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C# to VB translation problem?

Discussion in 'Programming & Webmastering' started by jchunn, Mar 12, 2008.

  1. Mar 12, 2008 at 6:15 AM #1

    jchunn New Member

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    In C#, this works, but I don't understand why:

    int i = ( int ) ( 0xff000000 | (r << 16) | (g << 8) | b );

    Shouldn't that cause an overflow?

    In the debugger, this:
    ( 0xff000000 | (r << 16) | (g << 8) | b )
    equals this: 4278190080

    But this:
    ( int ) ( 0xff000000 | (r << 16) | (g << 8) | b )
    equals this: -16777216

    What is going on there? The VB translation would be this:

    Dim i as Integer = CType((4278190080 Or (r << 16) Or (g << 8) Or b), Integer)
    But that returns an overflow error, since obviously the result (4278190080) is too large for an Integer, and unlike C#, it does not magically turn into -16777216.

    Any help would be greatly appreciated.

    Edit: r, g, and b all = 0 in this example.
     
    Last edited: Mar 12, 2008
  2. Mar 12, 2008 at 12:41 PM #2
    Kreij

    Kreij Senior Monkey Moderator Staff Member

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    In C# there are two keywords "checked" and "unchecked".
    Unchecked turns off overflow checking. (Checked is on by default)

    My compiler throws an error unless I wrap the statement in an unchecked condition
    Code:
    unchecked
{
    int i = (int)(0xff000000 | (0<<16) | (0<<8) | 0);
}
    In which case it returns the same negative value that you get.

    If you are not using the unchecked keyword, make sure that it is not being shut off globally in your compiler options or environment configuration.
     
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  3. Mar 12, 2008 at 2:21 PM #3

    jchunn New Member

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    That is the answer... thank you very much! Problem solved.
     

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