Enum as Flag using, setting and shifting

问题

I have two flags:

[Flags]
enum Flags
{
  A = 1,
  B = 2
};

I set them like this:

Mode = Flags.A | Flags.B; // default value
for(int i = 0; i < args.Length; i++) {
switch(args[i])
{
  case "--a":
  {
    if ((Mode & Flags.A) == Flags.A && (Mode & Flags.B) == Flags.B) 
     // both, default assume
    {
      Mode = Flags.A; // only A
    }
    else
    {
      Mode |= Flags.A; // append A
    }
    break;
  }
  case "--b":
  {
    if ((Mode & Flags.A) == Flags.A && (Mode & Flags.B) == Mode.B)
    {
      Mode = Flags.B;
    }
    else
    {
      Mode |= Flags.B;
    }
    break;
  }
} }

and use them later like this:

if((Mode & Flags.A) == Flags.A)
{
 //
}
if((Mode & Flags.B) == Flags.B)
{
 //
}

Major reservation: Both flags may be set. Or just one, in this case only one part of code is executed.

Is my code good? What is the best way to setup flags?

Upd: Is this less ugly then first, how do you think?

Mode = 0; // default value now is empty
for(int i = 0; i < args.Length; i++) {
switch(args[i])
{
  case "--a":
  {
    Mode |= Flags.A;
    break;
  }
  case "--b":
  {
    Mode |= Flags.B;
    break;
  }
} }
if(Mode == 0)
{
  Mode = Flags.A | Flags.B; // if no parameters are given, setup both flags
}

回答1:

Here's how I would set my flags:

Mode = 0;
for(int i = 0; i < args.Length; i++) {
    switch(args[i]) {
    case "--a":
        Mode |= Flags.A;
        break;
    case "--b":
        Mode |= Flags.B;
        break;
    }
}

If both flags should be on by default, I think it makes more sense to change the command line parameters to something like --not-a and --not-b. This would both reflect the default setting, and let you get rid of (Mode & Flags.A) == Flags.A && (Mode & Flags.B) == Flags.B, which is rather ugly, IMHO.

Then you can set your flags like this:

Mode = Flags.A | Flags.B;
for(int i = 0; i < args.Length; i++) {
    switch(args[i]) {
    case "--not-a":
        Mode &= ~Flags.A;
        break;
    case "--not-b":
        Mode &= ~Flags.B;
        break;
    }
}

Finally, if you have a lot of flags (instead of just two), it might be easier to set up your enum like this:

[Flags]
enum Flags
{
    A = 1,
    B = 1 << 1,
    C = 1 << 2,
    D = 1 << 3,
    E = 1 << 4,
    F = 1 << 5
};

回答2:

You can turn a "bit" off with the following wonderful statement:

Mode &= ~Flags.A;

I'd reccoment including a "null" value in your enum as well:

[Flags]
enum Flags
{
  Null = 0;
  A = 1,
  B = 2;
}

It will keep your life simpler! :-)

回答3:

The second version is much better - this is exactly what I would do. Change Mode == null to Mode == 0 though.

Mode = 0; // default value now is empty
for(int i = 0; i < args.Length; i++) {
    switch(args[i])
    {
        case "--a":
            Mode |= Flags.A;
            break;

        case "--b":
            Mode |= Flags.B;
            break;
    }
}

if(Mode == 0)
{
    Mode = Flags.A | Flags.B; // if no parameters are given, setup both flags
}

来源：https://stackoverflow.com/questions/581555/enum-as-flag-using-setting-and-shifting

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