SharePoint Fun

If you’re like me and you do a fair bit of object model programming, you’ve likely seen this error before. More specifically, the error is as follows:

Unhandled Exception: Microsoft.SharePoint.SPException: Invalid data has been used to update the list item. The field you are trying to update may be read only.

There are probably more than one cause for this particular issue, but in my case, it always comes down to an error in the way I’ve assigned a field of type Lookup.

So to set up our scenario, let’s say we have a custom list called Types, and it contains 3 items:

1. Employee
2. Contractor
3. Manager

Now let’s assume we’ve created a custom list called Users and it contains a column called User Type of type Lookup that points to the Types list using the Title column.

If we’re doing some updates to our list programmatically, we’ll need to be able to set the Type column. Your typical scenario is that you’re reading this from a form control and populating this field. In my case, I knew specifically what value I wanted to set. So let’s start with something like this:

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SPListItem item = list.Items.Add();
item["Type"] = "Employee";
item.Update();

Unfortunately, the above bit of code will fail with the error outlined at the beginning of this post. The data we’re attempting to assign is not valid. What we need to do is create another object, an SPFieldLookupValue object to be more specific, and use one of the constructors to build the lookup that we’ll then assign to our field.

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SPListItem item = list.Items.Add();
SPFieldLookupValue lookup = new SPFieldLookupValue(1, "Employee");
item["Type"] = lookup;
item.Update();

I’m not satisfied with the above, because it’s forcing me to hard code the id and the value. I am interested in some feedback from my readers — does anyone have a better approach to this problem? I’ll be awarding a free Black Ninja TSHIRT to the reader who can provide me the most elegant workaround.

Problem

You’ve developed some custom new, edit and display application pages that are stored in the _layouts directory. Let’s assume the filenames are newform.aspx, dispform.aspx and editform.aspx. You now want to change the properties of your custom list so that any new, edit or display requests point to your custom pages.

If you open up your site within SharePoint Designer and expand the Lists folder, from there you can access the properties of that custom list by right clicking on it and selecting Properties. Once the List Properties pane is open, click on the Supporting Files tab.

You’ll see there that you can actually choose what display, edit and new forms you want your list to be using. So if you’ve developed something custom, you would click Browse…, point to your new location and select the file. Here is where it starts to fail, I am only able to browse within the site itself and cannot navigate to my _layouts directory to select the files I mentioned above.

Solution

I suspected this was a limitation of the SharePoint Designer UI and not actually a limitation of the Object Model. I did a bit of fiddling with my SPList object and was not able to find anything that let me change those properties. If you look at the screenshot above, there is a key piece of information that’ll make the light turn on (at least it did for me). The Supporting Files tab has a drop down selector for the Content type specific forms. If you think about that a moment, you’ll remember that ALL lists within SharePoint inherit from a default content type.

So armed with that knowledge, I created an SPContentType object and took a look at it’s properties and methods. Sure enough, there are 3 properties I can set to change these forms: EditFormUrl, NewFormUrl and DisplayFormUrl. Here is some sample code I used to change the forms for my custom list called ‘My List’ that inherits from the Item content type:

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SPWeb web = SPContext.Current.Web;
 
web.AllowUnsafeUpdates = true;
 
SPList list = web.Lists["My List"];
 
SPContentType ct = list.ContentTypes["Item"];
 
ct.EditFormUrl = "_layouts/editform.aspx";
ct.NewFormUrl = "_layouts/newform.aspx";
ct.DisplayFormUrl = "_layouts/dispform.aspx";
 
ct.Update();
list.Update();

So in the example code above, I had to determine what default content type my list inherited from. In this case it was the Item content type.

NOTE: If you need to change the content type directly, you can definitely do that, however, I did a bit of testing and found that any lists ALREADY inheriting from that content type did not pickup my changes to the form locations. Any NEW list that I created that was inheriting from that content type did pickup the changes. In order for me to change the form locations for the existing list I had to use the code above. For reference, here is how you would change the content type directly. The code has only a subtle difference.

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SPWeb web = SPContext.Current.Web;
 
web.AllowUnsafeUpdates = true;
 
SPContentType ct = web.ContentTypes["Name Of Your Content Type"];
 
ct.EditFormUrl = "_layouts/editform.aspx";
ct.NewFormUrl = "_layouts/newform.aspx";
ct.DisplayFormUrl = "_layouts/dispform.aspx";
 
ct.Update();

As always, any questions, let me know!