POPInvCredItem inherits from POPInvCredItemView, and 'View' objects in Sage 200 can be thought of in the same way as SQL Server views (I know there are certain conditions under which SQL views are updateable but generally speaking you don't use views for CRUD operations).  In this case the metadata on the columns does explicitly mark them as being read-only.

What is it that you're trying to do? 

I was trying to amend PO item value before creating PO Invoice which is equivalent to the situation on the standard Sage 200 Record Invoice flow below:

Original value:

New value:

so I was thinking if I change it at this stage, maybe its okay, but its not

By the time you've got tto the form shown above you're working with a PurchaseInvoiceInstrument, not a POPinvCredItem.

Have a look at the example in the SDK:

https://my.sage.co.uk/Sage200SDKDocs/html/DOC0043_Example.html#BOOKMARK

You'll probably want to consider setting the NewDiscountedUnitPrice property of each POPInvCredItem if that needs changing (and do set it through the property, not throught the fields collection like in you example, that bypasses the business logic and you don't want to do that).  Then look at where the example is getting hold of the PurchaseInvoiceInstrument instance; once you've got hold of this then you can make the necessary changes to the values/nominal analysis etc. 

Thanks Chris. The Nominal Analysis is in the PurchaseInvoiceInstrument, hence I amend my value in this class.