Behind the Scenes in C#: The Compiler’s Magic with Lambdas and Yield Return

Lambda functions

The .NET runtime does not have in-line functions, all code in .NET must be in the form of methods that are part of classes — so how does lambda functions work?

using Timer = System.Timers.Timer;

var timer = new TimerExample();
timer.InitTimer();
        
Console.ReadLine();

public class TimerExample
{
    private Timer? _timer;
 
    public void InitTimer()
    {
        _timer = new Timer(1000);
        _timer.Elapsed += (sender,args) => Console.WriteLine("Hi");
        _timer.Enabled = true;
    }
}

The program will start spamming Hi every second. It clearly works. Here is the answer.

In .NET all code must be normal methods defined in classes, so how can that be? The answer is that the C# compiler rewrites you Lambda function as a normal method.

class TimerExample
{
    private Timer? _timer;
 
    private void HiddenMethodForLambda(
         object? sender, System.Timers.ElapsedEventArgs args)
   {
      Console.WriteLine("Hi");
   }
 
    public void InitTimer()
    {
        _timer = new System.Timers.Timer(1000);
        _timer.Elapsed += HiddenMethodForLambda;
        _timer.Enabled = true;
    }
}

The compiler will rearrange the code and move the body of the lambda function into a new method. This way, we can write the code in-line, and at runtime, it will be executed as a normal method.

Local variables

It was easy, but let's make things a little bit more complicated.

Lambda function also has the feature that they can use local variable from the method that defined them.

using Timer = System.Timers.Timer;

var timer = new TimerExample();
timer.InitTimer();
        
Console.ReadLine();

class TimerExample
{
    private Timer? _timer;
 
    public void InitTimer()
    {
        const string greet = "Hi";
        
        _timer = new Timer(1000);
        _timer.Elapsed += (sender,args) => Console.WriteLine(greet);
        _timer.Enabled = true;
    }
}

If we attempt to apply the same transformation to this code as we did in the previous example, we end up with two methods that share a local variable…