CODEWITHSUNDEEP
c#linq
otisonoza
otisonoza

Reputation: 1344

Why does modifying a list affect the filtered list?

Consider the following example:

List<String> myList = new List<string>();

myList.Add("Ford");
myList.Add("Porsche");

var filteredList = myList.Where(a => a.StartsWith("F"));

myList.Add("Ferrari");

foreach (string s in filteredList)
{
    Console.WriteLine(s);
}

The output is: 

Ford  
Ferrari

When I create the filtered list, the list only contains:

Ford

Why does modifying the original list affect the filtered one?

Upvotes: 1

Views: 78

Answers (2)

Tim S.
Tim S.

Reputation: 56536

LINQ is lazily evaluated. This means that filteredList does not contain Ford at the time you create it. All it contains is a reference to myList and the lambda you gave it. It's when you actually evaluate the list with your foreach that the filtering happens. Since the list now contains Ferrari, that is returned as well.

If you want to force the evaluation to happen earlier, you could use ToList().

var filteredList = myList.Where(a => a.StartsWith("F")).ToList();

This means that the evaluation will happen right then and there (and not again).

Upvotes: 7

Nathan A
Nathan A

Reputation: 11319

Because your filteredList is not a list. It's an IEnumerable generated by LINQ. It's not a list in of itself, but is a filter that's based off the original.

If you want your filteredList to be an independent list, do this:

var filteredList = myList.Where(a => a.StartsWith("F")).ToList();

Upvotes: 3

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