using System;
using System.Collections.Generic;

public static class LinqQueryOperations{

  public static IEnumerable<TTarget> MySelect<TSource,TTarget>
    (this IEnumerable<TSource> source, Func<TSource,TTarget> selector){
    foreach(TSource el in source)
      yield return selector(el);
  }

  public static IEnumerable<TSource> MyWhere<TSource>
    (this IEnumerable<TSource> source, Func<TSource,bool> predicate){
    foreach(TSource el in source)
      if (predicate (el)) yield return el;
  }

  public static TSource MyAggregate<TSource>
    (this IEnumerable<TSource> source, 
     Func<TSource,TSource,TSource> reducer){
     IEnumerator<TSource> e = source.GetEnumerator();
     bool sourceNotEmpty = e.MoveNext(); 
     if (sourceNotEmpty){
       TSource result = e.Current;
       while(e.MoveNext())  
          result = reducer(result, e.Current);       
       return result;
    } 
    else throw new InvalidOperationException("Source must be non-empty");
  }

  public static int MyCount<TSource>
    (this IEnumerable<TSource> source){
    return source
            .MySelect(p => 1)
            .MyAggregate((r,i) => r + i);
  }

}