C#

public static void Main(string[] args)
{
    if ( args.Length > 0 )
    {
        if ( args[0].ToLower().EndsWith(".foo") )
            //do something with foo file
            Console.WriteLine( "Openning Foofile named {0}", args[0] );
    }

    Console.WriteLine("Press any key to continue...");
    Console.ReadKey();
}

public static string FooFileName = null;

[STAThread]
public static void Main(string[] args)
{
    //check for the required file extension
    if ( args.Length > 0 && args[0].ToLower().EndsWith(".foo") )
    {
        //update the static member withe the file name
        FooFileName = args[0];
    }

    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Application.Run(new MainForm());
}

public MainForm()
{
    //
    // The InitializeComponent() call is required for Windows Forms designer support.
    //
    InitializeComponent();

    //check that the filename is not null
    if ( FooFileName != null )
    {
        //do something with the file
        MessageBox.Show( String.Format("FooFile Name: {0}", FooFileName) );
    }
}

[STAThread]
public static void Main(string[] args)
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);

    if ( args.Length > 0 && args[0].ToLower().EndsWith(".foo") )
    {
        //pass the the FooFile name to the MainForm constructor
        Application.Run(new MainForm( args[0] ));
    } else {
        //pass nothing to the MainForm constructor
        Application.Run(new MainForm( null ));
    }
}

public MainForm( string FooFileName )
{
    //
    // The InitializeComponent() call is required for Windows Forms designer support.
    //
    InitializeComponent();

    //
    // TODO: Add constructor code after the InitializeComponent() call.
    //

    //do something if a FooFileName was passed
    if ( FooFileName != null ) MessageBox.Show( String.Format("FooFile name: {0}", FooFileName) );
}