Warn users of double booking in SharePoint calendar in real time

Recently I came across this situation where in our room booking system we needed to warn users of parallel booking, but didn’t want to stop them from making the booking because in many cases they are agreed between parties.

Initially it didn’t look like a big deal because I had Client-side Object Model which I could use in JavaScript, but I couldn’t achieve the whole functionality in JavaScript. I would explain down my post why I couldn’t write the whole solution in JavaScript, and I had to go for Server-side code.

Finally I was able to achieve the following (click on the picture to see the big version)

The thing which makes this solution interesting is that Id’s of the date controls are handled differently by SharePoint in this page. If you inspect the source of the new item page of the calendar, you will notice that the controls id’s are kept in an array in JavaScript. You can see the array declaration in the source

 <script type="text/// <![CDATA[
">var g_strDateTimeControlIDs = new Array();
// ]]>

And then it’s used to store id’s of the calendar controls like start date

 <script type="text/javascript">
g_strDateTimeControlIDs["SPEventDate"] = "ctl00_m_g_089e7bbe_83d6_4e8a_ab8f_ca7ca3d3d0cc_ff31_ctl00_ctl00_DateTimeField_

And for end date

 <script type="text/javascript">
g_strDateTimeControlIDs["SPEndDate"] = "ctl00_m_g_089e7bbe_83d6_4e8a_ab8f_ca7ca3d3d0cc_ff41_ctl00_ctl00_DateTimeField_

Hours and Minutes drop downs’ id’s are same except “Hours” and “Minutes” appended in the end of the above id’s.

The final solution consists of the following components;

1. Changing ScriptManager tag in master page to allow page methods

2. Changing web.config to add PageParser element to allow server-side code in the page

3. Creating a new item page for calendar

4. Changing the newitem page

5. Writing some javascript

6. Writing a page method in C#

The only reason I used page method in C# is that I couldn’t run the following query in JavaScript, I am still trying for this, and if I get lucky I would love to remove the server-side code from this solution

SPQuery query = new SPQuery();

query.ExpandRecurrence = true;

query.Query = "<FieldRef Name='EventDate' />";

query.CalendarDate = new DateTime( startDateTime.Year, startDateTime.Month, startDateTime.Day);

1. ScriptManager tag is already in the v4.master, but it has page methods disabled, we need to first enable the page methods by changing the “EnablePageMethods” attribute to “true” and it should then look like this;

ScriptManager id="ScriptManager" runat="server" EnablePageMethods="true" EnablePartialRendering="true" EnableScriptGlobalization="false" EnableScriptLocalization="true" />

by enabling this you can add static page methods to an ASP.NET page and mark them as Web methods. You can then call these methods from script as if they were part of a Web service, but without creating a separate .asmx file.

If you don’t want to enable Page Methods in master page, then you can add “ScriptManagerProxy” control in your page, it’s because a page can contain only one ScriptManager control in its hierarchy and v4.master already has one.

2. Now change the web.cofig for the site to add a PageParser element, so that our page can call server-side code



<PageParserPath VirtualPath=”/Team/Lists/Calendar/CustomNewEvent.aspx” CompilationMode=”Always” AllowServerSideScript=”true” />




3. Create a custom new item page for calendar in SharePoint designer, it’s easy to do and I won’t be able to explain how to do it here.

4. Open the custom new item page in advance mode in SharePoint designer and scroll down to the place where there’s a table row (i.e) for “End Time” and add the following table row


 <span id="TimeOverlapLabel">  



I wanted to show the message below “End Time” but you can show it anywhere on the page and I did the styling in CSS, it’s all your choice.

Now search for place holder id “PlaceHolderAdditionalPageHead”, and the add the following inside the content tag

<asp:Content ContentPlaceHolderId=”PlaceHolderAdditionalPageHead” runat=”server”>

// <![CDATA[
// ]]>
<script type=”text/c#” runat=”server” src=”../../Script/calendar.cs”></script>
<script language=”javascript” type=”text/javascript”>
function OnLoad(){
// Calendar is the name of the Calendar list, I am passing the name of the list
// so that I can call this code from any Calendar library  
FindOverlappedEvents(“Calendar”); } 


I have only shown the code I have added inside the above content tag, there would be more already in there which you might not want to change.

