Object ref not set to instance of object in SPListItem extension method

Question

I am attempting to write an extension method for the SPListItem type to ease the pain of getting SPUserValues as strings. I am getting an "Object reference not set to an instance of an object" exception when I hit the line: msg = "field.AllowMultipleValues == " + field.AllowMultipleValues;

because field is null. I know that at the item level item[fieldName] has a value and that the field does exist as I can see the field definition in item.Fields.

Any suggestions about where I am going wrong?

    public static List<string> GetFieldValueUserLogin(this SPListItem item, string fieldName)
    {     

        try
        {
            if (item != null)
            {
                SPDiagnosticsService.Local.WriteTrace(0, new SPDiagnosticsCategory("_PAR_", TraceSeverity.Unexpected, EventSeverity.Information), TraceSeverity.Unexpected, "Getting value of " + fieldName, null);
                List<string> userNames = new List<string>();

                SPFieldUser field = item[fieldName] as SPFieldUser;

                string msg = field == null ? "field is null" : "field is not null";

                SPDiagnosticsService.Local.WriteTrace(0, new SPDiagnosticsCategory("_PAR_", TraceSeverity.Unexpected, EventSeverity.Information), TraceSeverity.Unexpected, msg, null);

                msg = "field.AllowMultipleValues == " + field.AllowMultipleValues;

                SPDiagnosticsService.Local.WriteTrace(0, new SPDiagnosticsCategory("_PAR_", TraceSeverity.Unexpected, EventSeverity.Information), TraceSeverity.Unexpected, msg, null);

//Etc ...
   }
}

Edit--- After a bit of caffeine I went back and realized I was kind of being an idiot. Appears to be my modus operandi. Here is my final method code.

public static List<string> GetFieldValueUserLogins(this SPListItem item, string fieldName)
{
    try
    {
        if (item != null)
        {
            List<string> userNames = new List<string>();

            SPFieldUserValueCollection uvc = new SPFieldUserValueCollection(item.Web, item[fieldName].ToString());
            foreach (SPFieldUserValue userValue in uvc)
            {
                userNames.Add(userValue.User.LoginName);
            }


            return userNames;
        }
        else
        {
            return null;
        }
    }
    catch (Exception ex)
    {
        SPDiagnosticsService.Local.WriteTrace(0, new SPDiagnosticsCategory("_PAR_", TraceSeverity.Unexpected, EventSeverity.Information), TraceSeverity.Unexpected, ex.Message + " - " + ex.StackTrace.ToString(), null);
        return null;
    }
}

Answer 1

I'll be honest I'm really confused with your code because your method name implies you are getting a value but the code looks like you are reading and possibly updating a field property. So to provide some feedback on the code you have there:

As you stated, item[fieldDisplayName] will get you a field value. (SPFieldUserValue, SPFieldUserValueCollection, string, etc.)

item.Fields[fieldInternalName] is what you need to get you a field. (SPField, SPFieldUser, SPFieldChoice, etc.)

Once you have the field, to get the value I would use the field Id and cast your values accordingly using item[field.Id] - this will eliminate any discrepencies / confusion between "should I now use the internal or display name?"

Answer 2

Have you tried to cast it as an SPFieldUserValue or an SPFieldUserValueCollection?

Answer 3

As C. Marius you need to lookup the internal name as it would have some other long values attached to it..

http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spfieldlookup.aspx

plus try this instead

SPFieldUser field =(SPFieldUser)item[fieldName];

also field.AllowMultipleValues shouldnt it be set i belive to true or false?

Gets or sets a Boolean value that specifies whether multiple values can be used in the lookup field.

true to specify that multiple values can be used in the field; otherwise, false. The default is false.

like so:

field.AllowMultipleValues = true;

or

field.AllowMultipleValues = false;

http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spfieldlookup.allowmultiplevalues(v=office.15).aspx

Answer 4

You should try :

1. Always using the InternalName of the field
2. Try validating the collection of Fields via spList.Fields.ContainsField(fieldName)