Roslyn fails to enforce definite assignment rule for a variable assigned in a local iterator function or local async function

Roslyn fails to enforce definite assignment rules for variables which are assigned in local iterator functions or local async functions which are called before the variables in question are used.

Since these types of local functions don’t execute all statements at the first call it can’t be proven that assignment was executed.

here are some examples:

1. Since I don’t iterate the result of Local function it’s body won’t execute at all and I will end up using unassigned local variable i;

using System;
using System.Collections.Generic;

public class C
{
  static void Main()
  {
    int i;

    Local();

    Console.WriteLine(i);

    IEnumerable<int> Local()
    {
      yield return 0;

      i = 10;
    }
  }
}

1. This case even marks statement ‘i = 10;’ as unreachable but still allows to use the variable in the enclosing method.

using System;
using System.Collections.Generic;

public class C
{
  static void Main()
  {
    int i;

    Local();

    Console.WriteLine(i);

    IEnumerable<int> Local()
    {
      yield break;

      i = 10;
    }
  }
}

1. It’s almost guaranteed that local variable i won’t be assigned by the time it’s used.

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

public class C
{
  static void Main()
  {
    int i;

    Local();

    Console.WriteLine(i);

    async Task Local()
    {
      await Task.Delay(1000*60*60);
      i = 10;
    }
  }
}

[agocke]

I’m going to disallow async and iterator functions from definitely assigning variables. The behavior is just too subtle.

Compiler data flow will be fixed to recognize these branches, though.

---

From: TessenR [e-mail] Sent: Monday, September 19, 2016 5:20:36 AM To: dotnet/roslyn Cc: Andy Gocke; Mention Subject: Re: [dotnet/roslyn] Roslyn fails to enforce definite assignment rule for a variable assigned in a local iterator function or local async function (#13762)

@agockehttps://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fgithub.com%2fagocke&data=02%7c01%7cangocke%40microsoft.com%7ce4b40b9577cd43bd24d608d3e0875c9b%7c72f988bf86f141af91ab2d7cd011db47%7c1%7c0%7c636098844388392240&sdata=R5kRtLRZ4tu2pEx1gc582382uQ%2fuWoXPThf9iNB42Eg%3d Is there any decision how this should be implemented? I will appreciate any clarification about expected behavior.

I personally think that compiler should not propagate writes from iterator/async local functions at all.

However if you are planning to propagate writes from iterator/async local functions I’d like to know expected results for some other cases which also don’t have compiler errors currently.

1. enumerating local iterator via foreach:

void M()
{
    int x;

    foreach(var y in Local()) { }

    System.Console.WriteLine(x);

    System.Collections.Generic.IEnumerable<int> Local()
    {
        yield return 5;
        x = 5;
    }
}

1. enumerating local iterator via foreach with breaks in the loop body:

void M()
{
    int x, y;

    foreach(var z in Local()) { break; }

    System.Console.WriteLine(x);
    System.Console.WriteLine(y);

    System.Collections.Generic.IEnumerable<int> Local()
    {
        x = 5; // executed
        yield return 5;
        y = 10; // not executed
    }
}

1. Enumerating part of the collection e.g. with linq Take method

using System.Linq; class C { void M() { int x,y;

    foreach(var z in Local().Take(2)) { }

    System.Console.WriteLine(x);
    System.Console.WriteLine(y);

    System.Collections.Generic.IEnumerable<int> Local()
    {
        yield return 5;
        x = 5;
        yield return 5;
        y = 5;
    }
}

}

1. Enumerating part of the collection when it has unknown number of elements before assignment

void M()
{
    int x;

    foreach(var z in Local().Take(int.MaxValue)) { } // is int.MaxValue enough?

    System.Console.WriteLine(x);

    System.Collections.Generic.IEnumerable<int> Local()
    {
        var random = new System.Random();
        int counter = 0;
        while(counter++ < random.Next())
            yield return 5;
        x = 5;
    }
}

1. assignment before/after first await in local async function with non-awaited call

void M()
{
    int x, y;

    Local();

    System.Console.WriteLine(x);
    System.Console.WriteLine(y);

    async System.Threading.Tasks.Task Local()
    {
        x = 5;
        await System.Threading.Tasks.Task.Delay(1000*60*60*24);
        y = 5;
    }
}

1. assignment if method is awaited:

async void M()
{
    int x;

    await Local();

    System.Console.WriteLine(x);

    async System.Threading.Tasks.Task Local()
    {
        await System.Threading.Tasks.Task.Delay(1000*60*60*24);
        x = 5;
    }
}

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[xieguigang]

Probably the LINQ operator in VisualBasic combine with lambda can be more simplify