Declaring an implicitly typed variable inside conditional scope and using it outside

c# foreach var conditional-statements implicit-typing
FMFF picture FMFF · Jan 6, 2012 · Viewed 15.2k times · Source

In the simplified code below, 

if(city == "New York City")
{
  var MyObject = from x in MyEFTable
                     where x.CostOfLiving == "VERY HIGH"
                     select x.*;

}
else
{
  var MyObject = from x in MyEFTable
                     where x.CostOfLiving == "MODERATE"
                     select x.*;

}

  foreach (var item in MyObject)
  {
     Console.WriteLine("<item's details>");
  }

The variable MyObject is not accessible outside conditional block. How can I iterate outside the if..else ?

Answer

Eric Lippert picture Eric Lippert · Jan 6, 2012

Let's clarify your confusing question. The problem is that you have two local variables, each of which has the same "unspeakable" type -- a sequence of anonymous type.

I would change your specific code like this:

string cost = city == "NYC" ? "HIGH" : "MODERATE";
var query = from row in table 
            where row.Cost == cost 
            select new { row.Population, row.Elevation };

However, if you still need to maintain the structure of the code as it is for some reason, you can do it like this:

static IEnumerable<T> SequenceByExample<T>(T t){ return null; }
...
var query = SequenceByExample(new { Population = 0, Elevation = 0.0 } );
if (whatever)
    query = ...
else
    query = ...

This is a variation on a trick called "cast by example" where you give an example of an anonymous type to a generic method. Method type inference then figures out what the return type is, and uses that as the type of the implicitly typed local. At runtime, it does nothing but create a useless object that then gets discarded quickly.

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