Removing read only attribute on a directory using C#

I was successfully able to remove read only attribute on a file using the following code snippet:

In main.cs

FileSystemInfo[] sqlParentFileSystemInfo = dirInfo.GetFileSystemInfos();

foreach (var childFolderOrFile in sqlParentFileSystemInfo)
{
    RemoveReadOnlyFlag(childFolderOrFile);
}

private static void RemoveReadOnlyFlag(FileSystemInfo fileSystemInfo)
{
    fileSystemInfo.Attributes = FileAttributes.Normal;
    var di = fileSystemInfo as DirectoryInfo;

    if (di != null)
    {
        foreach (var dirInfo in di.GetFileSystemInfos())
            RemoveReadOnlyFlag(dirInfo);
    }
}

Unfortunately, this doesn't work on the folders. After running the code, when I go to the folder, right click and do properties, here's what I see:

The read only flag is still checked although it removed it from files underneath it. This causes a process to fail deleting this folder. When I manually remove the flag and rerun the process (a bat file), it's able to delete the file (so I know this is not an issue with the bat file)

How do I remove this flag in C#?