binary search by predicate in csharp

main.cs
public static int BinarySearchByPredicate<T>(
    this IList<T> list, 
    Func<T, bool> predicate, 
    int startIndex = 0, 
    int endIndex = -1)
{
    endIndex = endIndex == -1 ? list.Count - 1 : endIndex;
 
    while (startIndex <= endIndex)
    {
        int mid = (startIndex + endIndex) / 2;
 
        if (predicate(list[mid]))
        {
            return mid;
        }
        else if (predicate(list[mid]))
        {
            endIndex = mid - 1;
        }
        else
        {
            startIndex = mid + 1;
        }
    }
 
    return -1;
}
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