Is there a better alternative than this to 'sw

Question

Seeing as C# can't switch on a Type (which I gather wasn't added as a special case because is-a relationships mean that more than one distinct case might apply), is there a better way to simulate switching on type than this?

void Foo(object o)
{
    if (o is A)
    {
        ((A)o).Hop();
    }
    else if (o is B)
    {
        ((B)o).Skip();
    }
    else
    {
        throw new ArgumentException("Unexpected type: " + o.GetType());
    }
}

Answer 1

I would either

• use method overloading (just like x0n), or
• use subclasses (just like Pablo), or
• apply the visitor pattern.

Answer 2

Create an interface IFooable, then make your A and B classes to implement a common method, which in turn calls the corresponding method you want:

interface IFooable
{
   public void Foo();
}

class A : IFooable
{
   //other methods ...

   public void Foo()
   {
      this.Hop();
   }
}

class B : IFooable
{
   //other methods ...

   public void Foo()
   {
      this.Skip();
   }
}

class ProcessingClass
{
public void Foo(object o)
{
   if (o == null)
      throw new NullRefferenceException("Null reference", "o");

   IFooable f = o as IFooable;
   if (f != null)
   {
       f.Foo();
   }
   else
   {
       throw new ArgumentException("Unexpected type: " + o.GetType());
   }
}
}

Note, that it's better to use "as" instead first checking with "is" and then casting, as that way you make 2 casts (expensive).

Answer 3

You're looking for Discriminated Unions which are a language feature of F#, but you can achieve a similar effect by using a library I made, called OneOf

https://github.com/mcintyre321/OneOf

The major advantage over switch (and if and exceptions as control flow) is that it is compile-time safe - there is no default handler or fall through

void Foo(OneOf<A, B> o)
{
    o.Switch(
        a => a.Hop(),
        b => b.Skip()
    );
}

If you add a third item to o, you'll get a compiler error as you have to add a handler Func inside the switch call.

You can also do a .Match which returns a value, rather than executes a statement:

double Area(OneOf<Square, Circle> o)
{
    return o.Match(
        square => square.Length * square.Length,
        circle => Math.PI * circle.Radius * circle.Radius
    );
}

Answer 4

I agree with Jon about having a hash of actions to class name. If you keep your pattern, you might want to consider using the "as" construct instead:

A a = o as A;
if (a != null) {
    a.Hop();
    return;
}
B b = o as B;
if (b != null) {
    b.Skip();
    return;
}
throw new ArgumentException("...");

The difference is that when you use the patter if (foo is Bar) { ((Bar)foo).Action(); } you're doing the type casting twice. Now maybe the compiler will optimize and only do that work once - but I wouldn't count on it.

Answer 5

Yes - just use the slightly weirdly named "pattern matching" from C#7 upwards to match on class or structure:

IObject concrete1 = new ObjectImplementation1();
IObject concrete2 = new ObjectImplementation2();

switch (concrete1)
{
    case ObjectImplementation1 c1: return "type 1";         
    case ObjectImplementation2 c2: return "type 2";         
}

Answer 6

I liked Virtlink's use of implicit typing to make the switch much more readable, but I didn't like that an early-out isn't possible, and that we're doing allocations. Let's turn up the perf a little.

public static class TypeSwitch
{
    public static void On<TV, T1>(TV value, Action<T1> action1)
        where T1 : TV
    {
        if (value is T1) action1((T1)value);
    }

    public static void On<TV, T1, T2>(TV value, Action<T1> action1, Action<T2> action2)
        where T1 : TV where T2 : TV
    {
        if (value is T1) action1((T1)value);
        else if (value is T2) action2((T2)value);
    }

    public static void On<TV, T1, T2, T3>(TV value, Action<T1> action1, Action<T2> action2, Action<T3> action3)
        where T1 : TV where T2 : TV where T3 : TV
    {
        if (value is T1) action1((T1)value);
        else if (value is T2) action2((T2)value);
        else if (value is T3) action3((T3)value);
    }

    // ... etc.
}

Well, that makes my fingers hurt. Let's do it in T4:

<#@ template debug="false" hostSpecific="true" language="C#" #>
<#@ output extension=".cs" #>
<#@ Assembly Name="System.Core.dll" #>
<#@ import namespace="System.Linq" #> 
<#@ import namespace="System.IO" #> 
<#
    string GenWarning = "// THIS FILE IS GENERATED FROM " + Path.GetFileName(Host.TemplateFile) + " - ANY HAND EDITS WILL BE LOST!";
    const int MaxCases = 15;
#>
<#=GenWarning#>

using System;

public static class TypeSwitch
{
<# for(int icase = 1; icase <= MaxCases; ++icase) {
    var types = string.Join(", ", Enumerable.Range(1, icase).Select(i => "T" + i));
    var actions = string.Join(", ", Enumerable.Range(1, icase).Select(i => string.Format("Action<T{0}> action{0}", i)));
    var wheres = string.Join(" ", Enumerable.Range(1, icase).Select(i => string.Format("where T{0} : TV", i)));
#>
    <#=GenWarning#>

    public static void On<TV, <#=types#>>(TV value, <#=actions#>)
        <#=wheres#>
    {
        if (value is T1) action1((T1)value);
<# for(int i = 2; i <= icase; ++i) { #>
        else if (value is T<#=i#>) action<#=i#>((T<#=i#>)value);
<#}#>
    }

<#}#>
    <#=GenWarning#>
}

Adjusting Virtlink's example a little:

TypeSwitch.On(operand,
    (C x) => name = x.FullName,
    (B x) => name = x.LongName,
    (A x) => name = x.Name,
    (X x) => name = x.ToString(CultureInfo.CurrentCulture),
    (Y x) => name = x.GetIdentifier(),
    (object x) => name = x.ToString());

Readable and fast. Now, as everybody keeps pointing out in their answers, and given the nature of this question, order is important in the type matching. Therefore:

• Put leaf types first, base types later.
• For peer types, put more likely matches first to maximize perf.
• This implies that there is no need for a special default case. Instead, just use the base-most type in the lambda, and put it last.