using the 'is' keyword in a switch in c#

Question

I'm currently adding some new extended classes to this code:

foreach (BaseType b in CollectionOfExtendedTypes) {
  if (b is ExtendedType1) {
    ((ExtendedType1) b).foo = this;

  }
  else if (b is ExtendedType2) {
    ((ExtenedType2) b).foo = this;

  } 
  else {
    b.foo = this;

  }
}

and was curious if there is a way to use the is keyword functionality in a switch statement?

Answer 1

The latest version of C# (7) now includes this functionality

Type pattern

The type pattern enables concise type evaluation and conversion. When used with the switch statement to perform pattern matching, it tests whether an expression can be converted to a specified type and, if it can be, casts it to a variable of that type. Its syntax is:

   case type varname 

Answer 2

This really looks like a situation for a good polymorphic implementation. If you override the appropriate methods in the derived classes, you may not need the checks in the loop at all.

Answer 3

Nope. See

C# switch statement limitations - why?

Answer 4

In C# it's not possible to use the "is" keyword as part of a switch statement. All case labels in a switch must evaluate to constant expressions. "is" is not convertible to a constant expression.

I definately feel the pain though when it comes to switching on types. Because really the solution you outlined works but it's a conveluted way of saying for x do y, and a do b. It would be much more natular to write it more like the following

TypeSwitch.Do(
    sender,
    TypeSwitch.Case<Button>(() => textBox1.Text = "Hit a Button"),
    TypeSwitch.Case<CheckBox>(x => textBox1.Text = "Checkbox is " + x.Checked),
    TypeSwitch.Default(() => textBox1.Text = "Not sure what is hovered over"));

Here's a blog post I wrote on how to achieve this functionality.

http://blogs.msdn.com/jaredpar/archive/2008/05/16/switching-on-types.aspx