Program 'xxx.exe' does not contain a static 'Main' method suitable for an entry point

In a WPF program I am getting an error "Program 'xxx.exe' does not contain a static 'Main' method suitable for an entry point". Others in my team compile the same code sucessfully. The App.xaml file has the BuildAction set to ApplicationDefinition. I get the same error using VS2010 Ultimate and VS2012 Ultimate.
Does anyone have any suggestions?