Dynamic Expressions in "Where" Clause - Linq to SQL

Question

I'm new in LINQ so I hope this isn't a stupid question:

I have a table with a lot of content presented in a datagrid, and I want the user to be able to filter the grid by using some combo boxes above the grid [like a search bar]

I created a method that takes the text in the combo boxes, and placing it the "Where" clause:

    public void find()
    {
        string disName;
        string statusName;


        disName = RMcmbDis.Text; //This Get the first string to filter
        statusName = RMcmbStatus.Text; // this get the second string to filter

//Here I gather all the data I need

        var allCNT = from x in cntDB.releases
                     join dis in cntDB.disciplines on x.discipline equals dis.discipline_id
                     join btch in cntDB.batches on x.batch_num equals btch.batch_id
                     join z in cntDB.status on x.status equals z.status_id

                     select new { dis.discipline_name, x.grade, x.batch_num, btch.batch_name, z.status_description, x.segment_leader, x.ped_leader, x.release_location, x.comments, x.QA_Verdict };

//Here I make the filtering

        var find = allCNT.Where(a => a.discipline_name == disName && a.status_description == statusName);


        dataGridView1.DataSource = find;
    }

Now I have a problem: I want the user to be able to leave one of the combo boxes empty and if he does so, this means he doesn't want to filter that criteria. [E.G - The combo "RMcmbDis" has "Math" and the Status combo ["RMcmbStatus"] is empty, so the grid will show only "Math" in all status'.

How Do I do that? Thanks guys... N.

Answer 1

You can just add Where() clauses if the condition you want is true...

var results = allCNT;

if (!string.IsNullOrEmpty(disName))
    results = result.Where(a => a.discipline_name == disname);

if (!string.IsNullOrEmpty(statusName))
    results = results.Where(a => a.status_description == statusName);

dataGridView1.DataSource = results;

See comment below for one option for handling lots of filters. Another option is to use a helper method:

T AddFilter<T>(IQueryable<T> results, string filterValue, Expression<Func<T, bool>> predicate)
{
    if(!string.IsNullOrEmpty(filterValue))
        return results.Where(predicate);
    return results;
}

Which you would use like this:

var results = allCNT;
results = AddFilter(results, disname, a => a.discipline_name == disname);
results = AddFilter(results, statusName, a => a.status_description == statusName);
results = AddFilter(results, whatever, a => a.whatever == whatever);
// ...
dataGridView1.DataSource = results;

Answer 2

You can add several Where clauses depending on what criteria you have, e.g.:

var find = allCNT;
if (!string.IsNullOrEmpty(disName))
{
    find = find.Where(a => a.discipline_name == disName);
}
if (!string.IsNullOrEmpty(statusName))
{
    find = find.Where(a.status_description == statusName);
}