Update item in IEnumerable

Question

I need to update a value in a IEnumerable list.

Here is a brief IEnumerable example:

IEnumerable<string> allsubdirs = new List<string>() { "a", "b", "c" }; 

Now if I want to add a timestamp to each item, this doesnt work:

allsubdirs.Select(a => a = a + "_" + DateTime.Now.ToString("hhmmss")).ToList(); 

Neither does this:

foreach (var item in allsubdirs)             item = item + "_" + DateTime.Now.ToString("hhmmss"); 

I made it work like this:

IEnumerable<string> newallsubdirs = allsubdirs.Select(a => a + "_" + DateTime.Now.ToString("hhmmss")).ToList();         allsubdirs = newallsubdirs; 

but this somehow seems like cheating. Whats the proper way of doing this please?

Answer 1

Linq is for querying, not updating. Linq queries return a new collection based on projections, filters etc. So your choices are:

• Save the "new" collection back to the variable (or a new variable, if necessary):

  allsubdirs = allsubdirs.Select(a => a = a + "_" + DateTime.Now.ToString("hhmmss")).ToList(); 

• Use a writable interface like IList<T> and a for loop:

  IList<string> allsubdirs = new List<string>() { "a", "b", "c" };  for(int i=0; i<allsubdirs.Count(); i++)     allsubdirs[i] = allsubdirs[i] + "_" + DateTime.Now.ToString("hhmmss"); 

The main difference is that Select does not modify the original collection, while the for loop does.

My opinion is that the Select is cleaner and is not "cheating" - you're just adding a projection on top of the original collection.

Answer 2

D Stanley has answered it correctly. I would like to add to his answer since the title is Update item in IEnumerable implying only a single item is to be updated.

As D Stanely explained in his answer:

Linq is for querying, not updating.

and

Use a writable interface like IList and a for loop

For updating a single item, you can retrieve the index of the item to be updated and use that index to update it.

For example:

IList<string> allsubdirs = new List<string>() { "a", "b", "c" }; int index = allsubdirs.IndexOf("a"); allsubdirs[index] = "d";