How to Read Lines from File and Turn Array of Strings into Dictionary in C# with line Number as Key?

I’m trying to read lines from a file (whose size is unknown) and convert the array values that start with "A" into a Dictionary with the line number as the key and the line contents as the value. I’m trying to think of a way of doing this without iterating over the array as that would be slow for large files. I’m currently putting in the line’s hashcode (assuming no duplicates, but I can use .Distinct if needed) as the key but get an error as shown below the code:

Code:

Dictionary<int, string> lines = File
  .ReadLines(inputFilePath)
  .Select(x => x.StartsWith("A"))
  .ToDictionary(x => x.GetHashCode, x => x.ToString());

The ToDictionary has the red error line with the following message:
"The Type arguments cannot be inferred from the usage. Try specifying the type arguments explicitly"

In that line, I’m just trying to get some distinct value for each Key, but I would like to get the line number if possible. I feel it’s something obvious I’m missing.

>Solution :

Let’s start from the example. You have a file like this:

Abracadabra
Boo-zoo
Sausages
Alakazam

And you want to obtain a dictionay: value is a file line which starts from A,
key is 1, 2, 3, ...

{1, "Abracadabra"}
{2, "Alakazam"}

So we can do it like this:

Dictionary<int, string> lines = File
  .ReadLines(inputFilePath)
  .Where(line => line.StartsWith("A"))
  .Select((value, index) => (value, index))
  .ToDictionary(pair => pair.index + 1, pair => pair.value);

The little trick is to obtain line index which we can do with a help of Select:

  .Select((value, index) => (value, index))

If you want to use line index before filtration, i.e.

{0, "Abracadabra"}
{3, "Alakazam"}

move Select ahead:

Dictionary<int, string> lines = File
  .ReadLines(inputFilePath)
  .Select((value, index) => (value, index))
  .Where(pair => pair.value.StartsWith("A"))
  .ToDictionary(pair => pair.index + 1, pair => pair.value);