LINQ: Select an object, but change some properties without creating a new object

Q: Using LINQ, if I wanted to perform some query and return the object from the query, but change only some of the properties in that object, how would I do this without creating a new object and manually set every property? Is this possible?

Example:

var list = from something in someList
           select x; // but change one property

This post is based off of my Stack Overflow post on the same topic.

You can do it via the LINQ extension methods pretty easily, as JaredPar answered:

var query = someList.Select(x => { x.SomeProp = "foo"; return x; })

But what if you wanted to use declarative syntax similar to my above example?  You could write a simple extension method to do that.   Example usage:

// select some monkeys and modify a property in the select statement
// instead of creating a new monkey and manually setting all properties
var list = from monkey in monkeys
           select monkey.Set(monkey1 =>
           {
               monkey1.FavoriteFood += " and banannas";
           });

This is much quicker and more concise than creating a new monkey object and manually setting all properties, like so:

// instead of
var list = from monkey in monkeys
           select new Monkey
           {
               Name = monkey.Name,
               FavoriteFood = monkey.FavoriteFood += " and banannas",
               FavoriteBeer = monkey.FavoriteBeer,
               FavoriteActivity = monkey.FavoriteActivity,
               EyeColor = monkey.EyeColor,
               HairColor = monkey.HairColor,
               LikesToFlingPoo = monkey.LikesToFlingPoo
               // etc...
           };

Answer: Write an extension method.  I wrote an extension method called Set that to facilitate this.  All it does is allow you to write a lambda expression inside your select statement that returns the same object that you pass it.  You can set any number of properties on that object without creating a new instance of it.  Here’s the source code:

namespace System.Linq
{
    public static class LinqExtensions
    {
        /// <summary>
        /// Used to modify properties of an object returned from a LINQ query
        /// </summary>
        public static TSource Set<TSource>(this TSource input, 
            Action<TSource> updater)
        {
            updater(input);
            return input;
        }
    }
}