Assigning External content type field column value using Client Object Model

I think I understand your problem.

In my example I have an external data column "Beneficiary Name", using a Beneficiary external content type (accessing a table of beneficiaries in a SQL table).

I want to set the "Beneficiary Name" property using the client object model. I know the name of the beneficiary but not the ID value.

It is a fairly simple solution. You just need to identify the name of the property SharePoint assigns to the ID field, in my case it is called "Beneficiary_ID". Then set the two properties as follows:

thisItem["Beneficiary_Name"] = "Charitable Trust";

thisItem["Beneficiary_ID"] = -1;

thisItem.Update();

Setting the ID property to -1 causes the server to do an automatic lookup for the ID from the value assigned to the item.