Calendar.js and Calendar.cs are the files I have added in the scripts folder on the root of the site, by selecting “All Files” in the left navigation in SPD and creating a folder on the root, you can of course choose to store them somewhere else but remember to use the correct path in the above code.

5. Now add the following code in Calendar.js, the way I create a new .js or .cs in SharePoint designer is by creating a new css files and then renaming it.  it’s because SharePoint designer gives you option to create only HTML, ASPX or CSS in a given folder.

var g_eventOverlapped = false;

function PreSaveAction()


// This is in case user presses save button right after selecting date and time

// Remember: our code runs on onblur even of date and time controls

if(g_eventOverlapped == true) {

 return confirm(“Your selected time overlaps with an existing booking!\nIf you want to change the time, press ‘Cancel'”);


 return true;


// This function is called from OnLoad of the form, it binds event handerl to the date and time controls

// onblur is the only event which works on these controls

function FindOverlappedEvents(listName)


var startDate = document.getElementById(g_strDateTimeControlIDs[“SPEventDate”]);

 var startHours = document.getElementById(g_strDateTimeControlIDs[“SPEventDate”]+”Hours”);

 var startMinutes = document.getElementById(g_strDateTimeControlIDs[“SPEventDate”]+”Minutes”);

 if(startDate != null && startHours != null && startMinutes != null)


 startDate.onblur = function(){validateDates(listName);};

 startHours.onblur = function() {validateDates(listName);};

 startMinutes.onblur = function() {validateDates(listName);};


 var endDate = document.getElementById(g_strDateTimeControlIDs[“SPEndDate”]);

 var endHours = document.getElementById(g_strDateTimeControlIDs[“SPEndDate”]+”Hours”);

 var endMinutes = document.getElementById(g_strDateTimeControlIDs[“SPEndDate”]+”Minutes”);

 if(endDate != null && endHours != null && endMinutes != null)


 endDate.onblur = function(){validateDates(listName);};

 endHours.onblur = function() {validateDates(listName);};

 endMinutes.onblur = function() {validateDates(listName);};


// validate initially when new item window opens, because by default date and time are selected



// This gets called every time onblur is fired on controls

function validateDates(listName)


var startDateCtl = document.getElementById(g_strDateTimeControlIDs[“SPEventDate”]);

 var startHoursCtl = document.getElementById(g_strDateTimeControlIDs[“SPEventDate”]+”Hours”);

 var startMinutesCtl = document.getElementById(g_strDateTimeControlIDs[“SPEventDate”]+”Minutes”);

 var startHour = startHoursCtl.options[startHoursCtl.selectedIndex].value

 var startMinutes = startMinutesCtl.options[startMinutesCtl.selectedIndex].value

 var endDateCtl = document.getElementById(g_strDateTimeControlIDs[“SPEndDate”]);

 var endHoursCtl = document.getElementById(g_strDateTimeControlIDs[“SPEndDate”]+”Hours”);

 var endMinutesCtl = document.getElementById(g_strDateTimeControlIDs[“SPEndDate”]+”Minutes”);

 var endHour = endHoursCtl.options[endHoursCtl.selectedIndex].value

 var endMinutes = endMinutesCtl.options[endMinutesCtl.selectedIndex].value

 // here is the call to the server-side method, which comes back to either OnCallComplete(result) or OnCallError(error)

PageMethods.GetCalendarOverlap(listName, startDateCtl.value, startHour,startMinutes,endDateCtl.value,endHour,endMinutes,OnCallComplete,OnCallError);


function OnCallComplete(result)


if(result != “”)


document.getElementById(“TimeOverlapLabel”).innerHTML = “Booking Error”;

document.getElementById(“TimeOverlap”).innerHTML = result;

// this makes sure that we’ve covered this event

g_eventOverlapped = true;








document.getElementById(“TimeOverlapLabel”).innerHTML = “”;


document.getElementById(“TimeOverlap”).innerHTML = “”;

