Starting with C# 7.0, there is basic pattern matching support. I want to look at using this to interop with F# Discriminated Unions, and see what consuming F# code from C# could look like at it's best.
Here is an example of an F# Discriminated Union (hereafter DU)
type Record = { Name: string; Age: int }
type Abc =
| A
| B of double
| C of Record
For those unfamiliar with DUs, the Abc
type defined above can be either A
, B
, or C
(and nothing else), and in the case of A
there is no "payload", with B
and C
there is a payload that comes with it.
So given that this compiles down to a sealed class, and that the cases become classes, we can use the new type pattern in C# to match this with:
void HandleAbc(Abc abc)
{
switch (abc)
{
case Abc.B _:
Console.WriteLine("B");
break;
case Abc.C c:
Console.WriteLine(c.Item.Name);
break;
}
}
There are some things to notice here:
1. It's really nice
The code here is clean I'd say, we are matching on type and can either ignore the result of the cast using _
or we can take it as a named variable (like with c
in this example).
2. We don't get the safety we have in F
In F#, when handling DUs the compiler ensure that we have handled all cases, in C#, these safety checks aren't enforced.
3. What about case A
!
Yeah, so this is where it's not perfect. The F# compiler does generate a type for case A
however it is marked as internal and therefore is not accessible for us to match against, however, there are a few options, I'll let you pick which is your favourite:
void HandleAbc(Abc abc)
{
switch (abc)
{
// Option 1:
case var x when x.IsA:
Console.WriteLine("A");
break;
// Option 2:
case var x when x == Abc.A:
Console.WriteLine("A");
break;
}
switch (abc.Tag)
{
// Option 3:
case abc.Tags.A:
Console.WriteLine("A");
break;
}
}
Let me know in the comments what your preference is, or is there a better option?
Thanks for reading!
Top comments (2)
Pattern matching was one of my favorite parts of F#.
With records looking like they'll be in c# 8.1 or 8.2, it makes me wonder how close DUs are.
Can you not do
case var x when x is Abc.A:
? Nevermind,Abc.A
looks like it's the value not the type, how odd...Yeah, DUs are the biggest thing I miss when going back to C#, I would love that.
You're right, the type that gets generated is a little wierd and inuntuative to consume from C#, that's why I created this post, I've been back to it every now and then over the last few months.