Question

I have a custom list which inherits from Generic.List<T> like this:

public class TransferFileList<T> : List<TransferFile> { .. }

When I set (where ‘Files‘ is a TransferFileList<T>):

var files = uploadResponse.Files.Where(x => !x.Success).ToList()

the ‘files‘ object resolves as System.Collections.Generic.List<TransferFile>, not TransferFileList<T>, which is what I would expect as it was what was being filtered through the Where, so how could I successfully return a list of TransferFileList<T> into ‘files’?

I did try:

var files = uploadResponse.Files.Where(x => !x.Success).ToList() 
as TransferFileList<TransferFile>;

but using that safe cast, it just resolves as null.

Thanks guys and gals.

Answer 1

First, I have to ask why you are inheriting from List<T>? 99% of the time that’s a bad idea.

If you want to extend the functionality of a list, use extension methods:

public static something PrintErrors(this List<TransferFile> list)
{
    //do your printing logic
}

On to the answer: ToList() operates on an IEnumerable<T> and converts the members of the sequence to a List of the same type. Since you inherit from List<T> which implements IEnumerable<T>, that’s what happens there.

Where() works the same way – operates on an IEnumerable<T> and returns an IEnumerable<T>.

To get some arbitrary list-like object back, like you have, you need to add the items in a sequence to your custom list, like so:

var myFiles = new TransferFileList<TransferFile>();
myFiles.AddRange(originalFileList.Where(condition));