Question

I’m trying to implement cascading controls using the following LINQ query expression.

The idea is that I have three option lists represented by the tables OptionA, OptionB and OptionC and a view called OptionIndex with one column each for OptionA_ID, OptionB_ID, OptionC_ID and that table has of all the combinations of tags from the option lists that are in use. Left outer joining the OptionIndex on the option list produces a boolean for the Disabled attributed in the option tag.

How do I make the on clause, which is .Where(…) in the following sample code, allow for any combination of the controls being used?

For example, lets say the user initially selects option value 123 in OptionA. The code to return the Values, Labels and Disabled booleans for OptionC would look like the following:

from t1 in OptionCs
from t2 in OptionIndexes.Where(x => t1.OptionC_ID == x.OptionC_ID && new List<int> { 123 }.Contains(x.OptionA_ID)).DefaultIfEmpty()
group new {t1, t2} by new { t1.OptionC_ID, t1.Label } into g
select new { g.Key.OptionC_ID, g.Key.Label, Disabled = g.Count(t => t.t2.OptionC_ID == null) > 0 }

Then lets say the user selects option values 456 and 789 in OptionB. The code to return the Values, Labels and Disabled booleans for OptionC change to:

from t1 in OptionCs
from t2 in OptionIndexes.Where(x => t1.OptionC_ID == x.OptionC_ID && new List<int> { 123 }.Contains(x.OptionA_ID) && new List<int> { 456, 789 }.Contains(x.OptionB_ID)).DefaultIfEmpty()
group new {t1, t2} by new { t1.OptionC_ID, t1.Label } into g
select new { g.Key.OptionC_ID, g.Key.Label, Disabled = g.Count(t => t.t2.OptionC_ID == null) > 0 }

To make the example code easier to understand I used new List<int>. In the actual project, however I would be passing the integers from the option list in as integer arrays from the controls themselves.

The trick is somehow making the query expression dynamic so that it can represent any combination of 0 to N multi-select controls being used or passing something that tells the join to accept any value for any given control such as

{x.OptionB_ID.Any}.Contains(x.OptionB_ID)

What is the best way to handle this?

Thanks!

Answer 1

Distilling your issue down to a simple example, consider this list of integers:

List<int> l = new List<int> { 1, 25, 3, 99, -23, 0, 15, 75 };

Say that you want to conditionally filter this list based on external criteria. Sometimes you want positive numbers, sometimes you want numbers smaller than 50, sometimes you want numbers divisible by 5, or any combination of these. Applying all filters with a static expression would look like this:

l.Where(n => n > 0).Where(n => n < 50).Where(n => n % 5 == 0);

To apply any or all of these dynamically, just build the LINQ query in pieces:

// These switches simulate your external conditions.
bool conditionA = true;
bool conditionB = false;
bool conditionC = true;

IEnumerable<int> myList = l;
if (conditionA) { myList = myList.Where(n => n > 0     ); }
if (conditionB) { myList = myList.Where(n => n < 50    ); }
if (conditionC) { myList = myList.Where(n => n % 5 == 0); }

With the switches set as in my example, the output is 25, 15, 75.

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Side note: if you are not aware of it, use LINQPad to experiment with things like this. It is a fantastic tool for essentially executing code interactively, be it LINQ code or not. When I built the above sample, I inserted myList.Dump(); calls after each of the last 4 lines so I could see how each filter was applied. Here is the output: