LINQ indexOf a particular entry

Well you can use Array.IndexOf:

int index = Array.IndexOf(HeaderNamesWbs, someValue);

Or just declare HeaderNamesWbs as an IList<string> instead - which can still be an array if you want:

public static IList<string> HeaderNamesWbs = new[] { ... };

Note that I'd discourage you from exposing an array as public static, even public static readonly. You should consider ReadOnlyCollection:

public static readonly ReadOnlyCollection<string> HeaderNamesWbs =
    new List<string> { ... }.AsReadOnly();

If you ever want this for IEnumerable<T>, you could use:

var indexOf = collection.Select((value, index) => new { value, index })
                        .Where(pair => pair.value == targetValue)
                        .Select(pair => pair.index + 1)
                        .FirstOrDefault() - 1;

(The +1 and -1 are so that it will return -1 for "missing" rather than 0.)

I'm late to the thread here. But I wanted to share my solution to this. Jon's is awesome, but I prefer simple lambdas for everything.

You can extend LINQ itself to get what you want. It's fairly simple to do. This will allow you to use syntax like:

// Gets the index of the customer with the Id of 16.
var index = Customers.IndexOf(cust => cust.Id == 16);

This is likely not part of LINQ by default because it requires enumeration. It's not just another deferred selector/predicate.

Also, please note that this returns the first index only. If you want indexes (plural), you should return an IEnumerable<int> and yield return index inside the method. And of course don't return -1. That would be useful where you are not filtering by a primary key.

  public static int IndexOf<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
     
     var index = 0;
     foreach (var item in source) {
        if (predicate.Invoke(item)) {
           return index;
        }
        index++;
     }

     return -1;
  }

If you want to search List with a function rather than specifying an item value, you can use List.FindIndex(Predicate match).

See https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.findindex?view=netframework-4.8

Right List has IndexOf(), just declare it as ILIst<string> rather than string[]

public static IList<string> HeaderNamesWbs = new List<string>
                                   {
                                      WBS_NUMBER,
                                      BOE_TITLE,
                                      SOW_DESCRIPTION,
                                      HARRIS_WIN_THEME,
                                      COST_BOGEY
                                   };

int index = HeaderNamesWbs.IndexOf(WBS_NUMBER);

MSDN: List(Of T).IndexOf Method (T)