I remove item from ArrayList in foreach loop and get follwing exception.

Collection was modified; enumeration operation may not execute.

How can I remove items in foreach

Following is my code:

/*
 * Need to remove all items from 'attachementsFielPath' which does not exist in names array.
 */

try
{
    string attachmentFileNames = txtAttachment.Text.Trim(); // Textbox having file names.
    string[] names = attachmentFileNames.Split(new char[] { ';' });

    int index = 0;

    // attachmentsFilePath is ArrayList holding full path of fiels user selected at any time.
    foreach (var fullFilePath in attachmentsFilePath)
    {
        bool isNeedToRemove = true;

        // Extract filename from full path.
        string fileName = fullFilePath.ToString().Substring(fullFilePath.ToString().LastIndexOf('\\') + 1);

        for (int i = 0; i < names.Length; i++)
        {
        // If filename found in array then no need to check remaining items.
        if (fileName.Equals(names[i].Trim()))
        {
            isNeedToRemove = false;
            break;
        }
        }

        // If file not found in names array, remove it.
        if (isNeedToRemove)
        {
        attachmentsFilePath.RemoveAt(index);
        isNeedToRemove = true;
        }

        index++;
    }
}
catch (Exception ex)
{
    throw ex;
}