Using the iterator variable of foreach loop in a lambda expression - why fails?

Solution 1:

Welcome to the world of closures and captured variables :)

Eric Lippert has an in-depth explanation of this behaviour:

• Closing over the loop variable considered harmful
• Closing over the loop variable, part two

basically, it's the loop variable that is captured, not it's value. To get what you think you should get, do this:

foreach (var type in types)
{
   var newType = type;
   var sayHello = 
            new PrintHelloType(greeting => SayGreetingToType(newType, greeting));
   helloMethods.Add(sayHello);
}

Solution 2:

As a brief explanation that alludes to the blog postings that SWeko referenced, a lambda is capturing the variable, not the value. In a foreach loop, the variable is not unique on each iteration, the same variable is used for the duration of the loop (this is more obvious when you see the expansion the compiler performs on the foreach at compile time). As a result, you've captured the same variable during each iteration, and the variable (as of the last iteration) refers to the last element of your set.

Update: In newer versions of the language (beginning in C# 5), the loop variable is considered new with each iteration, so closing over it does not produce the same problem as it did in older versions (C# 4 and prior).

Solution 3:

You can fix it by introducing additional variable:

...
foreach (var type in types)
        {
            var t = type;
            var sayHello = new PrintHelloType(greeting => SayGreetingToType(t, greeting));
            helloMethods.Add(sayHello);
        }
....