As Integer
Dim par As DAO.Parameter
For i = 0 To QD.Parameters.Count - 1
Set par = QD.Parameters(i)
' This is the key line: Eval "evaluates" the form field and gets the value
par.Value = Eval(par.Name)
Next i
EVal_Params_Exit:
On Error Resume Next
Exit Sub
EVal_Params_Err:
MsgBox Err.Description, vbExclamation, "Runtime-Error " & Err.Number & " in EVal_Params"
Resume EVal_Params_Exit
End Sub
With that you can open a recordset on the query, and its .EOF property will tell you whether there are double bookings:
Dim DB As DAO.Database
Dim QD As DAO.QueryDef
Dim RS As DAO.Recordset
Set DB = CurrentDb
Set QD = DB.QueryDefs("Double Booked")
' Get form field values into query parameters
Call EVal_Params(QD)
Set RS = QD.OpenRecordset(dbOpenSnapshot)
If RS.EOF Then
' no doubles
Else
' there are double bookings
End If
RS.Close
Edit
If the query parameters are not references to form controls, but "normal" named parameters, you'd do instead:
Set QD = DB.QueryDefs("Double Booked")
QD.Parameters("Your VanID") = someValue
QD.Parameters("Your Collection Date") = someOtherValue
Set RS = QD.OpenRecordset(dbOpenSnapshot)