Expression.Compile

C# course · Advanced · Lesson 40/83

Learn to build delegates at runtime with Expression.Compile.

Expression.Compile

Compile() turns an expression tree into a runnable delegate at runtime. With var fn = expr.Compile(); you get a Func<T> that you can call like fn(5).

The conversion emits IL at runtime and wraps it in a delegate; this has an up-front cost, but subsequent calls are fast. It is a key tool for dynamic code generation, rule engines and expression-based query providers.

When to use Compile?

Compile is used when you build an expression tree at runtime. For example, if you combine conditions from user input into a filter, you compile the result into a delegate and run it over data.

Rule engines and dynamic query systems are typical users. If the expression tree is known at compile time, you don't need Compile — the Expression<T> can already be a delegate.

Exercise Build a delegate with Compile and invoke it

Define an expression tree with Expression<Func<int, int>> square = x => x * x;. Build a delegate with Func<int, int> fn = square.Compile();. Print fn(5) with Console.WriteLine.

Show example solution

Define the expression tree with Expression<Func<int, int>> square = x => x * x;. Build the delegate with Func<int, int> fn = square.Compile();. Print with Console.WriteLine(fn(5));.
Example solution:
using System;
using System.Linq.Expressions;

class Program
{
    static void Main()
    {
        Expression<Func<int, int>> square = x => x * x;
        Func<int, int> fn = square.Compile();
        Console.WriteLine(fn(5));
    }
}

Test yourself

Complete the Compile call

var fn = expr.____();

Show answer

var fn = expr.Compile();

What does Compile return?
- An expression tree
- A string
- A Type object
- A runnable delegate
Show answer

A runnable delegate — Correct. Compile returns a runnable delegate.
Match the method to its purpose

Compile

Produces a delegate
Invoke

Calls the delegate
NodeType

Gives the operation kind
Parameters

Gives the parameter list

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Expression.Compile