// this makes sure that we’ve covered this event

g_eventOverlapped = false;




function OnCallError(error)


if(error !== null)


document.getElementById(“TimeOverlap”).innerHTML = “There’s an error checking for duplicate events”;


6. Create Calendar.cs in the Scripts folder and add the following code;


public static string GetCalendarOverlap(string listName, string startDate, string startHour, string startMinutes, string endDate, string endHour, string endMinutes)




SPWeb web = SPContext.Current.Web;

SPList calendarList = web.Lists[listName];

// Construct a query that expands recurring events

SPQuery query = new SPQuery();

// If I could run the following CAML in JavaScript using SharePoint Object Model, I wouldn’t have gone for server side code.

query.ExpandRecurrence = true;

query.Query = “<FieldRef Name=’EventDate’ />”;

startHour = startHour.TrimEnd(“:”.ToCharArray());

string[] startDateParts = startDate.Split(“/”.ToCharArray());

DateTime startDateTime = new DateTime(int.Parse(startDateParts[2]), int.Parse(startDateParts[1]), int.Parse(startDateParts[0]), int.Parse(startHour), int.Parse(startMinutes), 0);

endHour = endHour.TrimEnd(“:”.ToCharArray());

string[] endDateParts = endDate.Split(“/”.ToCharArray());

DateTime endDateTime = new DateTime(int.Parse(endDateParts[2]), int.Parse(endDateParts[1]), int.Parse(endDateParts[0]), int.Parse(endHour), int.Parse(endMinutes), 0);

query.CalendarDate = new DateTime( startDateTime.Year, startDateTime.Month, startDateTime.Day);

// Returns all items (including recurrence instances) that

// would appear in the calendar view for the current day

SPListItemCollection calendarItems = calendarList.GetItems(query);

DateTime eventStartDateTime;

DateTime eventEndDateTime;

string eventTitle;

bool timeOverlap = false;

string events = string.Empty;

foreach (SPListItem item in calendarItems)


eventStartDateTime = DateTime.Parse(item[“EventDate”].ToString());

eventEndDateTime = DateTime.Parse(item[“EndDate”].ToString());

eventTitle = (string)item[“Title”];

timeOverlap = false;

//if newstartdate >= eStartDate and newStartDate < eEndDate

if(startDateTime >= eventStartDateTime && startDateTime < eventEndDateTime)


timeOverlap = true;


//if newEndDate > eStartDate and newEndDate <= eEndDate

else if(endDateTime > eventStartDateTime && endDateTime <= eventEndDateTime)


timeOverlap = true;


// if eStartDate >= newStartDate and eStartDate < newEndDate

else if(eventStartDateTime >= startDateTime && eventStartDateTime < endDateTime)


timeOverlap = true;


// if eEndDate > newStartDate and eEndDate <= newEndDate

else if(eventEndDateTime > startDateTime && eventEndDateTime <= endDateTime)


timeOverlap = true;


// if newStartDate = eStartDate and newEndDate = eEndDate

else if (startDateTime == eventStartDateTime && endDateTime == eventEndDateTime)


timeOverlap = true;



if(timeOverlap == true)


// This HTML decoration is better done at the client side, but I was being lazy and did it here

events += “ Title : “+ eventTitle +”

events += “Start Date: “+ item[“EventDate”].ToString() +”
End Date: ” + item[“EndDate”].ToString() + ”

events += “Created By: ” + item[“Author”].ToString().Split(“#”.ToCharArray())[1] + ”




if(events != string.Empty)


events = “Time Overlap Warning!<br/>Your selected time overlaps with the following booking(s);<br/><br/>”+events;


return events;


catch(Exception ex)


throw ex;



  1. #1 by albert on September 6, 2011 - 2:52 am

    just wondering, can we simplify a lot of steps by using Custom Field Type approach?

    • #2 by Aleem on September 7, 2011 - 4:01 pm

      Thanks for the idea, I would love to modify this for a simpler solution.

  2. #3 by Tshepo on August 27, 2014 - 4:05 pm

    Thank you very much, you saved me. I had to create a custom list and add to a lookup field which already existed.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: