Retrieve Look Up Guid Value

Hello,

Please assist, I am new to Plugins, my code is not completed.

Scenario

- Plugin fired and executes on the 'House' entity
- When triggered, I would like to retrieve the ( LookUpGuid value - purchaser) from the Product entity.
- Note, there is not relationship between Product and House Entity.

// Current Code

// Retrive Product Entity
Entity Product = service.Retrieve("product", productid, new ColumnSet("purchaser"));
// Get LookUp Purchaser
EntityReference entityRef = (EntityReference)Product.Attributes["PurchaserId"];

// Get LookUp Value
var Purchaser = .... 

-- (Reference - example) sufiawan.wordpress.com/.../c-crm-retrieve-entity-from-lookup-field-entityreference

Many Thanks