Based on your code, I think the issue is caused by your connection string syntax. Please change it to:
string connString = @"Data Source= \Program Files\Microsoft SQL Server Compact Edition\v3.5\Samples\restaurantdb.sdf";
For more information, please refer to the links below
Connection strings for SQL Server Compact Editionhttp://www.connectionstrings.com/sql-server-2005-ce
Since you are developing a smart device project, I would suggestion you post your questions in:
Smart Device Developmenthttp://social.msdn.microsoft.com/Forums/en-US/category/smartdevicedevelopment
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