Evaluate Expressions in Switch Statements in C#

I have to implement the following in a switch statement:

switch(num)
{
  case 4:
    // some code ;
    break;
  case 3:
    // some code ;
    break;
  case 0:
    // some code ;
    break;
  case < 0:
    // some code ;
    break;
}

Is it possible to have the switch statement evaluate case < 0? If not, how could I do that?

I know that this topic is pretty old but if someone still looking for the answer now in C# 7 it's possible. Here is an example:

switch (value)
{
     case var expression when value < 0:
         //some code
         break; 

     case var expression when (value >= 0 && value < 5):
         //some code
         break;

     default:
         //some code
         break;
}

Note: the answer below was written in 2009. Switch patterns were introduced in C# 7.

---

You can't - switch/case is only for individual values. If you want to specify conditions, you need an "if":

if (num < 0)
{
    ...
}
else
{
    switch(num)
    {
        case 0: // Code
        case 1: // Code
        case 2: // Code
        ...
    }
}

you can do this

switch (mark)
{
    case int n when n >= 80:
        Console.WriteLine("Grade is A");
        break;
	
    case int n when n >= 60:
        Console.WriteLine("Grade is B");
        break;
	
    case int n when n >= 40:
        Console.WriteLine("Grade is C");
        break;
	
    default:
        Console.WriteLine("Grade is D");
        break;
}

You could do something like this at the end of your switch statement:

default:
    if(num < 0)
    {
        ... // Code
    }
    break;

If your num can't be less than zero:

public int GetSwitch(int num) { return num < 0 ? -1 : num; }
switch(GetSwitch(num))
{
case 4: // some code ; break;
case 3:// some code ; break;
case 0: // some code ; break;
case -1 :// some code ; break;
}

If it can, use some other "non-existent" number such as int.MinValue.

In a twist of C# fate, this has come all the way back around. If you upgrade to C# 9.0, your original switch statement will now compile! C#9.0 has added Relational patterns to pattern matching in general, which includes switch statements.

You can now do some really funky stuff, like this:

      var num = new Random().Next();
      
      switch(num)
      {
        case < 0:
          // some code ;
          break;
        case 0:
          // some code ;
          break;
        case > 0 and < 10:
          // some code ;
          break;
        case > 20 or (< 20 and 15):
          // some code ;
          break;
      }

Note the use of literal 'and' and 'or' in the last case, to allow && and || type expressions to compile.

To use C# 9, make sure the XmlNode LangVersion is set to Latest or 9.0 in the csproj file, in a property group

e.g.

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <Platforms>AnyCPU;x86</Platforms>
    <Configurations>Debug;Release;Mock</Configurations>
	<LangVersion>Latest</LangVersion>
  </PropertyGroup>

Enter 2021.

var res = num switch
{
    4 => "It's four",
    3 => "It's three",
    0 => "It's zero",
    < 0 => "It's negative",
    _ => "It's something else"
};

Boom.

(To be fair, switch expressions are a C# 8.0 feature, so not really 2021. But still few people seem to know about it.)

You will have to use if, wether you want or not. Switch is only capable of comparing your value to constant values.

The only way I could think of (and I really don't recommand it), would be as follows:

int someValue;

switch (Math.Max(someValue, -1))
{
    case -1:
        // will be executed for everything lower than zero.
        break;

    case 0:
       // will be executed for value 0.
       break;

    case 1:
       // will be executed for value 1.
       break;

    default:
       // will be executed for anything else.
       break;
}

The other way around would be possible also (relating to Jon Skeet's answer):

switch(num)
{
  case a:
      break;
  default:
      if( num < 0 )
      {}
   break;
}

What you could do is to use a delegate like this.

		var actionSwitch = new Dictionary<Func<int, bool>, Action>
		{
			 { x => x < 0 ,		() => Log.Information("less than zero!")},
			 { x => x == 1,	    () => Log.Information("1")},
			 { x => x == 2,		() => Log.Information("2")},
			 { x => x == 3,		() => Log.Information("3")},
			 { x => x == 4,		() => Log.Information("4")},
			 { x => x == 5,	    () => Log.Information("5")},
		};

		int numberToCheck = 1;

        //Used like this
		actionSwitch.FirstOrDefault(sw => sw.Key(numberToCheck)).Value();

Just swap out what you wan´t to perform where the Log.Information("x") is and you have your "switch" statement.

You will need to some error handling if you check for a key that is "found" by the Func.

If you like to see the Func version of the switch I just wrote a blog post with Switch example chapter.

But if you can use C# 7 it will give you better switch capabilities.

You cannot use comparisons in switches like you could in VB, you have 2 options here, replace the value you switch on with a known value and use that or - if you mean all other cases - you can use the default clause:

switch(num)
{
  case 4:
    // some code ;
    break;
  case 3:
    // some code ;
    break;
  case 0:
    // some code ;
    break;
  default:
    // some code ;
    break;
}

Note that this does not exactly like you asked for: any values other than 0,3,4 will end up in the deafult: clause.

I've run into the following pattern recently, and while I abhor it, I can't argue that it's not practical:

switch(0)
{
  case 0 when x < 0:
    ...
    break;
  case 0 when a > 5 && x == 0:
    ...
    break;
}

The use of dummy expressions ((0) in the switch, and case 0) is absolutely terrible, and I'd hate for it to become an idiom, but hey - it's concise and very clear. For sure it's not some hack that completely obscures the meaning and needs arcane knowledge, but C# would do well to obviate the need for it. I'd like the following to be legal:

switch                   // maybe () here, if the grammar would demand
{
  case when x < 0:       // I like the case to stay
    ...
  case when a > 5 && x == 0:
    ...
}