Concept & use-cases; incremental generators (overview)

What is a generator?

Partial class idea

using System;

// Partial lets code live in multiple files; generators often emit a second part.
public partial class Person
{
  // "User" code (hand-written)
  public string FirstName;
  public string LastName;
}

// Imagine another file is generated with helpers.
public partial class Person
{
  // This region simulates "generated" members.
  public string FullName()
  {
    // simple boilerplate a generator could write
    return (FirstName ?? """") + " " + (LastName ?? """");
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Person p = new Person();
    p.FirstName = "Ada";
    p.LastName = "Lovelace";
    Console.WriteLine(p.FullName());
  }
}

Attribute markers

using System;
using System.Reflection;

// Marker attribute a generator would search for
[AttributeUsage(AttributeTargets.Class)]
public sealed class AutoNotifyAttribute : Attribute { }

// A class we want to augment (emulating generator input)
[AutoNotify]
public class Settings
{
  public string Theme; // we wish to auto-generate a property wrapper
}

// In real generators, Roslyn would find [AutoNotify] and emit code.
// Here we only "discover" the attribute at runtime (emulation).

public class Program
{
  public static void Main(string[] args)
  {
    Type t = typeof(Settings);
    object[] at = t.GetCustomAttributes(typeof(AutoNotifyAttribute), false);
    Console.WriteLine(at.Length > 0 ? "Marker found" : "No marker");
  }
}

Code template demo

using System;
using System.Text;

// Tiny template that prints C# code a generator could add.
public static class Template
{
  public static string EmitAutoProperty(string className, string fieldName)
  {
    // Very small example; real generators would use syntax trees.
    StringBuilder sb = new StringBuilder();
    sb.AppendLine("public partial class " + className);
    sb.AppendLine("{");
    sb.AppendLine("  public string " + Upper(fieldName) + "");
    sb.AppendLine("  {");
    sb.AppendLine("    get { return this." + fieldName + "; }");
    sb.AppendLine("    set { this." + fieldName + " = value; /* notify here */ }");
    sb.AppendLine("  }");
    sb.AppendLine("}");
    return sb.ToString();
  }

  private static string Upper(string name)
  {
    if (string.IsNullOrEmpty(name)) return name;
    if (name.Length == 1) return name.ToUpperInvariant();
    return char.ToUpperInvariant(name[0]) + name.Substring(1);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    // Show what the "generated" code could look like
    string code = Template.EmitAutoProperty("Settings", "theme");
    Console.WriteLine(code);
  }
}

Incremental overview

Where they shine

Generator role

Recap