Question

I need to access the current and previous element in an IQueryable object. If I had an int array, I would do the following:

var array = new int[]{0,1,2,3,4}; for(var i = 1; i<array.Length ; i++) {     method1(array[i-1], array[i]); } 

I don’t know to do the same with IQueryable, since it does not implement IList.

Answer 1

Using extension methods makes this fairly easy.

public static class IEnumerableExtensions {   public static IEnumerable<ValueWithPrevious<T>> WithPrevious<T>(this IEnumerable<T> @this)   {     using (var e = @this.GetEnumerator())     {       if (!e.MoveNext())         yield break;        var previous = e.Current;        while (e.MoveNext())       {         yield return new ValueWithPrevious<T>(e.Current, previous);         previous = e.Current;       }     }   } }  public struct ValueWithPrevious<T> {   public readonly T Value, Previous;    public ValueWithPrevious(T value, T previous)   {     Value = value;     Previous = previous;   } } 

Usage:

var array = new int[] { 1, 2, 3, 4, 5 }; foreach (var value in array.WithPrevious()) {   Console.WriteLine('{0}, {1}', value.Previous, value.Value);   // Results: 1, 2   //          2, 3   //          3, 4   //          4, 5 }