OK I Was finally able to execute the code as below which doesnt give any error.
However i just now need help in ensuring that first value is saved on the page that opens up for new casenumber which is lookup field called attach_casenumber__c which is what user selects on the apex page that pops up and when use hits saves, the class should execute.

Currently, since initailly if the field attach_casenumber__c is not blank on the apex page, it still executes class code and attaches
emails and attachments.

How do i ensure in apex page that first save action on the button is done and only then the class code should execute.

thanks


Class:
public class escalationcd {
//private final Case casenow;
private final ApexPages.StandardController c;

public escalationcd(ApexPages.StandardController c) {this.c = c;}

Public PageReference getcaseemailmessage(){
List<Case> mescal = new List<Case>([Select Id,attach_casenumber__c from Case where Id = :c.getId()]);

map<Id,Id> oldnewCaseIdmap =new map<Id,Id>();
Map<Id, EmailMessage> emailMessageMap = new Map<Id, EmailMessage>{};

//Case casenow = (case)c.getRecord();
//string attv = casenow.attach_casenumber__c;

list<Case> caseemlist=new list<Case>();
for(Case cs:mescal) {
if(cs.attach_casenumber__c!=null ) {
oldnewCaseIdmap.put(cs.id,cs.attach_casenumber__c);
}
}

if(oldnewCaseIdmap.size()!=0) {
//get list of emails from old/current case
caseemlist=[Select (Select ParentId,fromaddress,fromname,subject,textbody,htmlbody,toaddress,ccaddress,status,messagedate From EmailMessages)
From Case where Id in:oldnewCaseIdmap.keySet()];
}
for(Case c:caseemlist) {
for(EmailMessage em:c.EmailMessages) {
Emailmessage message = new Emailmessage (fromname = em.fromname,subject = em.subject,textbody = em.textbody,htmlbody = em.htmlbody,toaddress = em.toaddress,
fromaddress = em.fromaddress,ccaddress = em.ccaddress,status = em.status,messagedate = em.messagedate,ParentId=oldnewCaseIdmap.get(c.id));
//emlisttoinsert.add(message);
emailMessageMap.put(em.Id, message);
}
}




Attachment[] attList = new list <Attachment>();
Attachment[] insertAttList = new list <Attachment>();
if(emailMessageMap.values().size()>0) {
insert emailMessageMap.values();

attList = [select Id, Name, Body, ParentId from Attachment where ParentId in :emailMessageMap.keySet()];

for (Attachment a : attList) {
EmailMessage em = emailMessageMap.get(a.ParentId);
Attachment att = new Attachment(Name = a.Name, Body = a.Body, ParentId = em.Id);
insertAttList.add(att);
}
}

if(insertAttList.size() > 0) {
insert insertAttList;
}
return null;
}

}

Page:

<apex:page standardController="Case" extensions="escalationcd">

<apex:form >
<apex:pageBlock >
<apex:pageBlockSection >
<apex:inputField value="{!case.casenumber}"/>
<apex:inputField value="{!case.attach_casenumber__c}"/>

<apex:commandButton action="{!save}" value="Save!"/>
<apex:page standardController="Case" action="{!getcaseemailmessage}" extensions="escalationcd">
</apex:page>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>