使用Linq select和String.Join插入行号

Question

Hi all and thanks in advance,

I have a question about how (or if) the following should be done. I have a list of items being returned and for certain pieces I am using a string.join to compile those parts into a single string on the page but now I need to add line numbers to this display.

Currently this is how the data is displaying and how I want it to look after numbers are added:

 Title (before)             Title (after)
  ---------                 ---------
  Book1                     1. Book1
  Book2                     2. Book2

The data is being displayed in a List View in a single Eval statement for each item (Title,Program) like so:

 <asp:ListView runat="server" ID="lvResults" AutoGenerateColumns="False" ItemPlaceholderID="resultsPlaceHolder">
      <EmptyDataTemplate>
           <div style="width: 100%; text-align: center;">No results available.</div>
      </EmptyDataTemplate>
      <LayoutTemplate>
           <table id="tblResults" summary="">
               <tbody>
                    <asp:PlaceHolder runat="server" ID="resultsPlaceHolder"></asp:PlaceHolder>
               </tbody>
           </table>
      </LayoutTemplate>
      <ItemTemplate>
           <tr>
               <td>
                    Text: <%# Eval("Text") %><br/>
                    Description: <%# Eval("Description") %><br/>
                    <br class="clear"/>
               </td>
               <td>
                    Title<br/><%# Eval("Title") %>
               </td>
               <td>
                    Program<br/><%# Eval("Value") %>
               </td>
            </tr>
      </ItemTemplate>
   </asp:ListView>

I'm populating the List View with this:

 List<MyBooks> newBooklist = oldBookList.Select(i => new MyBooks() { Id = i, Text = oldBookList.Where(o => o.Id == i).Select(o => o.Text).FirstOrDefault(), Description = oldBookList.Where(o => o.Id == i).Select(o => o.Description).FirstOrDefault(), //tried getting index here but output is wrong "{ index = 0, title = .. }" Title = string.Join("<br/>", oldBookList.Where(o => o.Id == i).Select((o,index) => new { index, o.Title }).Distinct()), Value = string.Join("<br/>", oldBookList.Where(o => o.Id == i).Select(o => o.Value).Distinct()), }).ToList(); 

I'd like to fix the above so that I can add in the numbers into the text as part of the string.join operation. To that end, I tried following suggestions here How To Project a Line Number Into Linq Query Result and here How to get index using LINQ? [duplicate] but both examples seem to deal with have a separate property to dump the value into which I don't have in my scenario.

Another idea that I found was to consider looping through the list Titles and adding the text before I pull this list that populates the List View but I'm concerned that would be an expensive operation, would it be?

The one thing I would really prefer not to do is to add these numbers to the SQL query because this query is used in other places.

[UPDATE]

As asked in the comment below, I wanted to have numbers but I needed the numbers to further subgroup by Title and Program. The best way to achieve this finally turned out to be just iterating through the list before hand with a foreach statement to set the numbers.

  int dt = 0; int oldId = 0; foreach (var d in oldList) { if (oldId != d.Id) { dt = 0; oldId = d.Id; } dt++; d.Title = dt + ". " + d.Title; d.Program = dt + ". " + d.Program; } 

Answer 1

Using enumerable.Range should get you what you want.

List<MyBooks> newBooklist = from i in Enumerable.Range(0, oldBookList.Count)
{
    Id = i, 
    Text = oldBookList[i].Text,
    Description = oldBookList[i].Description, 
    ....

}).ToList();

You use the select overload in your OP, so I assumed that you knew about it. But in that format it would look like this:

List<MyBooks> newBooklist = oldBookList.Select((book, i) => new MyBooks()
{
    Id = i, 
    Text = book.Text,
    Description = book.Description, 
    ....

}).ToList();

Answer 2

I think Jon Skeet might have the answer for you Can I have an incrementing count variable in LINQ?

Basically there is an overload of Select that provides an index, and you can use that index in your string.Join calls.

stuff.Select((value, index) => new { index, value.Name });