UnionConverter: Accept custom discriminator without catchAllCase (#51)

CHANGELOG.md

@@ -13,6 +13,8 @@ The `Unreleased` section name is replaced by the expected version of next releas
 ### Removed
 ### Fixed
 
+- `UnionConverter`: Support overriding discriminator without needing to nominate a `catchAllCase` [#51](https://github.com/jet/FsCodec/pull/51)
+
 <a name="2.1.0"></a>
 ## [2.1.0] - 2020-05-10
 

src/FsCodec.NewtonsoftJson/UnionConverter.fs

@@ -72,8 +72,9 @@ module private Union =
 type UnionConverter private (discriminator : string, ?catchAllCase) =
     inherit JsonConverter()
 
-    new() = UnionConverter("case")
-    new(discriminator: string, catchAllCase: string) = UnionConverter(discriminator, ?catchAllCase = match catchAllCase with null -> None | x -> Some x)
+    new() = UnionConverter("case", ?catchAllCase=None)
+    new(discriminator: string) = UnionConverter(discriminator, ?catchAllCase=None)
+    new(discriminator: string, catchAllCase: string) = UnionConverter(discriminator, ?catchAllCase=match catchAllCase with null -> None | x -> Some x)
 
     override __.CanConvert (t : Type) = Union.isUnion t
 

tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs

@@ -297,6 +297,26 @@ let ``Implementation ensures no internal errors escape (which would render a Web
     test <@ (CaseD "hi") = d @>
     test <@ false = gotError @>
 
+module CustomDiscriminator =
+
+    [<JsonConverter(typeof<UnionConverter>, "esac")>]
+    type DuWithCustomDiscriminator =
+        | Known
+        | Catchall
+
+    [<Fact>]
+    let ``UnionConverter handles custom discriminator`` () =
+        let json = """{"esac":"Known"}"""
+        test <@ DuWithCustomDiscriminator.Known = JsonConvert.DeserializeObject<_> json @>
+
+    [<Fact>]
+    let ``UnionConverter can complain about missing case with custom discriminator without catchall`` () =
+        let aJson = """{"esac":"CaseUnknown"}"""
+        let act () = JsonConvert.DeserializeObject<DuWithCustomDiscriminator>(aJson, settings)
+
+        fun (e : System.InvalidOperationException) -> <@ -1 <> e.Message.IndexOf "No case defined for 'CaseUnknown', and no catchAllCase nominated" @>
+        |> raisesWith <@ act() @>
+
 module ``Unmatched case handling`` =
 
     [<Fact>]