Practical use of expression trees

Question

Expression trees are a nice feature, but what are its practical uses? Can they be used for some sort of code generation or metaprogramming or some such?

Answer 1

You can use them to build your own linq provider for a website like Google or Flickr or Amazon, your own website or another data provider.

Answer 2

Marc Gravell has used them to great effect in MiscUtil to implement generic operators.

Answer 3

Implementation of LINQ providers is mostly done by processing expression trees. I'm also using them to remove literal strings from my code:

• Monorail
• NHibernate

Answer 4

As Jon notes, I use them to provide generic operators with .NET 3.5. I also use them (again in MiscUtil) to provide fast access to non-default constructors (you can't use Delegate.CreateDelegate with constructors, but Expression works fine).

Other uses of manually created expression trees:

• object cloning
• dynamic LINQ sorting
• as a compiler

But really, Expression is a very versatile way of writing any dynamic code. Much simpler than Reflection.Emit, and for my money, simpler to understand than CodeDOM. And in .NET 4.0, you have even more options available. I show the fundamentals of writing code via Expression on my blog.

Answer 5

I use them to create dynamic queries, whether it be for sorting or filtering the data. As an example:

IQueryable<Data.Task> query = ctx.DataContext.Tasks;

if (criteria.ProjectId != Guid.Empty)
      query = query.Where(row => row.ProjectId == criteria.ProjectId);

if (criteria.Status != TaskStatus.NotSet)
      query = query.Where(row => row.Status == (int)criteria.Status);

if (criteria.DueDate.DateFrom != DateTime.MinValue)
      query = query.Where(row => row.DueDate >= criteria.DueDate.DateFrom);

if (criteria.DueDate.DateTo != DateTime.MaxValue)
     query = query.Where(row => row.DueDate <= criteria.DueDate.DateTo);

if (criteria.OpenDate.DateFrom != DateTime.MinValue)
     query = query.Where(row => row.OpenDate >= criteria.OpenDate.DateFrom);

var data = query.Select(row => TaskInfo.FetchTaskInfo(row));

Answer 6

Originally by Jomo Fisher, Gustavo Guerra published a revised version of the static string dictionary.

Where through Expression trees, a dynamic expression that provides a really (read: ridiculously) Dictionary.

The implementation creates a dynamic decision tree that select the corrent value according to the length of the input string, then by the first letter, then the second letter and so on.

This ultimately runs much faster than the equivalent Dictionary.