Switch statement

Maha

http://www.dotnetperls.com/switch

In the above webpage under the CAUTION heading it says that expression in the switch must be int/char/string.

In the following program different type is used in the switch expression. Could you explain the difference please? Problem is highlighted.

using System;
enum Priority
{ Zero, Low, Medium, Important, Critical }

class Program
{
static void Main()
{
// New local variable of the Priority enum type.
Priority priority = Priority.Zero;

// Set priority to critical on Monday.
if (DateTime.Today.DayOfWeek == DayOfWeek.Monday)
priority = Priority.Critical;

// Write this if the priority is important.
if (IsImportant(priority))
Console.WriteLine("The problem is important.");

// See if Low priority is important.
priority = Priority.Low;
Console.WriteLine(IsImportant(priority));

// See if Important priority is.
priority = Priority.Important;
Console.WriteLine(IsImportant(priority));

Console.ReadKey();
}

static bool IsImportant(Priority priority)
{
// Switch on the Priority enum.
switch (priority)
{
case Priority.Low:
case Priority.Medium:
case Priority.Zero:
default:
return false;
case Priority.Important:
case Priority.Critical:
return true;
}
}
}

10 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes12y ago
Accepted answer
In C# there isn't actually a conversion defined between the int and bool types. For example this is illegal:

int i = 1;
bool b = (bool)i;

and so is this:

if(i)
{
// do something)
}

So, 1 and 0 are int literals and true and false are bool literals.

It's just a convenient convention that we regard 1 as equivalent to true and 0 as equivalent to false as this enables us to map values between the two types.

So, in the latest program, I'm making use of this convention by assigning true to 'sheLovesMe' if num is 1 or false to it if num is 0.

Note in particular that sheLovesMe is not becoming equal to num. It's becoming equal to the bool expression (num == 1) which is a different matter.
Maha12y ago
Thank you for your explanation.
Maha12y ago
It is not understandable that bool type sheLovesMe become equal to int type num.
Vulpes12y ago
No 'num' is always an int - you can't change the type of variables dynamically in C# like you can in some other languages.

But (num== 1) is an expression of type bool because num is either equal to 1 or it isn't.
Maha12y ago
So in this case num is taking two different types according to requirement.
Vulpes12y ago
Yes, it is.

'num' is an int which is either 0 or 1 chosen at random.

(num == 1) is a bool expression which is true if num is equal to 1 or false otherwise (in this case, if num equals 0).

So we're assigning this expression to the bool variable 'sheLovesMe' and then switching on it and printing out the appropriate message to the console.
Maha12y ago
Thank you for adding further information.

In the following code num is bool type as well as int type. Could you explain this variations.

bool sheLovesMe = (num == 1);

Usually we say 0 is false and 1 is true. I wish to know whether this concept is used in the above program.
Vulpes12y ago
Just for a bit of fun, here's an example of a switch on a bool:

using System;

class Program
{
static void Main()
{
Random r = new Random();
int num = r.Next(2);
bool sheLovesMe = (num == 1);

switch(sheLovesMe)
{
case true :
Console.WriteLine("She loves me :)");
break;

case false :
Console.WriteLine("She loves me not :(");
break;
}

Console.ReadKey();
}
}
Maha12y ago
Thanks
Vulpes12y ago
It does in fact say under the Types heading that you can switch on enums as well which is what is happening in this example.

This is reasonable enough given that enums always have an underlying integral type.

Although it's completely undocumented, I think you can also switch on the 'bool' type as well but, given there are only two possible values: true and false, an if/else is always better :)