How can I use LINQ to avoid nested loops?

Question

I've been reading about LINQ to Objects, and now my colleagues want me to present it to them.

Now, I have an OK understanding of the operators and the syntax choices, but I've heard you can avoid heavy nested loops by using LINQ. I'm having trouble coming up with a good set of "before and after" code listings to demonstrate this though.

I found a great example of sorting and grouping with and without LINQ in Magennis' book, and he also has an example of writing xml. But what about those nested loops? Is this even a realistic claim, given that we usually need a foreach loop or two to iterate over the results of the query anyway?

If anyone can explain this idea to me (ideally with specific examples), I would greatly appreciate it.

Answer 1

Say you've got a lot of products, such as:

var products = new List<Product>
    {
        new Product { Id = 1, Category = "Electronics", Value = 15.0 },
        // etc.
    };

And you want to find all products with a value > $100.0, grouped by category, you could do this using foreach:

var results = new Dictionary<string, List<Product>>();

foreach (var p in products)
{
    if (p.value > 100.0)
    {
        List<Product> productsByGroup;

        if (!results.TryGetValue(p.Category, out productsByGroup))
        {
            productsByGroup = new List<Product>();
            results.Add(p.Category, productsByGroup);
        }
        productsByGroup.Add(p);
    }
}

Or, you could simply use the LINQ methods:

var results = products.Where(prod => prod.Value > 100.0)
                  .GroupBy(prod => prod.Category);

Or using LINQ expression syntax:

var results = from p in products 
                  where p.Value > 100.0
                  group p by p.Category;

Much more concise and less error-prone.

Answer 2

Here's a type of nested loop you can remove with Linq.

foreach(SomeClass item in Items)
{
    foreach(SomeOtherClass subItem in item.SubItems)
    {
        // ...
    }
}

It can be turned into:

foreach(SomeOtherClass subItem in Items.SelectMany(i => i.SubItems))
{
}

Using the SelectMany extension method on IEnumerable.

One place where this is quite useful is for nested loop double-break scenarios.