Linq to Entity with Visual Studio 2008

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C#

LINQ

I have used Linq to Entity with Visual Studio 2008 but now i have met a problem about table's foreign key. Please help me! Thank you very much
Description: I want to get combobox's value to transmit for table's foreign key.

this in case table Products has foreign key concern to table Categories in Northwind Database

I have code :

Products _producAdd = new Products();
_producAdd.CategoriesReference.EntityKey.EntityKeyValues[0].Value =       cbbCategory.SelectedValue.ToString();//mistake at this row

Please help me !