C# 8.0 switch expression based on input type

Question

Is it possible to create switch expression in C# 8 based on input type?

My input classes looks like this:

public class A1
{
    public string Id1 {get;set}
}

public class A2 : A1
{
    public string Id2 {get;set}
}

public class A3 : A1
{
    public string Id3 {get;set;}
}

I want to run diffrent methods based on input type (A1, A2, or A3):

var inputType = input.GetType();
var result = inputType switch
{
       inputType as A1 => RunMethod1(input); // wont compile, 
       inputType as A2 => RunMethod2(input); // just showing idea
       inputType as A3 => RunMethod3(input);

}

But it wont work. Any ideas how to create switch or switch expression based on input type?C

Try starting the switch with most specific type first (A3) and work your way down to least specific (A1) — Thomas, Oct 13 '20 at 08:10

Johnathan Barclay · Accepted Answer · 2020-10-13T08:17:08.723

5

You could use pattern matching, checking for the most specific types first.

GetType is unneccesary:

var result = input switch
{
    A2 _ => RunMethod1(input),
    A3 _ => RunMethod2(input),
    A1 _ => RunMethod3(input)    
};

However, a more OO approach would be to define a method on the types themselves:

public class A1
{
    public string Id1 { get; set; }
    public virtual void Run() { }
}

public class A2 : A1
{
    public string Id2 { get; set; }
    public override void Run() { }
}

Then it's simply:

input.Run();

edited Oct 13 '20 at 08:17

answered Oct 13 '20 at 08:13

Johnathan Barclay

18,599
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Nice idea, but your code wont compile. Im getting this error: `The Pattern has already been handled by previous arm of switch expression` – michasaucer Oct 13 '20 at 08:17
1

@michasaucer It compiles fine for me: https://dotnetfiddle.net/1dGHUz – Johnathan Barclay Oct 13 '20 at 08:20
Your answer compiles for me too... So now i need figuring out why it wont in my scenerio (i just pasted base example here in SO). Thanks! – michasaucer Oct 13 '20 at 08:22
@JohnathanBarclay if you change the instance to `A2` or `A3` [it wont compile](https://dotnetfiddle.net/H4w59Y) see the second point of "notes" in my answer. – Jamiec Oct 13 '20 at 08:27
@Jamiec Yes, I probably should have explicitly typed the variable in my fiddle. – Johnathan Barclay Oct 13 '20 at 08:29
@JohnathanBarclay i didnt mark your post as answer because Jamiec was faster but still you deserve for upvote – michasaucer Oct 13 '20 at 08:29
@michasaucer Not to be pedantic, but I answered first ;) – Johnathan Barclay Oct 13 '20 at 08:30
@JohnathanBarclay sure, my bad, i edited, thanks again – michasaucer Oct 13 '20 at 08:31

Jamiec · Answer 2 · 2020-10-13T08:28:40.167

You can, but in an inheritance hierarchy like that you need to start with the most specific and move down towards the least:

A1 inputType = new A2();
var result = inputType switch
{
    A3 a3 => RunMethod(a3),
    A2 a2 => RunMethod(a2),
    A1 a1 => RunMethod(a1)
};

Note that

inputType is an instance, not an instance of Type
inputType is typed as the base class but can be an instance of any A1-3. Otherwise you'll get a compiler error.

Live example: https://dotnetfiddle.net/ip2BNZ

C# 8.0 switch expression based on input type

2 Answers2