A better way to obtain fields values of an SPListItem
.NET, Best Practices, Development, Productivity, SharePointObtaining the value of a particular field (SPField) of a list item (SPListItem) is quite awkward. First of all you have to pass either the index, Id or the InternalName of an existing field. If the field exists a value of the object type is being returned. In some cases that value can be null. If it’s not you can cast it to its origin type as in most cases defined by the SPField.FieldValueType Property. Imagine doing all that each time you want to retrieve a field value. Isn’t there a better way?
Introducing the TryGetValue extension method
To simplify the task as described above I have created an extension method using generics:
What is it doing?
The method does exactly the same what I describe above: it retrieves the value of the given field for the current list item. Because it might just happen that you’re passing an invalid id, the whole process of retrieving and processing the value is wrapped in a try catch clause.
The method encapsulates the whole process of retrieving and process the value what leads to simplification of your code. Compare the standard approach of retrieving a field value with how you would do the same using the extension method:
See the difference? Using the extension method all you need is one line of code. The value you get is type safe and you know is different than null.
Under the hood
I have chosen to make it an extension method to make using it more intuitive. Getting the value of the particular field of the particular list item is what you want to do: why not calling the method like that?
Furthermore I have decided to use generics to provide the type safety of the retrieved value. Like that you have just one method which works for every single type out there including your custom field types:
To provide you some feedback about the value I have decided to use the try pattern. The method tries to retrieve the value of the particular field. It returns true if succeeded and false if failed. To prevent you from making an extra call to get the value I have decided to use an out parameter so you can directly use the value in your custom code.
As you’ve probably noticed in the example above I’m referring to fields using classes instead of typing the internal names. Using the latest version of Imtech Fields Explorer you can automatically generate wrapper classes for content types so you will never have to use any internal name again.
The TryGetValue extension method: do you love it or hate it?
November 14th, 2008 at 3:07 pm
Excellent code. I am sure it will save a lot of people a lot of hassle, including me :)
One question/remark.
You say at one point "Because it might just happen that you’re passing an invalid id, the whole process of retrieving and processing the value is wrapped in a try catch clause."
Is there a specific reason why you would not use the following:
if (listItem.Fields.ContainsField("MyDateField"))
{
// Enter retrieval code here
}
This seems to me a typical situation in which you can save yourself (and the CLR) the trouble of using Exceptions. But I might be missing something.
Regards,
Erik
November 14th, 2008 at 4:23 pm
Thanks for the tip Erik, I've definitely missed that one. Adding the extra check you mentioned would keep you off from getting an extra Exception but I still like the idea of having it all wrapped in a try..catch clause 'just in case'.
November 24th, 2008 at 8:23 pm
The only problem in using the ContainsField method is it only accepts a string that contains either the display name or the internal name of the field. The wrapper classes use IDs.
By the way Love It :-)
November 24th, 2008 at 8:33 pm
Thanks TJ :)
December 3rd, 2008 at 7:39 pm
Quick question on your sample caller code:
MyContentType.MyDateField
Do you have custom classes in your code that inherit from SPContentType to do this or is there something else at play?
December 3rd, 2008 at 9:11 pm
@Jim: It's a wrapper class generated using Imtech Fields Generator v1.5 (http://blog.mastykarz.nl/imtech-fields-explorer-v1410-inconvenient-site-columns-update/).
January 23rd, 2009 at 4:15 pm
Thank you!
July 6th, 2009 at 9:19 pm
Great, Thanks. question though! I have to look at all the SPListItems that are in the same list and show only those items that are not repeatetive. (I might have 3 splistitem within the same list that have the same specific field). What I want to do is to show only 1 of those splistitem instead of 3!
Any idea????
I REALLY appriciate any help. Thank you
July 6th, 2009 at 10:50 pm
@nancy: Unfortunately CAML doesn't provide any mechanism for selecting distinct values. What you could is to take care that you're retrieving items in a best possible way (I covered some samples here: http://blog.mastykarz.nl/performance-of-content-aggregation-queries-on-multiple-lists-revisited/) and then do the distinct selection. If the query is being run frequently it might be a good idea to cache the results as it will probably be a resource-expensive operation (depending on the total number of items of course).
November 19th, 2010 at 7:02 pm
I think a try/catch{} is bad practice especially if you're just suppressing the exception to return false. You're better off doing something like:
public static bool TryGetValue(this SPListItem listItem, Guid fieldId, out T value)
{
var oValue = listItem[fieldId];
if (oValue != null && oValue is T)
{
value = (T)oValue;
return true;
}
value = default(T);
return false;
}
November 19th, 2010 at 9:12 pm
@Demis: Fully agreed, although the try..catch is there also to suppress errors that might occur while trying to retrieve a value of a non-existing field.
March 14th, 2011 at 7:35 pm
I have very little knowledge in c#, can someone convert this to VB and how can I actually use it? Please help!
Thanks so much in advance!
March 14th, 2011 at 9:40 pm
@denisse: Have you tried using http://www.developerfusion.com/tools/convert/csharp-to-vb/?
October 16th, 2011 at 11:44 pm
Hi Waldek,
I've installed the Imtech Field Explorer addon but receive the following exception when trying to load any URL.
"The Web application at … could not be found. Verify that you have typed the URL correctly…"
I have no problem accessing using other third party tools. Any idea??
October 17th, 2011 at 10:26 am
@Panoone: Is SharePoint installed on your local machine or are you trying to access a remote site? Imtech Fields Explorer works only with local sites.
October 18th, 2011 at 11:07 am
Hi Waldek, running of the server. Odd. Seems like a permission issue, perhaps as a result if running within the context if VS? Yet I have no problem with WSPBuilder, DB connects to SP through VS, or any stand-alone tools. It's an SP farm with single server + dedicated SQL box.
October 18th, 2011 at 11:10 am
Forgot to mention this is a MOSS farm. :)
October 18th, 2011 at 11:25 am
@Panoone: If you're running with UAC enabled, have you tried running the tool as admin?
October 18th, 2011 at 11:31 am
No UAC but will try a Run As anyway. Do I need to append the internal port to the URL fir a host header web app?
October 18th, 2011 at 1:29 pm
@Panoone: Only if it's on a port different than 80.
October 18th, 2011 at 10:34 pm
Nope. It's on 80. Oh well. Will give it another shit this morning and see what happens. Is there a debug mode at all?
October 19th, 2011 at 6:41 am
@Panoone: No, there is no debug mode. Good luck!
October 20th, 2011 at 11:07 pm
I think I cracked the problem. We recently upgraded our SQL server to 2008. I tired opening a DB connection to the server through VS and iot failed with the message:
"This server is not supported. Only servers up to MS SQL 2005 are supported."
October 21st, 2011 at 6:23 am
Ugh. Legacy dev VM with RTM installed. For some reason SP1 was never offered through Windows Update. Maybe a WSUS setting for web servers. Anyways…after finally locating the SP1 update and waiting am hour for it to complete, I ran an update check again. Lo and behold there were a vast number. ;)
Will try again when it's all over. I'm done for this week.
January 10th, 2012 at 1:51 pm
@Demis Bellot correction for Waldek solution.
Guys, You ommit one thing, null value is a good value for reference types (for eq string ;)). Both of you threat null as not optained value (return false)