Update item in IEnumerable

问题

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?

回答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.

回答2:

Your first try

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

returns a new IEnumerable but does not change the old one. For that use

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

回答3:

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";

回答4:

Try this:-

var result= allsubdirs.Select(x => String.Format("{0}_{1}", x,
                                      DateTime.Now.ToString("hhmmss")));

If you don't want to store it in another variable, consider using ToList().

回答5:

As mentioned in the existing answers, an IEnumerable collection cannot be modified by trying to assign a new value to one of its elements.

However if the elements contained in the collection are of a mutable type, you could modify them. But for your example that is not possible because a string is immutable.

So if you want to modify the existing elements in an IEnumerable, the element itself should be a type that supports modification operations, as in the below example.

public class MyItem
{
    private string _value;

    public MyItem(string content)
    {
        _value = content;
    }
    public void Append(string other)
    {
        _value += other;
    }
    public void ReplaceWith(Func<string, string> replacer)
    {
        _value = replacer(_value); 
    }
}

Then you could use it like so:

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

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

来源：https://stackoverflow.com/questions/29676220/update-item-in-ienumerable

标签

linq