how to replace Collections function errors in C#...

Function Display_Attachment_List(ActivityID as Integer) as String
    If AttachmentCollections.ContainsKey(ActivityID) Then
        Dim Attachments as HashTable
        Attachments = AttachmentCollections(ActivityID)

        Dim sb as New StringBuilder
        Dim this_entry as DictionaryEntry
        Dim this_attachment as HashTable

        sb.Append("<table cellspacing=0 cellpadding=2 width=100%>")

        For each this_entry in Attachments
            this_attachment = this_entry.value

if (AttachmentCollections.ContainsKey(ActivityID))
    {
		Hashtable Attachments = default(Hashtable);
		Attachments = AttachmentsCollections[ActivityID];

		StringBuilder sb = new StringBuilder();
		DictionaryEntry this_entry = default(DictionaryEntry);
		Hashtable this_attachment = default(Hashtable);

		sb.Append("<table cellspacing="0" cellpadding="2" width="100%>");<br" mode="hold" />
		foreach ( this_entry in Attachments) {
			this_attachment = this_entry.value;