stepping through a collection object in a foreach loop

[David C]

This is very strange. Say I have code like this. I am simply looping
through a collection object in a foreach loop.

Course course = new Course();

foreach(Student s in course.Students)
{
Console.WriteLine(s.StudentID);
}

When I run the code without debugging, it runs fine. Then when I step
through the loop, I get this error.

An unhandled exception of type 'System.InvalidOperationException'
occurred in mscorlib.dll

Additional information: Collection was modified; enumeration operation
may not execute.


The implmentation of course.Students? Very straightforward.

public ArrayList Students
{
get
{
if (students == null)
students = new ArrayList();

for(int i=0; i<5; i++)
{
Student s = new Student();
s.StudentID = i;
students.Add(s);
}
return students;
}
}

I want Students to be refreshed every time it's called. Now if I do
this (see below), this code will make sure that the Students gets
populated only once. That is not what I want.

What is very strange is that I've been doing this the past 3 years.
Stepped through loops about a trillion types and my implmentation has
always been like the one above.

What I have is a fresh installation of .NET. This is just strange.

public ArrayList Students
{
get
{
if (students == null)
{
students = new ArrayList();

for(int i=0; i<5; i++)
{
Student s = new Student();
s.StudentID = i;
students.Add(s);
}
}
return students;
}
}

 

[Guest]

Hi David,
the reason for your error is that while you are looping throught the
Students arraylist i.e.

Course course = new Course();

foreach(Student s in course.Students)
{
Console.WriteLine(s.StudentID);
}

The Students array list was modified which is not allowed. Are you running
this code in a multithreaded environment. Something must have called
course.Students on the same instance of course while another thread was in
the foreach loop.

I would place a lock around the setting and getting of the students
arraylist to make sure it cannot be modified whilst you are looping through
it something like:

public ArrayList Students
{
get
{
if (students == null)
students = new ArrayList();

lock(students.SyncRoot)
{
for(int i=0; i<5; i++)
{
Student s = new Student();
s.StudentID = i;
students.Add(s);
}
}
return students;
}
}

and


Course course = new Course();

lock(course.Students.SyncRoot)
{
foreach(Student s in course.Students)
{
Console.WriteLine(s.StudentID);
}
}

Hope that helps
Mark R Dawson
http://www.markdawson.org

 

[Daniel Jin]

This is very strange. Say I have code like this. I am simply looping
through a collection object in a foreach loop.

Course course = new Course();

foreach(Student s in course.Students)
{
Console.WriteLine(s.StudentID);
}

When I run the code without debugging, it runs fine. Then when I step
through the loop, I get this error.

An unhandled exception of type 'System.InvalidOperationException'
occurred in mscorlib.dll

Additional information: Collection was modified; enumeration operation
may not execute.


The implmentation of course.Students? Very straightforward.

public ArrayList Students
{
get
{
if (students == null)
students = new ArrayList();

for(int i=0; i<5; i++)
{
Student s = new Student();
s.StudentID = i;
students.Add(s);
}
return students;
}
}

I want Students to be refreshed every time it's called. Now if I do
this (see below), this code will make sure that the Students gets
populated only once. That is not what I want.

you are not refreshing anything. every time the get accessor is
called, you add 5 more Student objects to it.

when the debugger is running, in order to display the information in
the debugger, it will invoke the get accessor and cause 5 more objects
to be added to the ArrayList, and foreach loop fails.

 

[Ignacio Machin \( .NET/ C# MVP \)]

Hi,

You cannot modify the collection when you are iterating.

if you want to update the collection elements you would have to implement
the iterator yourself.


you could implement IEnumerator.Current like:

object IEnumerator.Current
{
get
{
//update the student
students[ i].Update(); //or whatever method you use
return students[ i++];
}
}




cheers,

 

[SharpCoderMP]

public ArrayList Studentsand also you shouldn't do things like this in a getter

 

[David C]

Thank you all for your replies.

I had a friend run the code with his install of VS.NET 2003, and can
step through this without a problem.

So looks like there is a problem with my instance of VS.NET 2003. I
wouldn't know how to began to address that.