Dynamic WHERE clause in LINQ

Question

What is the best way to assemble a dynamic WHERE clause to a LINQ statement?

I have several dozen checkboxes on a form and am passing them back as: Dictionary<string, List<string>> (Dictionary<fieldName,List<values>>) to my LINQ query.

public IOrderedQueryable<ProductDetail> GetProductList(string productGroupName, string productTypeName, Dictionary<string,List<string>> filterDictionary)
{
    var q = from c in db.ProductDetail
            where c.ProductGroupName == productGroupName && c.ProductTypeName == productTypeName
            // insert dynamic filter here
            orderby c.ProductTypeName
            select c;
    return q;
}

Answer 1

I had same question ( User defined filter for linq ), and @tvanfosson told me about Dynamic Linq ( http://code.msdn.microsoft.com/csharpsamples ).

Answer 2

I have similar scenario where I need to add filters based on the user input and I chain the where clause.

Here is the sample code.

var votes = db.Votes.Where(r => r.SurveyID == surveyId);
if (fromDate != null)
{
    votes = votes.Where(r => r.VoteDate.Value >= fromDate);
}
if (toDate != null)
{
    votes = votes.Where(r => r.VoteDate.Value <= toDate);
}
votes = votes.Take(LimitRows).OrderByDescending(r => r.VoteDate);

Answer 3

You can also use the PredicateBuilder from LinqKit to chain multiple typesafe lambda expressions using Or or And.

http://www.albahari.com/nutshell/predicatebuilder.aspx

Answer 4

I came up with a solution that even I can understand... by using the 'Contains' method you can chain as many WHERE's as you like. If the WHERE is an empty string, it's ignored (or evaluated as a select all). Here is my example of joining 2 tables in LINQ, applying multiple where clauses and populating a model class to be returned to the view. (this is a select all).

public ActionResult Index()
    {
        string AssetGroupCode = "";
        string StatusCode = "";
        string SearchString = "";

        var mdl = from a in _db.Assets
                  join t in _db.Tags on a.ASSETID equals t.ASSETID
                  where a.ASSETGROUPCODE.Contains(AssetGroupCode)
                  && a.STATUSCODE.Contains(StatusCode)
                  && (
                  a.PO.Contains(SearchString)
                  || a.MODEL.Contains(SearchString)
                  || a.USERNAME.Contains(SearchString)
                  || a.LOCATION.Contains(SearchString)
                  || t.TAGNUMBER.Contains(SearchString)
                  || t.SERIALNUMBER.Contains(SearchString)
                  )
                  select new AssetListView
                  {
                      AssetId = a.ASSETID,
                      TagId = t.TAGID,
                      PO = a.PO,
                      Model = a.MODEL,
                      UserName = a.USERNAME,
                      Location = a.LOCATION,
                      Tag = t.TAGNUMBER,
                      SerialNum = t.SERIALNUMBER
                  };


        return View(mdl);
    }

Answer 5

alt text http://www.scottgu.com/blogposts/dynquery/step2.png

You need something like this? Use the Linq Dynamic Query Library (download includes examples).

Check out ScottGu's blog for more examples.

Answer 6

A simple Approach can be if your Columns are of Simple Type like String

public static IEnumerable<MyObject> WhereQuery(IEnumerable<MyObject> source, string columnName, string propertyValue)
{
   return source.Where(m => { return m.GetType().GetProperty(columnName).GetValue(m, null).ToString().StartsWith(propertyValue); });
}