Get Calendar List Item GUID

Question

I downloaded code for a reservation event receiver from here: 
http://blog.sharepointsydney.com.au/post/Setting-up-multiple-calendars-for-meeting-room-bookings-prevent-double-booking.aspx
However, on the ItemUpdating it throws an "Object Reference Not Set to an Instance of an Object" error.  By commenting out parts of the code and re-deploying I have narrowed the issue down to the line that gets the item's GUID:
string guid_internal = collItems.List.Fields["GUID"].InternalName;
When I modify it to something like "UniqueId" I get the "Value does not fall within expected range" error.  Is there a better way to obtain the GUID of the calendar list item - or am I missing something?  Full code below:
using System;
using Microsoft.SharePoint;
namespace Webcoda.WSS.Calendar.Events
class PreventDoubleBooking: SPItemEventReceiver
/// <summary>
/// This event is triggered when the user adds a new item
/// </summary>
/// <param name="properties"></param>
public override void ItemAdding(SPItemEventProperties properties)
//Our query string variable
string strQuery = null;
try
//Get the Sharepoint site instance
using (SPWeb oWebsite = new SPSite(properties.SiteId).OpenWeb(properties.RelativeWebUrl))
//Get the collection of properties for the Booking item
SPListItemCollection collItems = oWebsite.Lists[properties.ListTitle].Items;
//Get the Calendar List that we will be querying against
SPList calendar = oWebsite.Lists[properties.ListId];
//Get the internal name of the fields we are querying.
//These are required for the CAML query
string start_internal = collItems.List.Fields["Start Time"].InternalName;
string end_internal = collItems.List.Fields["End Time"].InternalName;
string MeetingRoom_Internal = collItems.List.Fields["Meeting Room"].InternalName;
//Get the query string parameters
string start_str = properties.AfterProperties[start_internal].ToString();
string end_str = properties.AfterProperties[end_internal].ToString();
string MeetingRoom_str = properties.AfterProperties[MeetingRoom_Internal].ToString();
//Construct a CAML query
SPQuery query = new SPQuery();
//Create the CAML query string that checks to see if the booking we are attemping
//to add will overlap any existing bookings
strQuery = string.Format(@"
<Where>
<And>
<Or>
<Or>
<And>
<Leq>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Leq>
<Gt>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Gt>
</And>
<And>
<Lt>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Lt>
<Geq>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Geq>
</And>
</Or>
<Or>
<And>
<Leq>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Leq>
<Geq>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Geq>
</And>
<And>
<Geq>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Geq>
<Leq>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Leq>
</And>
</Or>
</Or>
<Eq>
<FieldRef Name='Meeting_x0020_Room' />
<Value Type='Choice'>{2}</Value>
</Eq>
</And>
</Where>
<OrderBy>
<FieldRef Name='EventDate' />
</OrderBy>
", start_str, end_str, MeetingRoom_str);
//Set the query string for the SPQuery object
query.Query = strQuery;
//Execute the query against the Calendar List
SPListItemCollection existing_events = calendar.GetItems(query);
//Check to see if the query returned any overlapping bookings
if (existing_events.Count > 0)
//Cancels the ItemAdd action and redirects to error page
properties.Cancel = true;
//Edit the error message that will display on the error page
properties.ErrorMessage += "This booking cannot be made because of one or more bookings in conflict. <BR><BR>";
//Here you can loop through the results of the query
//foreach (SPListItem oListItem in existing_events)
properties.ErrorMessage += "Please go back and schedule a new time.";
catch (Exception ex)
//Cancels the ItemAdd action and redirects to error page
properties.Cancel = true;
//Edit the error message that will display on the error page
properties.ErrorMessage = "Error looking for booking conflicts: " + ex.Message;
/// <summary>
/// This event is triggered when the user edits an calendar item
/// </summary>
/// <param name="properties"></param>
public override void ItemUpdating(SPItemEventProperties properties) {
string strQuery = null;
try {
//Get the Sharepoint site instance
using (SPWeb oWebsite = new SPSite(properties.SiteId).OpenWeb(properties.RelativeWebUrl)) {
//Get the collection of properties for the Booking item
SPListItemCollection collItems = oWebsite.Lists[properties.ListTitle].Items;
//Get the Calendar List that we will be querying against
SPList calendar = oWebsite.Lists[properties.ListId];
//Get the internal name of the fields we are querying.
//These are required for the CAML query
string start_internal = collItems.List.Fields["Start Time"].InternalName;
string end_internal = collItems.List.Fields["End Time"].InternalName;
string MeetingRoom_Internal = collItems.List.Fields["Meeting Room"].InternalName;
string guid_internal = collItems.List.Fields["GUID"].InternalName;
//Get the query string parameters
string start_str = properties.AfterProperties[start_internal].ToString();
string end_str = properties.AfterProperties[end_internal].ToString();
string MeetingRoom_str = properties.AfterProperties[MeetingRoom_Internal].ToString();
string guid_str = properties.AfterProperties[guid_internal].ToString();
//Construct a CAML query
SPQuery query = new SPQuery();
//Create the CAML query string that checks to see if the booking we are attemping
//to change will overlap any existing bookings, OTHER THAN ITSELF
strQuery = string.Format(@"
<Where>
<And>
<And>
<Or>
<Or>
<And>
<Leq>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Leq>
<Gt>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Gt>
</And>
<And>
<Lt>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Lt>
<Geq>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Geq>
</And>
</Or>
<Or>
<And>
<Leq>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Leq>
<Geq>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Geq>
</And>
<And>
<Geq>
<FieldRef Name='EventDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{0}</Value>
</Geq>
<Leq>
<FieldRef Name='EndDate' />
<Value Type='DateTime' IncludeTimeValue='TRUE'>{1}</Value>
</Leq>
</And>
</Or>
</Or>
<Eq>
<FieldRef Name='Meeting_x0020_Room' />
<Value Type='Choice'>{2}</Value>
</Eq>
</And>
<Neq>
<FieldRef Name='GUID' />
<Value Type='GUID'>{3}</Value>
</Neq>
</And>
</Where>
<OrderBy>
<FieldRef Name='EventDate' />
</OrderBy>
", start_str, end_str, MeetingRoom_str, guid_str);
//Set the query string for the SPQuery object
query.Query = strQuery;
//Execute the query against the Calendar List
SPListItemCollection existing_events = calendar.GetItems(query);
//Check to see if the query returned any overlapping bookings
if (existing_events.Count > 0)
//Cancels the ItemAdd action and redirects to error page
properties.Cancel = true;
//Edit the error message that will display on the error page
properties.ErrorMessage += "This booking cannot be made because of one or more bookings in conflict. <BR><BR>";
//Here you can loop through the results of the query
//foreach (SPListItem oListItem in existing_events)
properties.ErrorMessage += "Please go back and schedule a new time.";
catch (Exception ex)
//Cancels the ItemAdd action and redirects to error page
properties.Cancel = true;
//Edit the error message that will display on the error page
properties.ErrorMessage = "Error looking for booking conflicts: " + ex.Message;

Answer 1

Hi there,
Please verify the internal name of column which you have hardcoded in the code i.e 
string start_internal = collItems.List.Fields["Start Time"].InternalName;
string end_internal = collItems.List.Fields["End Time"].InternalName;
I have used the Room reservation template from MSDN which has provided by MS under the code name of "Fantastic 40" along with below James Finn article.
http://www.codeproject.com/Articles/30983/SharePoint-Reservations
It worked for me for reservation.