Can I "unenumerate" a set of data?

Question

User1642115476 posted

Hello,

I know that once you enumerate a linq query, you cannot enumerate it again. So if I do this:

var d = GetData().ToList();

d.Where(some condition).GroupBy(some property).Select(new object);

I will get an exception.

I'm wondering if there's a way to "un-enumerate" a set of data. Something like:

var d = GetData().ToList();

d.Unenumerate();

d.Where(some condition).GroupBy(some property).Select(new object);

...such that this won't throw an exception.

Answer 1

User303363814 posted

Can you show some actual code, tell us what you want to do and what error you are actually getting?

Answer 2

User-1838255255 posted

Hi gib9898_00,

var d = GetData().ToList();

According to your description, ToList() method is use for convert data set inherited by IEnumerable. So you could use Linq to operate selected data. Please refer to this tutorial:

Introduction to LINQ Queries (C#):

https://msdn.microsoft.com/en-us/library/bb397906.aspx 

if you un-enumerate your datasource, you will can not use linq to operate datasource.

Do you want to show query data?

I hope you could give us more detailed description?

Best Regards,

Eric Du

Answer 3

User1642115476 posted

Sorry, I made a mistake in the code above. Here's how I get the error:

var d = GetData();

var l = d.ToList();

var e = d.Where(some condition).GroupBy(some property).Select(new object); // <-- ERROR: "The result of a query cannot be enumerated more than once."

Answer 4

User475983607 posted

The following is returning a query.

var d = GetData();

The following code executes the query and places the results in l.

var l = d.ToList();

The following throws an error because the query executed in the previous line of code.

var e = d.Where(some condition).GroupBy(some property).Select(new object);

Either query the list, l, or query the result of GetData()

var e = l.Where(some condition).GroupBy(some property).Select(new object);

or

var e = GetDate().Where(some condition).GroupBy(some property).Select(new object);

Please see the previously posted link for more information.

Introduction to LINQ Queries (C#):

https://msdn.microsoft.com/en-us/library/bb397906.aspx 

Answer 5

User303363814 posted

Sorry, I made a mistake in the code above

The solution is to use the first code you posted!