Public ReadOnly Default Property Item ( _
    iIndex As Integer _
) As String

Dim instance As SPAttachmentCollection
Dim iIndex As Integer
Dim value As String

value = instance(iIndex)

public string this [
    int iIndex
] { get; }

Dim siteCollection As SPSite = SPContext.Current.Site
Dim subSites As SPWebCollection = siteCollection.AllWebs
Dim site As SPWeb

For Each site In  subSites

    Dim list As SPList = site.Lists("Announcements")
    Dim listItems As SPListItemCollection = list.Items
    Dim listItem As SPListItem

    For Each listItem In  listItems

        Dim attachments As SPAttachmentCollection = 
            listItem.Attachments
        Dim i As Integer

        For i = 0 To attachments.Count - 1

            Label1.Text += attachments(i)

        Next i

    Next listItem

Next site 

SPSite oSiteCollection = SPContext.Current.Site;
SPWebCollection collWebsites = oSiteCollection.AllWebs;

foreach (SPWeb oWebsite in collWebsites)
{
    SPList oList = oWebsite.Lists["Announcements"];
    SPListItemCollection collListItems = oList.Items;

    foreach (SPListItem oListItem in collListItems)
    {
        SPAttachmentCollection collAttachments = oListItem.Attachments;

        for (int i=0; i<collAttachments.Count; i++)
        {
            Label1.Text += collAttachments[i];
        }
    }
}