在C＃中将代码从if语句更改为switch语句

Question

I've never used switch case instead of if/else if , and I'm wondering how to use it. I would really appreciate the help! The task is to put in an amount of wind in a textbox(tbVindstyrke) and the code should tell the user what amount of Watt per hour(W/t) the wind is generating, in a windmill. It should post the result in a label (lbWattprodusert).

I have got it to work with an if -statement, put as I have understood, this takes up a lot of the computers processioning power (or something). So, I would like to switch it up to a switch -statement.

double Vs = 0;
private void btSjekkW_Click(object sender, EventArgs e)
{
    Vs = Convert.ToDouble(tbVindstyrke.Text);
    if (Vs >= 0 && Vs <= 2.4)
        lbWattProdusert.Text = 0 + " W/t";
    else if (Vs >= 2.5 && Vs <= 3.3)
        lbWattProdusert.Text = 2 + " W/t";
    else if (Vs >= 3.4 && Vs <= 5.4)
        lbWattProdusert.Text = 10 + " W/t";
}

Answer 1

switch statements work with constant values.

So, this is valid:

var val = 2;
switch (val)
{
    case 1:
        // Do something if val is 1.
        break;
    case 2:
        // Do something if val is 2.
        break;
    default:
        // Do something for all values of val other than 1 or 2.
        break;
}

But you want to convert an if-else that deals with ranges. That too with double ranges. This is not possible since a switch doesn't allow you to work with a range.

If your ranges are int , you could hypothetically write a case for each value in the rage, but that makes absolutely no sense.

Say, you want to do something if the value is between int 1-3, and something else if it's between 4-6. You could write something like the follwing, but that would be nonsensicle. You'd be better off sticking to an if-else .

var val = 2;
switch (val)
{
    case 1:
    case 2:
    case 3:
        // Do something if val is between 1-3.
        break;
    case 4:
    case 5:
    case 6:
        // Do something if val is between 4-6.
        break;
    default:
        // Do something for all other values of val
        break;
}

Answer 2

A switch statement is useful when you want to choose between many options based on a single value. When you want to work against ranges, as you have done in your example, an if-statement is needed.

Here's an example of a switch statement... you can see that you specify the value to switch on at the start and you can then perform different actions based on equality.

string color = "red";

switch (color)
{
    case "red":
        // do something
        break;
    case "green":
        // do something
        break;
    case "blue":
        // do something
        break;
    default:
        throw new ColorUnknownException(color);
}

Very often, a switch-statement is a sign of a missing design pattern... but that is a conversation for another day.

Answer 3

switch doesn't work with float and double values . However, as all your range limits are multiples of 0.3, you can use the trick of dividing the value by 0.3 to achieve what you seek as follows:

int Vi;
double Vs = 0;
private void btSjekkW_Click(object sender, EventArgs e)
{
    Vs = Convert.ToDouble(tbVindstyrke.Text);
    Vi = (int)(Vs / 0.3);
    switch (caseSwitch)
    {
        case 0:
        case 1:
        case 2:
        case 3:
        case 4:
        case 5:
        case 6:
        case 7:
        case 8:
            lbWattProdusert.Text = 0 + " W/t";
            break;
        case 9:
        case 10:
        case 11:
            lbWattProdusert.Text = 2 + " W/t";
            break;
        case 12:
        case 13:
        case 14:
        case 15:
        case 16:
        case 17:
        case 18:
            lbWattProdusert.Text = 10 + " W/t";
          default:
            // what to do if Vs > 5.4
            break;
      }
}