How to get the index of an item in a list in a single step?

Question

How can I find the index of an item in a list without looping through it?

Currently this doesn\'t look very nice - searching through the list for the same item twice

Answer 1

Here's a copy/paste-able extension method for IEnumerable

public static class EnumerableExtensions
{
    /// <summary>
    /// Searches for an element that matches the conditions defined by the specified predicate,
    /// and returns the zero-based index of the first occurrence within the entire <see cref="IEnumerable{T}"/>.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="list">The list.</param>
    /// <param name="predicate">The predicate.</param>
    /// <returns>
    /// The zero-based index of the first occurrence of an element that matches the conditions defined by <paramref name="predicate"/>, if found; otherwise it'll throw.
    /// </returns>
    public static int FindIndex<T>(this IEnumerable<T> list, Func<T, bool> predicate)
    {
        var idx = list.Select((value, index) => new {value, index}).Where(x => predicate(x.value)).Select(x => x.index).First();
        return idx;
    }
}

Enjoy.

Answer 2

That's all fine and good -- but what if you want to select an existing element as the default? In my issue there is no "--select a value--" option.

Here's my code -- you could make it into a one liner if you didn't want to check for no results I suppose...

private void LoadCombo(ComboBox cb, string itemType, string defVal = "")
{
    cb.DisplayMember = "Name";
    cb.ValueMember = "ItemCode";
    cb.DataSource = db.Items.Where(q => q.ItemTypeId == itemType).ToList();

    if (!string.IsNullOrEmpty(defVal))
    {
        var i = ((List<GCC_Pricing.Models.Item>)cb.DataSource).FindIndex(q => q.ItemCode == defVal);
        if (i>=0) cb.SelectedIndex = i;
    }
}