Dynamically adding or removing items from a C# collection in a loop.

Strictly speaking, there is no way to dynamically add or remove items from a Collection in a C# Foreach loop. There are at least three ways I know of that this can be faked though, do you know any others?

The "Right" Way

for(i = list.Count()-1;i>=0;i--)
{
item=list[i];
if (item.Delete) list.Remove(item);
}

This way cycles through the list backwards with a plain old For loop. Doing this forwards could be problematic if the size of the collection changes, but backwards should always be safe.

The "Easy" Way

foreach(var item in list.ToList())
{
if(item.Delete) list.Remove(item);
}

Simply create an entirely new list from the first one. I say "Easy" rather than "Right" as creating an entirely new list probably comes at a performance premium over the previous method (I haven't bothered with any benchmarking.) I generally prefer this pattern, it can also be useful in overcoming Linq-To-Entities limitations.

The "Strange" Way

var outList = new List(CustomObject);
foreach(var item in list)
{
if(!item.Delete) outList.Add(item);
}
list = outList;

I encountered this particular pattern in early versions of the Monkey programming language. Monkey originally had no way of removing items from a collection (No longer an issue), so the only way you could do it is to create a smaller list to supplant the larger one. This is probably the clumsiest and slowest of the three methods, but it has the unusual (but probably useless) advantage of not needing a remove method.

The Linq Way

list.RemoveAll(p=>p.Delete);

The namespace 'global namespace' already contains a definition for 'xxxxx'

While troubleshooting a handler in ASP.NET suddenly, I encountered with following error. I started looking around and reverted the changes however error didn't go and then I found the reason for that :)

Server Error in '/' Application.
--------------------------------------------------------------------------------

Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0101: The namespace 'global namespace' already contains a definition for 'xxxx'

I had renamed and kept the versions of the same .cs file for the handler in the same directory. You should also check whether you have a copy of the file with different name but using the same class thus having duplicate file. I took the files out of the directory and kept them somewhere else as backup, hence solved the problem. You can also save your precious time either by deleting the file or excluding it from the project.