How to set value in experission for scheme in biztalk orchestration?

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VS2010

biztalk

XSD

workflow

When I want to compile, an error shown:
#error: The expression you have entered is not valid.
Plus in Visual Studio 2010 this message is shown:
'if' conditional must have boolean type.

and my code for expression is:

Message_2.Article.Approved = false;

and this is my XML code in scheme Article.xsd:

- <xs:element name="Approved">
- <xs:simpleType>
  <xs:restriction base="xs:boolean" /> 
  </xs:simpleType>
  </xs:element>