How do lambdas scope to local variables?

Anthony Russell Published at Dev

Anthony Russell

So my understanding of how the compiler handles lambdas is limited.

My understanding is that the compiler takes your lambda and turns it into a real method.

If that's the case then how does it scope to local variables?

    public async Task<dynamic> GetWebStuff()
    {
        dynamic ret = "";

        WebClient wc = new WebClient();          

        wc.DownloadStringCompleted += async (s, a) => 
        {
            ret = await Newtonsoft.Json.JsonConvert.DeserializeObject(a.Result.ToString());
        };

        wc.DownloadString("http://www.MyJson.com");

        return ret;
    }

The above example will set the return value of ret to the caller which is a dynamic object of deserialized JSON.

How does that happen though if the compiler takes that completed event lambda and abstracts it into its own method? How does it know to set the ret value?

It's like me saying this (which obviously wont work)

        public async Task<dynamic> GetWebStuff()
        {
            dynamic ret = "";

            WebClient wc = new WebClient();

            wc.DownloadStringCompleted += wc_DownloadStringCompleted;            

            wc.DownloadString("google.com");

            return ret;
        }

        void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            ret = await Newtonsoft.Json.JsonConvert.DeserializeObject(e.Result.ToString());
        }

Selman Genç

It does that creating an anonymous class. For example consider this code:

int x = 0;

Action action = () => x = 2;

action();

Console.Write(x);

And the generated class :

enter image description here

IL code of the <Main>b__2 method which sets the value of x:

    .method assembly hidebysig instance void 
        '<Main>b__2'() cil managed
{
  // Code size       10 (0xa)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  ldc.i4.2
  IL_0002:  stfld      int32 ConsoleApplication1.Program/'<>c__DisplayClass0'::x
  IL_0007:  br.s       IL_0009
  IL_0009:  ret
} // end of method '<>c__DisplayClass0'::'<Main>b__2'

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