Why is this error "local variable cannot be declared in this scope because it ... is already used"?

Question

I am trying to run the code from the blog article by Eric Lippert "Why does a recursive lambda cause a definite assignment error?"

but instead of running (or giving the compilation "definite assignment error" I am getting:

A local variable named 't' cannot be declared in this scope because it would give a different meaning to 't', which is already used in a 'parent or current' scope to denote something else

Why?
Where is it already used in parent or current scope?
Tried to rename it having gotten the same error
How should I change the code to launch this code?

using System;
namespace LippertRecursiveLambdaError
{
  class Program
  {
    static void Main(string[] args)
    {
      Tree<int> tree = new Tree<int>
      (3, new Tree<int>
             (4, new Tree<int>(1), null), 
          new Tree<int>(5));
      PrintTree(tree);
      Console.ReadLine();
    }

    delegate void Action<A>(A a);
    delegate void Traversal<T>(Tree<T> t, Action<T> a);

  static void DepthFirstTraversal<T>(Tree<T> t, Action<T> a)
  {
    if (t == null) return;
    a(t.Value);
    DepthFirstTraversal(t.Left, a);
    DepthFirstTraversal(t.Right, a);
  }
  static void Traverse<T>(Tree<T> t, Traversal<T> f, Action<T> a)
  {
    f(t, a);
  }
  static void Print<T>(T t)
  {
    System.Console.WriteLine(t.ToString());
  }
  /*static void PrintTree<T>(Tree<T> t)
  {
    Traverse(t, DepthFirstTraversal, Print);
  }*/

  static void PrintTree<T>(Tree<T> t)
  {
    //Traversal<T> df = (t, a)=>          **************
    Traversal<T> df = null;
    //========================================
//The next line gives compilation error
//A local variable named 't' cannot be declared in this scope 
//because it would give a different meaning to 't', 
//which is already used in a 'parent or current' scope to denote something else       
    df = (t,
      a) =>
     {
       if (t == null) return;
       a(t.Value);
       df(t.Left, a);
       df(t.Right, a);
     };
  Traverse(t, df, Print);
  }//PrintTree
  }//class
  class Tree<T>
  {
    public Tree<T> Left;
    public Tree<T> Right;
    public T Value;

    public Tree(T value) 
    { 
       Value = value; 
    }
    public Tree(T value, Tree<T> left, Tree<T> right) 
    { 
        Value = value; 
        Left = left; 
        Right = right; 
    }
  } 
}//namespace

Whoops, you are absolutely right, that is a typo. My bad. Sorry about that. — Eric Lippert
@Eric_Lippert, thanks, wanted to find you in StackOverflow to inform abt this question but it was answered superfast — Fulproof

Lews Therin Lews Therin · Accepted Answer · 2013-02-18T11:28:29

  static void PrintTree<T>(Tree<T> t)
  {
    //Traversal<T> df = (t, a)=>          **************
    Traversal<T> df = null;
    //========================================

    df = (t,      a) =>
     {
       if (t == null) return;
       a(t.Value);
       df(t.Left, a);
       df(t.Right, a);
     };
    }

That's because Tree<T> t is one declaration And (t,a) => is another declaration.. practically the same thing as saying:

int someFunction(T t, U a)
//Assuming int is the return type

Anyway to fix: change t to another identifier.. n for example

df = (n,a) =>{
           if (n == null) return;
           a(n.Value);
           df(n.Left, a);
           df(n.Right, a);
         };
        }

Why is this error "local variable cannot be declared in this scope because it ... is already used"?

1 Answers