How to Create a Linq Statement from a String

Posted: 11/4/2009 1:21:26 PM

Almost two years ago ScottGu posted some really awesome code that lets you create dynamic where clauses in Linq - meaning you could use a string builder to create your where clause just like the good 'ole days when ad-hoc sql was all the rage. You're going to be better off if you can build a predicate using lambdas, but sometimes its a real pain the the rear to build a proper predicate (Func<T, bool> or Predicate<T>). Building a predicate properly typically becomes difficult when you're giving your users some mechanism to build their own filter for their data.

The problem I had with ScottGu's code is that the extension methods only worked off IQueryable<T>, and well, sometimes you would like to create a dynamic where clause for a datasource that didn't come from an IQueryProvider like LinqToSql. Sometime you want to create a dynamic where clause for a plain old IEnumerable<T>.

So, I took the time to dig through ScottGu's pretty complicated code and created a handful of methods that help you generate useful dynamic functions - including dynamic "OrderBy" and "Where" extension methods for IEnumerable<T>.

I've attached a single common file that includes both ScottGu's code, as well as my own.

Download my code here: (15 KB)

IEnumerable<Person> allPeople = GetAllPeople();
foreach(var person in allPeople.Where("Age > 10").OrderBy("Age"))
//notice the above arguments are strings and not lambdas
//this makes it a lot easier to build everything ad-hoc
Console.WriteLine(person.Age + " - " + person.FirstName);
I also included a method called CreateValueExpression, that will evaluate an expression on any object and return the result.

var fullName = DynamicLinqExtensions.CreateValueExpression<Person,string>("First + \" \" + Last");
//outputs "Jon von Gillern"

Console.WriteLine(fullName(new Person { First = "Jon", Last = "von Gillern" }));

You can find the Gu's original blog post here:

As always, Kicks and Shoutouts are appreciated!

Tags: Tips and Tricks