Expression trees and how to use them to manipulate expressions at runtime

Expression trees offer a unique capability in C#: the ability to treat code as data and manipulate it at runtime. They are central to the functionality of LINQ, allowing us to use the same query syntax for in-memory objects and external data sources. Let’s explore this fascinating feature. At a high level, an expression tree is a data structure that represents some code in a tree-like format, where each node is an expression. Expression trees are constructed from lambda expressions and allow you to inspect the code within the lambda as data.

To illustrate this, consider a simple lambda expression:

Func<int, int, int> add = (a, b) => a + b;

Now, let’s rewrite it as a binary expression:

ParameterExpression a = Expression.Parameter(typeof(int), "a");
ParameterExpression b = Expression.Parameter(typeof(int), "b");
ParameterExpression c = Expression.Parameter(typeof...