Has foreach's use of variables been changed in C# 5?

In this answer http://stackoverflow.com/a/8649429/1497 Eric Lippert says that "FYI we are highly likely to fix this in the next version of C#; this is a major pain point for developers" with regards to how the foreach loops uses the variable.

In the next version each time you run through the "foreach" loop we will generate a new loop variable rather than closing over the same variable every time. This is a "breaking" change but in the vast majority of cases the "break" will be fixing rather than causing bugs.

I have not been able to find anything indicating that this change has been made yet. Is there any indication that this is how the foreach loop will work in C# 5?

Answers


This is a change to the C# language, not the .NET framework. Therefore, it only affects code compiled under C# 5.0, regardless of the .NET framework version on which that code will execute.

C# 5.0

Section 8.8.4 of the specification makes it clear that this change has been made. Specifically, page 249 of the C# 5.0 specification states:

foreach (V v in x) embedded-statement

is then expanded to:

{
    E e = ((C)(x)).GetEnumerator();
    try {
        while (e.MoveNext()) {
            V v = (V)(T)e.Current;
            embedded-statement
        }
    }
    finally {
        … // Dispose e
    }
}

And later:

The placement of v inside the while loop is important for how it is captured by any anonymous function occurring in the embedded-statement.

C# 4.0

This change to the specification is clear when comparing with the C# 4.0 specification which states (again, in section 8.8.4, but this time, page 247):

foreach (V v in x) embedded-statement

is then expanded to:

{
    E e = ((C)(x)).GetEnumerator();
    try {
        V v;
        while (e.MoveNext()) {
            v = (V)(T)e.Current;
            embedded-statement
        }
    }
    finally {
        … // Dispose e
    }
}

Note that the variable v is declared outside the loop instead of inside, as it is with C# 5.0.

Note

You can find the C# specification in the installation folder of Visual Studio under VC#\Specifications\1033. This is the case for VS2005, VS2008, VS2010 and VS2012, giving you access to specifications for C# 1.2, 2.0, 3.0, 4.0 and 5.0. You can also find the specifications on MSDN by searching for C# Specification.


Need Your Help

Graphing different sets of data on same graph within a ‘for’ loop MATLAB

matlab for-loop plot regression

I just have a problem with graphing different plots on the same graph within a ‘for’ loop. I hope someone can be point me in the right direction.

About UNIX Resources Network

Original, collect and organize Developers related documents, information and materials, contains jQuery, Html, CSS, MySQL, .NET, ASP.NET, SQL, objective-c, iPhone, Ruby on Rails, C, SQL Server, Ruby, Arrays, Regex, ASP.NET MVC, WPF, XML, Ajax, DataBase, and so on.