Evaluate Expressions in Switch Statements in C#

Question

I have to implement the following in a switch statement:

switch(num)
{
  case 4:
    // some code ;
    break;
  case 3:
    // some code ;
    brea         


        

Answer 1

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
        ...
    }
}

Answer 2

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

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

Answer 3

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

Answer 4

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.

Answer 5

In 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>

Answer 6

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.