LINQ utility functions like IndexOf, ForEach, ItemAt that should have been included in LINQ

LinqUtil.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

 public static class LinqUtil
 {
 	public static void AddRange<T>(this ICollection<T> collection, IEnumerable<T> items)
        {
            foreach (T item in items)
            {
                collection.Add(item);
            }
        }
        
 	public static void AddRangeIf<T>(this ICollection<T> collection, IEnumerable<T> items, Func<T, bool> predicate)
	{
		foreach (var item in items) {
			if (!predicate (item))
				continue;
			collection.Add (item);
		}
	}
	
 	 public static string ToListText<T>(this IEnumerable<T> values)
        {
            StringBuilder builder = new StringBuilder();
            int i = 0;
            foreach (T val in values)
            {
                if (i++ > 0)
                    builder.Append(',');

                builder.Append(val != null ? Convert.ToString(val) : "NULL");
            }
            return builder.ToString();
        }
        
	public static void ForEach<T>(this IEnumerable<T> enumerator, Action<T> action)
	{
		 foreach (T element in enumerator)
		 {
		     action(element);
		 }
	}
	
	public static int IndexOf<T>(this IEnumerable<T> enumerator, T item)
	{
		 int index = 0;
		 foreach (var it in enumerator)
		 {
		     if (it.Equals(item))
		         return index;
		
		     index++;
		 }
		 return -1;
	}
	
	public static T ItemAt<T>(this IEnumerable<T> enumerator, int index)
	{
		 int idx = 0;
		 foreach (var item in enumerator)
		 {
		     if (idx++ == index)
		         return item;
		 }
		 return default(T);
	}
     
	public static void Shuffle<T>(this T[] array, int seed = 0)  
	{  
		System.Random rng = (seed != 0) ? new System.Random(seed) : new System.Random();  
		int n = array.Length;  
		while (n > 1) {  
			 n--;  
			 int k = rng.Next(n + 1);  
			 T value = array[k];  
			 array[k] = array[n];  
			 array[n] = value;  
		}  
	}

	public static T[] WithElement<T>(this T[] array, T element)
	{
	 	if (array == null)
	 	{
	 		return array = new T[] { element };
	 	}
	
	 	int index = array.Length;
	 	Array.Resize(ref array, index+1);
	 	array[index] = element;
	 	return array;
	}
	
	public static T[] Remove<T>(this T[] items, T item) 
	{
		var list = items.ToList();
		for(int i = list.Count-1; i >= 0; i--)
		{
			if (list[i].Equals(item))
			{
				list.RemoveAt(i);
				break;
			}
		}
		return list.ToArray();
	}
 }