Collection was modified. Enumeration operation may not execute.

public async Task<bool> DeleteFolder(Folder folder) 
{
    if (folder.SubFolders.Any()) 
    {
        foreach (Folder f in folder.SubFolders) 
        {
            await DeleteFolder(f);
        }
    }

    if (folder.Files.Any()) 
    {
        foreach (Core.Domain.Entities.File file in folder.Files) 
        {
            DisconnectAndNullifyMetadataAndSharing(file);
            _db.Files.Remove(file);
        }
    }

    folder.ParentFolder = null;
    folder.ParentFolderId = null;

    DisconnectAndNullifyMetadataAndSharing(folder);
    _db.Folders.Remove(folder);

    return await _db.SaveChangesAsync() > 0;
}

// Utility Function
private void DisconnectAndNullifyMetadataAndSharing(BaseForFileFolder entity) 
{
    if (entity.Metadata != null)
    {
        Metadata metadata = entity.Metadata;
        metadata.File = null;

        entity.Metadata = null;
        entity.MetadataId = null;
    }
    if (entity.Sharing != null)
    {
        Sharing sharing = entity.Sharing;
        sharing.File = null;

        entity.Sharing = null;
        entity.SharingId = null;
    }
}

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at CloudStoragePlatform.Infrastructure.Repositories.FoldersRepository.DeleteFolder(Folder folder) in C:\Users\rusha\source\repos\Cloud Storage Platform\CloudStoragePlatform.Infrastructure\Repositories\FoldersRepository.cs:line 135