I was adding content types to a document library using C# and got a Null reference exception when calling SPList.ContentTypes.Add(contentType). The stack was:

at Microsoft.SharePoint.SPFile.UpdateInternal(Boolean migrationSemantics, Boolean reGhost)
   at Microsoft.SharePoint.SPContentTypeCollection.AddContentType(SPContentType contentType, Boolean updateResourceFileProperty, Boolean checkName, Boolean setNextChildByte)
   at Microsoft.SharePoint.SPContentTypeCollection.Add(SPContentType contentType)
   at Harness.BernadoHarness.Execute() in C:\Project\BernadoHarness.cs:line 24
   at Harness.Program.Main(String[] args) in C:\Project\Program.cs:line 34
   at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

My code to add the content type was fairly innocent:

using (var site = new SPSite("http://myServer/subSite1"))
{
	using (var web = site.OpenWeb())
	{
		SPList list = web.Lists["Documents Library"];
		SPContentType contentType = null;

		contentType = web.AvailableContentTypes["Meeting Agenda"];
		list.ContentTypes.Add(contentType);
	}
}

It turns out that it was because I have 2 ItemEventReceivers (ItemUpdating and ItemAdded) on the document library and these were raised when the content type was being added to the document library. When these events are raised as part of the content type being added, the properties.ListItem would be null. My 2 receivers do some work with the properties.ListItem object, but did not anticipate that this could be null, hence they felt over and raised a Null exception to the SPList.ContentTypes.Add() method. Adding a null check to my 2 receivers fixed the issue.

Now, the interesting point here is why adding a content type to the library would raise the ItemUpdating and ItemAdded events? This is because adding a content type to a document library may involve adding/updating the document template file to the resource folder of the content type, which is a sub-folder of the library – and hence raise the item events.