Question

I found this code snippet on a different post that I don’t quite understand and would appriciate someone explaining.

private bool[] GetPageNumbersToLink(IPagedResult result)
{
   if (result.TotalPages <= 9)
      return new bool[result.TotalPages + 1].Select(b => true).ToArray();

   ...

C# Array initialization – with non-default value

My take on this:

new bool[result.TotalPages + 1]

this creates an array of bools with length equal to that of the totalpages, the default value being false on all of them.

.Select(b => true)

this uses a linq query to return only the items that are true, i.e. none of them

.ToArray();

linq returns IEnumerable or something, so it needs to go back to a new array, of length 0 as none were selected in the first place.

I think that’s what it’s doing, but it doesn’t make any sense.

Answer 1

(EDIT: Oops, misread the post (unless it was edited without me noticing).)

No, that’s not what it’s doing. The result is effectively:

Enumerable.Repeat(true, result.TotalPages + 1).ToArray();

The Select call doesn’t depend on the value that it’s selecting at all – it’s just returning true whatever the input is. Select doesn’t perform filtering, just projection.

In other words:

// Array filled with (result.TotalPages + 1) x false
new bool[result.TotalPages + 1]

// Sequence of length (result.TotalPages + 1) x true
.Select(b => true)

// Array filled with (result.TotalPages + 1) x true
.ToArray();

Anyway it’s inefficient. It’s occasionally unfortunate that .NET doesn’t provide something like:

public static T[] CreateFilledArray<T>(T value, int size)
{
    T[] ret = new T[size];
    for (int i=0; i < size; i++)
    {
        ret[i] = value;
    }
    return ret;
}