Declare IEnumerable<T> variable for use later on

Question

I have a LINQ query such as:

using(csv.......)
{
    csv.Select(x => new Data() { ID = x[27], Raw = String.Join(",", x) });
}
//Do some stuff

//Now I want to use the result of the query
foreach(var item in ??)
{

}

The query returns IEnumerable within my using statement but how do I declare and then assign a variable so I can use it later on in a foreach statement?

Solution 1

The type of the query is IEnumerable<Data> which you can declare outside the using block. However, from the comments it is obvious that you cannot enumerate after myObject has been disposed. If you don't want to pull all elements into a list before leaving the using block your only option is to do the enumeration inside the using block. Pulling data from a database requires an open connection to that database etc.

using(myObject.......) 
{ 
  var query = csv.Select(x => new Data() { ID = x[27], Raw = String.Join(",", x) }); 
  //Do some stuff 

  //Now I want to use the result of the query
  foreach (var item in query)
    ...
} 

Solution 2

You should take well advantage of IEnumerable. The beauty of this is your query is not evaluated until you are need. Once you do a ToList() your query is executed fully. It is well suggested to use IEnumerable as much as possible.

You just can define IEnumerable<Data> in the outer scope like this

IEnumerable<Data> myDataList;
using(myObject.......)
{
    myDataList =csv.Select(x => new Data() { ID = x[27], Raw = String.Join(",", x) });
}
// do your stuffs
// and just use the IEnumerable<Data> in your foreach loop
foreach(var item in myDataList)
{
    // here item is typeof(Data)
}

Solution 3

Declare a variable outside the using block.

Call ToList() or ToArray() to execute the query.

List<Data> data=null;
using(myObject.......)
{
    data=csv.Select(x => new Data() { ID = x[27], Raw = String.Join(",", x) }).ToList();
}

If it's possible to execute the query after returning from the using clause you could do:

IEnumerable<Data> data=null;
using(myObject.......)
{
    data=csv.Select(x => new Data() { ID = x[27], Raw = String.Join(",", x) });
}

But if that's the case, why are you putting it into the using clause in the first place?

If the query depends on myObject not being disposed you have two choices:

• Use ToList() to execute the query immediately and load it all into memory.
• dispose myObject later.