Inappropriate usage of SPContentType.Fields

Description

SPContentType contains two collections, Fields (of type SPFieldCollection) and FieldLinks (of type SPFieldLinkCollection). Even if the object model appears to support addition or deletion of fields using the Fields collection, an exception is thrown if you try to do so. Lists and Webs contain the actual fields with field data. A content type, on the other hand, only holds Field Reference, which simply points at the corresponding field in the list or web. This gets a bit confusing, because content types have both an SPFieldLinkCollection and an SPFieldCollection.

Inappropriate usage:

var ct = web.ContentTypes[contentTypeName];
ct.Fields.Add("NewField", SPFieldType.Boolean, true);

Exception: This functionality is unavailable for field collections not associated with a list.

Just to remember: Content Type is the definition for a list, but it does not contain any data. Since Fields contain data, Content Types (disassociated from a list) do not have Fields, they have FieldRefs.

Resolution

Add field reference to Content Type:

//Check if the Field reference does not exists already
if (!contentType.Fields.ContainsField(field.Title))
{
    contentType.FieldLinks.Add(new SPFieldLink(field));
    contentType.Update();
}
else
{
    //Do Nothing
}

Delete field reference from Content Type:

//Check if the Field reference exists
if (contentType.Fields.ContainsField(field.Title))
{
    contentType.FieldLinks.Delete(field.Title);
    contentType.Update();
}
else
{
    //Do Nothing
}