how to handle when Dictionary element does not exist

[Jeff0803]

I made a class which is inherited from dictionary like following.

public class MessageListDict : Dictionary<string, List<Message>>
{
//Add new elemet if key exists already or update existing element if key does not exist
//returns
//bool
// true : Add or update succeeded
// false : some error occurred
public bool AddNewMessage(string key, Message message)
{
bool ret = true;
List<Message> messagelist;
try
{
if (this.ContainsKey(key))
{
messagelist = (List<Message>)this[key];
}
else
{
messagelist = new List<Message>();
}
messagelist.Add(message);
this.Add(key, messagelist);
}
catch
{
ret = false;
}
return (ret);
}
public bool IsExistKey(string key)
{
bool ret = true;
try
{
if (!this.ContainsKey(key))
{
ret = false;
}
}
catch
{
ret = false;
}
return (ret);
}
//Removes message list from the Dictionary
//returns
//bool
// true : key exists and removed element successfully
// false : key doesn't exist
public bool RemoveMessageList(string key)
{
bool ret = true;

if (this.ContainsKey(key))
{
this.Remove(key);
}
else
{
ret = false;
}
return (ret);
}
//Remove all elements from the Dictionary
public void RemoveAllMessageLists()
{
this.Clear();
}

//Get messagelist
//returns
//List<Message>
// not null : if key exists, return message list
// null : if key does not exist, return null
public List<Message> GetMessageList(string key)
{
List<Message> messagelist = null;
if (this.ContainsKey(key))
{
messagelist = this[key];
}
return (messagelist);
}

//Get message
//returns
//string
// not null string : if key exists, return message
// null string : if key does not exist, return null string
public string GetMessage(string key, string sid)
{
List<Message> messagelist;
if (this.ContainsKey(key))
{
messagelist = this[key];
}
else
{
return("");
}

Message message = messagelist.Find(delegate(Message m)
{
return m.SID == sid;
});
return (message.MessageBody);
}
}

The Message class is like the followings.

public class Message
{
private short m_direction;
private DateTime m_message_datetime;
private string m_message_body;
private string m_firstname;
private string m_sid;
private string m_phoneno;
public Message(string sid, short direction, DateTime message_datetime, string message_body, string firstname, string phoneno)
{
m_sid = sid;
m_direction = direction;
m_message_datetime = message_datetime;
m_message_body = message_body;
m_firstname = firstname;
m_phoneno = phoneno;
}
public string SID
{
get { return m_sid; }
set
{
m_sid = value;
}
}
public string MessageBody
{
get { return m_message_body; }
set
{
m_message_body = value;
}
}
public DateTime MessageDateTime
{
get { return m_message_datetime; }
set
{
m_message_datetime = value;
}
}
public Int16 Direction
{
get { return m_direction; }
set
{
m_direction = value;
}
}
public string FirstName
{
get { return m_firstname; }
set
{
m_firstname = value;
}
}
public string PhoneNo
{
get { return m_phoneno; }
set
{
m_phoneno = value;
}
}
}

From the client, I consume this class like following.

MessageListDict messagelistdict = new MessageListDict();
.
.
.
.
List<Message> messagelist;
.
.
.
messagelist = messagelistdict.GetMessageList(key);


The error occurs in MessageListDict.GetMessageList(string key)'s last line, that is, return messagelist when key does not exist in the dictionary.

How could I handle when dictionary key doesn't exist and return null?

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