Proposal: Multiple possibilities return value

[hez2010]

If I want a function to return value of different types, I have to write like this:

public void m()  // can constraint the type of returned value but have to write a lot of duplicated code
{
     if (....) 
     {
         int x;
         x = a();
     }
     else 
     {
         string x;
         x = a();
     }
}
public int a()
{
      return 5;
}
public string a()
{
      return "t";
}

or

public object a() // cannot constraint the type of returned value
{
      if (...) return 5;
      else return "t";
}

I think the code below will be useful and can both constraint the type of returned value and reduce duplicated code:

public string|int a()
{
      if (...) return 5;
      else return "t";
}

[HaloFour]

So, if you have var value = a(), what is the type of value? How can you use it?

[DavidArno]

What you are proposing is often implemented as Discriminated Unions, which is championed in #113.

[jnm2]

Sounds like #399.

I have a OneOf<T1, T2> type I use for ad-hoc discriminated unions (the way ValueTuple is an ad-hoc product type).

[hez2010]

@HaloFour
The type of value is object when the variable is implicitly declared and there's no difference between object and T1|T2|....

[Happypig375]

Syntax 1:
It has a lot of problems. If the type when var is used is object, will the overload returning an int or the one returning an string be called?

Syntax 2: It is already valid today. What are you proposing?

Syntax 3: Duplicate of #399.

[theunrepentantgeek]

This proposes a TypeScript idiom that makes a lot of sense in the dynamically typed world of JavaScript; it doesn't make any sense to me in C#.

Shapes and Extensions (#164) would provide all of the benefits of this proposal, and a lot more, but in a form that's type safe and rigorous, as suits the nature of C#.

[Austin-bryan]

@hez2010 From what I understand, you can't overload on return types for very good reasons. It had to do with the compiler never being able to know which when they meant. All of these examples use your first syntax example. Examples:

a();    // does string or int get called?

...

public void Foo(int ...) { ... }
public void Foo(string s) { ... }
...
Foo(a())    // which overload is called and which a is called?

...

object mysteryType = a();    // Which a gets called? 
int x = mysteryType;    // ...and will this work? It depends, is mysteryType an int or string? I don't know and neither does the compiler

etc., I'm probably missing a lot here.

In the meantime, just use dynamic, it's basically the same thing except that dynamic can also be Foo, but it'll still get your job done.

[alexgman]

@hez2010 From what I understand, you can't overload on return types for very good reasons. It had to do with the compiler never being able to know which when they meant. All of these examples use your first syntax example. Examples:

a();    // does string or int get called?

...

public void Foo(int ...) { ... }
public void Foo(string s) { ... }
...
Foo(a())    // which overload is called and which a is called?

...

object mysteryType = a();    // Which a gets called? 
int x = mysteryType;    // ...and will this work? It depends, is mysteryType an int or string? I don't know and neither does the compiler

etc., I'm probably missing a lot here.

In the meantime, just use dynamic, it's basically the same thing except that dynamic can also be Foo, but it'll still get your job done.

can you please give an example of how to use dynamic as a union type solution? for example, if i want to have a discriminated union of "mystring1" and "mystring2" how do i do that with dynamic?

[DarthVella]

The dynamic type/keyword is used to force type and member checking of objects to be deferred until runtime and can be used to implement crude, unchecked type unions (indiscriminate unions?) between unrelated objects with similarly named properties and methods. It cannot be used to create an enforced union of string values as can be done in Typescript. If that's what you're after, I'd suggest creating an enum and calling the ToString method on it.