How to handle null values in LINQ with multiple where clauses - asp.net

Question

I have a LINQ query with multiple values in a where clause. The values in where clause are from Filtering options from checkboxlist. Checkboxlist can return Null (or empty string in the e.g.) and that means I don't need where clause in the query as selecting none on checkboxlist means selecting all. I didn't know how to write one good LINQ that can handle that so I ended up using multiple IF statements with multiple queries as below. I tested it and works fine for now with 2 parameters in the where clause. But in reality, I need more parameters to pass in to the query and it will get messy having many IF statements to do that job. How can I handle that in one good LINQ query?
Function FilterCol(ByVal col As List(Of ProductDetails), Optional SelectedCategory As List(Of String) = Nothing, Optional SelectedBrand As List(Of String) = Nothing) As List(Of ProductDetails)
Dim strSelectedCategory As String = String.Join(",", SelectedCategory .ToArray())
Dim strSelectedBrand As String = String.Join(",", SelectedBrand .ToArray())
If strSelectedCategory = "" And strSelectedBrand = "" Then
Return col
ElseIf strSelectedCategory = "" Then
Dim res1 As IEnumerable(Of StatsDetails) = From x In col Where strSelectedBrand.Contains(x.Brand) Select x
Return res1.ToList
ElseIf strSelectedBrand = "" Then
Dim res2 As IEnumerable(Of StatsDetails) = From x In col Where strSelectedCategory.Contains(x.Category) Select x
Return res2.ToList
Else
Dim res As IEnumerable(Of StatsDetails) = From x In col Where strSelectedCategory.Contains(x.Category) And strSelectedBrand.Contains(x.Brand) Select x
Return res.ToList
End If
End Function

Answer 1

If you have multiple variable conditions for your LINQ query, consider using Dynamic LINQ Library. It will allows you to combine the conditions into a string variable in advance based on supplied parameters and then use that variable as condition for LINQ Query - this is something akin to Dynamic SQL in SQL Server.

Answer 2

This is easier to do using Extension Method syntax. I don't think doing it as one LINQ statement is a good idea.
What you can do is first create your base query using just the table. I'm going to do it in C#, but it won't be complex to translate to VB--I don't want to confuse you or myself!
var baseQuery = col;
Now that you've got your base table, you can start adding wheres based on your conditions.
if(condition1 == true)
{
baseQuery = baseQuery.Where(this is your where condition);
}
if(condition2 == true)
{
baseQuery = baseQuery.Where(this is another condition);
}
So, you can chain your Wheres together, allowing you to narrow down your query based on the conditions.
Finally, you return your query's results, with the ToList. This avoids the code duplication that you had in your conditions and everything's easier to understand and maintain.

Answer 3

Use lambda exp like this:
Dim FnValueExists =
Function(v As String, a As List(Of String)) a Is Nothing OrElse
a.Count = 0 OrElse
a.Contains(v)
Then in your linq, just test for the value like below:
Dim res As IEnumerable(Of StatsDetails) =
From x In col
Where FnValueExists(x.Category, SelectedCategory) And
FnValueExists(x.Brand, SelectedBrand)
Select x
The main code that determines if item x will be selected or not, is that a Is Nothing OrElse a.Count = 0 OrElse a.Contains(v) line in the FnValueExists lambda expression.
In the case above, if SelectedCategory and/or SelectedBrand list is Nothing or empty (Count=0), or contains the value, the item will still be selected.
So if users did not select any categories, you can set SelectedCategory to Nothing or just an empty list, therefore FnValueExists(x.Category, SelectedCategory) will always return true.