Syntax receiver

Learn to scan the AST during compilation with SyntaxReceiver.

Syntax receiver

A syntax receiver is an object that scans the syntax tree during compilation and collects interesting nodes. The generator calls the receiver for each syntax tree; the receiver accumulates, for example, class declarations that carry a certain attribute.

This is like compile-time reflection: you inspect the structure of code without running it. The collected nodes are later used for code generation.

Collecting and counting

A receiver usually holds a List<string> and adds an item for each matching node. At the end of compilation this list is passed to the generator, which emits code for each item.

In this lesson we simulate the receiver with a ClassCollector class: the OnClass method adds a name, and the Count property gives the total.

Exercise Collect with ClassCollector

Define a ClassCollector class. Give it a public List<string> Classes { get; } = new(); property, a public void OnClass(string name) method (with Classes.Add(name);), and a public int Count => Classes.Count; property. In Main, add three class names and print Console.WriteLine(collector.Count).

Show example solution

class ClassCollector { public List<string> Classes { get; } = new(); public void OnClass(string name) { Classes.Add(name); } public int Count => Classes.Count; }. In Main: collector.OnClass three times, Console.WriteLine(collector.Count);
Example solution:
using System;
using System.Collections.Generic;

class ClassCollector
{
    public List<string> Classes { get; } = new();
    public void OnClass(string name) => Classes.Add(name);
    public int Count => Classes.Count;
}

class Program
{
    static void Main()
    {
        var collector = new ClassCollector();
        collector.OnClass("UserDto");
        collector.OnClass("OrderDto");
        collector.OnClass("AccountDto");
        Console.WriteLine(collector.Count);
    }
}

Test yourself

Complete the OnClass call

collector.____;

Show answer

collector.OnClass("UserDto");

Match each syntax receiver concept

Syntax tree

Parsed code tree
OnClass

Class node event
Classes list

Collected class names
What does a syntax receiver collect?
- Runtime errors
- User input
- Syntax tree nodes
- Network packets
Show answer

Syntax tree nodes — Correct. The receiver collects syntax tree nodes.

Next lesson: AddSource

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