Syntax Error on Connection String - But Connection String work in other code.

The example use the following for the connection string, but when I used my own connection string VS give me SYNTAX Error
on the connection string.  I used the second connection string in other forms and it work but I get SYNTAX error when used in a READONLY.
Example:
Private ReadOnly Property connectionString() as string  
     Get        
  Return       
   "Server=.\sqlexpress;database=northwind;trusted_connection=True"     <===== Example in VS
  End Get
End Property
My Connection string work in other code but not in the Readonly Property. When I used the below code in a Readonly Property I get  Syntax error on data source statement.  WHY???   
Private ...