Programatically updating SharePoint custom field type column value [closed]

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I have created a custom field type (say 'Sample') in SharePoint which is based on 'multiline text' field type. Now I have created a new column(say 'test') of type 'Sample' in a list. I created some list items.

I can edit the column value successfully from SharePoint UI (through edit form). But when I try to modify the value of 'test' column programatically for any list item, value of 'test' column for that list item becomes null/empty.

Any idea why this probem is occuring?? Below is the filedtypes xml that I am using

<?xml version="1.0" encoding="utf-8"?>
<FieldTypes>
 <FieldType>
  <Field Name="TypeName">Sample</Field>
  <Field Name="ParentType">Note</Field>
  <Field Name="TypeDisplayName">Sample</Field>
  <Field Name="TypeShortDescription">Sample</Field>
  <Field Name="UserCreatable">TRUE</Field>
  <Field Name="ShowInListCreate">TRUE</Field>
  <Field Name="ShowInSurveyCreate">TRUE</Field>
  <Field Name="ShowInDocumentLibraryCreate">TRUE</Field>
  <Field Name="ShowInColumnTemplateCreate">TRUE</Field>
  <Field Name="FieldTypeClass">
   SharePoint.Sample.FieldType, SharePoint.Sample, Version=1.0.0.0, Culture=neutral, PublicKeyToken=c5560aa45b5518dc
  </Field>
  <Field Name="FieldEditorUserControl">
   /_controltemplates/FieldEditor.ascx
  </Field>
   <PropertySchema>
   <Fields>
    <Field Name="DisplayedListBoxProperty" DisplayName="DisplayedListBoxProperty"
     Type="Text" Hidden="True">
     <Default>"abc"</Default>
    </Field>
   </Fields>
  </PropertySchema>
 </FieldType>
</FieldTypes>

Code that I am using to edit:

SPSite site = new  SPSite("site url")
SPWeb web = site.OpenWeb();
SPList list = web.Lists["MyList"];
SPListItem item  = list.Items[0];
item["test"] = "xyz";    //becomes null after update
item["numCol"] = "34";  //Gets updated to new value 34 after update
web.AllowUnsafeUpdates = true;
item.Update();

One Important thing: This开发者_运维技巧 code runs in itemupdating eventhadler of another list.

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With your code, there is problem with your Custom Field Type with "FieldTypeClass". Try this 'SharePoint.Sample.FieldType, SharePoint.Sample, Version=1.0.0.0, Culture=neutral, PublicKeyToken=c5560aa45b5518dc'

hope this will helps you.

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Well I found the problem area. There is a function called GetValidatedString() which is used for any validation on the value. This function gets called even in case when you update value programmatically. It was creating a problem.