Using the SelectMany() God function.

SelectMany God

void Main()
{
	// An implementation of the Where method using the SelectMany() method.
	TestWhere();
	
	// An implementation of Join using the SelectMany() method.
	TestJoin();
}

public static IEnumerable<T> Where<T>(IEnumerable<T> stuffs, Func<T, bool> predicate)
{
	return 
		from i in stuffs
		from i2 in ( predicate(i) ? new T[] { i } : new T[0] )
		select i2;
}

public static IEnumerable<V> Join<T, TKey, U, V>(
	IEnumerable<T> innerCollection,
	IEnumerable<U> outerCollection,
	Func<T, TKey> innerKeySelector,
	Func<U, TKey> outerKeySelector,
	Func<T,U,V> resultSelector) {
	
	var outerLookup = outerCollection.ToLookup(o => outerKeySelector(o));
	
	return
		from i in innerCollection
		from o in outerLookup[innerKeySelector(i)]
		select resultSelector(i, o);
}

void TestWhere() {

	var numbers = Enumerable.Range(0, 10);
		
	var results = Where(numbers, n => (n & 1) == 1);
	
	CollectionAssert.AreEqual(new[] {1, 3, 5, 7, 9}, results);
}

void TestJoin() {

	var customers = new[] 
	{
		new { ID = 1, Name = "John" },
		new { ID = 2, Name = "Mary" },
		new { ID = 3, Name = "Hank" }
	};
	
	var purchases = new[]
	{
		 new { Product = "Tablet", CustomerID = 1 },
		 new { Product = "iPhone", CustomerID = 2 },
		 new { Product = "LINQPad", CustomerID = 2 },
		 new { Product = "ReSharper", CustomerID = 3 }
	};
	
	var results = Join(
	
		customers, 		purchases,
		c => c.ID, 		p => p.CustomerID, 
		
		(c,p) => new { c.ID, c.Name, p.Product } );
	
	var expectedResults = 
		new [] {
			new { ID = 1, Name = "John", Product = "Tablet" },
			new { ID = 2, Name = "Mary", Product = "iPhone" },
			new { ID = 2, Name = "Mary", Product = "LINQPad" },
			new { ID = 3, Name = "Hank", Product = "ReSharper" },
		};
		
	CollectionAssert.AreEqual(expectedResults, results);
}