Unable to cast an F# Object in C#

Question

Given this F#:

namespace DU_Sample  

type StateA = { Counter: int }  
type StateB = { Counter: int; Pass: bool }  

type DU =  
| A of StateA  
| B of StateB

And this C#:

[TestMethod]  
public void TestMethod1()  
{  
  var stateB = GetStateB();  
  Assert.IsTrue( stateB.IsB );  
  //Assert.IsTrue( ((DU_Sample.StateB)stateB).Pass );  // nope  
  //var nutherB = DU_Sample.DU.NewB( stateB );  // nope  
  Assert.IsTrue( ( (dynamic)stateB ).Item.Pass );  // pass  
}  

private static DU_Sample.DU GetStateB()  
{  
  var stateB = new DU_Sample.StateB( 0, true );  
  return DU_Sample.DU.NewB( stateB );  
}

How can I cast the Discriminated Union type to one of it's parts in order to access that part's properties?

Why is `GetStateB` returning something other than a `StateB` instance? How confusing.. — ildjarn, Jun 17 '16 at 19:55
You should consider the answer from here: http://stackoverflow.com/questions/17254855/what-is-the-simplest-way-to-access-data-of-an-f-discriminated-union-type-in-c — Ringil, Jun 17 '16 at 20:27
@ildjarn - this is a simplified example; I want to pass the "parent" DU type around but be able to cast... — dudeNumber4, Jun 17 '16 at 21:09
@JeffMercado - You can't access the .Item either - I only see it while debugging. — dudeNumber4, Jun 17 '16 at 21:09
@JoelMueller - because I'd like to try to introduce F# into a C# code base if possible. We're never going to just cut over. This is just a sample. — dudeNumber4, Jun 17 '16 at 21:10
@dudeNumber4 : The "parent" DU type is `DU_Sample.DU`, but `GetStateB` by its name should return a `DU_Sample.StateB`. — ildjarn, Jun 17 '16 at 23:31
@ildjarn - True. Again, contrived sample. Just trying to isolate the particular problem I was experiencing. — dudeNumber4, Jun 18 '16 at 02:55

Gus · Accepted Answer · 2016-06-17T20:47:07.470

1

In both cases you have to (down) cast and then use the property Item:

Assert.IsTrue((stateB as DU_Sample.DU.B)?.Item.Pass ?? false); 
var nutherB = DU_Sample.DU.NewB(((DU_Sample.DU.B)stateB).Item)

edited Jun 17 '16 at 20:47

answered Jun 17 '16 at 20:42

Gus

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Unable to cast an F# Object in C#

1 Answers1