Declaring an empty Queryable?

c#, iqueryable, linq

Solution

IQueryable<type of p> fData = null;

If you want to use the query later (iow after the if):

var fData = Enumerable.Empty<type of p>().AsQueryable();

Update:

Now for using with anonymous types:

IQueryable<T> RestOfMethod<T>(IQueryable<T> rData)
{
  var fData = Enumerable.Empty<T>().AsQueryable(); // or = rData;

  if(x)
    fData = fData.Concat(rData.Where(u => ...));
  if(y)
    fData = fData.Concat(rData.Where(u => ...));

  return fData;
}

// original code location
var rData = some query;
var fData = RestOfMethod(rData);

Update 2:

As pointed out, the above does not actually work, as the predicate of `Where` does not know the type. You could refactor it some more to include the predicates in the parameters, example:

IQueryable<T> RestOfMethod<T>(IQueryable<T> rData, 
  Expression<Func<T,bool>> pred1,
  Expression<Func<T,bool>> pred2) 
{ ... }

Update 3: (perhaps hacky)

var fData = rData.Take(0); // should be cheap.

Problem

How do I declare such a variable? ``` var rData = from nc in ctx.NEWSLETTER_CLIENTS join ni in ctx.NEWSLETTER_INDICES on nc.INDEX_NUM equals ni.INDEX_NUM select new { ClientID = nc.CLIENT_ID, Email = nc.CLIENT_EMAIL_ADDRESS, Index = nc.INDEX_NUM, MainClass = ni.MAIN_CLASS, SubClass = ni.SUB_CLASS, App1 = ni.VALUE_1, App2 = ni.VALUE_2, App3 = ni.VALUE_3, App4 = ni.VALUE_4 }; // Now I need to declare on a variable named fData under the function scope, // so I can later use it: var fData = ...; //What do I declare here? if(x) fData = fData.Concat(rData.Where(u => ...)); if(y) fData = fData.Concat(rData.Where(u => ...)); // etc ```

Original source