[#180] Allow overriding generic types

Author: Tarmil

src/FSharp.SystemTextJson/Helpers.fs

@@ -3,6 +3,7 @@
 #nowarn "44" // JsonSerializerOptions.IgnoreNullValues is obsolete for users but still relevant for converters.
 
 open System
+open System.Collections.Generic
 open System.Reflection
 open System.Text.Json
 open System.Text.Json.Serialization
@@ -111,31 +112,38 @@ let rec tryGetNullValue (fsOptions: JsonFSharpOptionsRecord) (ty: Type) : obj vo
 let isNullableFieldType (fsOptions: JsonFSharpOptionsRecord) (ty: Type) =
     tryGetNullValue fsOptions ty |> ValueOption.isSome
 
+let private tryGetTypeOrGeneric (ty: Type) (types: IDictionary<Type, _>) =
+    let mutable res = Unchecked.defaultof<_>
+    if isNull types then
+        ValueNone
+    elif types.TryGetValue(ty, &res) then
+        ValueSome res
+    elif ty.IsGenericType && types.TryGetValue(ty.GetGenericTypeDefinition(), &res) then
+        ValueSome res
+    else
+        ValueNone
+
 let overrideOptions (ty: Type) (defaultOptions: JsonFSharpOptions) =
     let inheritUnionEncoding (options: JsonFSharpOptions) =
         if options.UnionEncoding.HasFlag(JsonUnionEncoding.Inherit) then
             options.WithUnionEncoding(defaultOptions.UnionEncoding)
         else
             options
 
-    let applyAttributeOverride () =
-        if defaultOptions.AllowOverride then
-            match
-                ty.GetCustomAttributes(typeof<IJsonFSharpConverterAttribute>, true)
-                |> Array.tryHead
-                with
-            | Some (:? IJsonFSharpConverterAttribute as attr) -> attr.Options |> inheritUnionEncoding
-            | _ -> defaultOptions
+    let overrides = defaultOptions.Overrides defaultOptions
+
+    match tryGetTypeOrGeneric ty overrides with
+    | ValueSome options ->
+        inheritUnionEncoding options
+    | ValueNone when defaultOptions.AllowOverride ->
+        let attrs = ty.GetCustomAttributes(typeof<IJsonFSharpConverterAttribute>, true)
+        if attrs.Length > 0 then
+            let attr = attrs[0] :?> IJsonFSharpConverterAttribute
+            inheritUnionEncoding attr.Options
         else
             defaultOptions
-
-    let overrides = defaultOptions.Overrides defaultOptions
-    if isNull overrides then
-        applyAttributeOverride ()
-    else
-        match overrides.TryGetValue(ty) with
-        | true, options -> options |> inheritUnionEncoding
-        | false, _ -> applyAttributeOverride ()
+    | ValueNone ->
+        defaultOptions
 
 let isWrappedString (ty: Type) =
     TypeCache.isUnion ty

tests/FSharp.SystemTextJson.Tests/Test.Union.fs

@@ -1248,6 +1248,39 @@ module NonStruct =
                 .ToJsonSerializerOptions()
         Assert.Equal("""{"tag2":"A","x":123,"y":"abc"}""", JsonSerializer.Serialize(Override.A(123, "abc"), o))
 
+    [<Fact>]
+    let ``should apply explicit overrides on generic type`` () =
+        let o =
+            JsonFSharpOptions
+                .Default()
+                .WithUnionInternalTag()
+                .WithUnionNamedFields()
+                .WithOverrides(fun o -> dict [ typedefof<Result<_, _>>, o.WithUnionTagName("Result") ])
+                .ToJsonSerializerOptions()
+        Assert.Equal("""{"Result":"Ok","ResultValue":"abc"}""", JsonSerializer.Serialize(Ok "abc", o))
+
+    [<Fact>]
+    let ``should apply explicit override on specific type over generic type`` () =
+        let o =
+            JsonFSharpOptions
+                .Default()
+                .WithUnionInternalTag()
+                .WithUnionNamedFields()
+                .WithOverrides(fun o ->
+                    dict
+                        [ typeof<Result<string, string>>, o.WithUnionTagName("SpecificResult")
+                          typedefof<Result<_, _>>, o.WithUnionTagName("GenericResult") ]
+                )
+                .ToJsonSerializerOptions()
+        Assert.Equal(
+            """{"GenericResult":"Ok","ResultValue":42}""",
+            JsonSerializer.Serialize((Ok 42: Result<int, string>), o)
+        )
+        Assert.Equal(
+            """{"SpecificResult":"Ok","ResultValue":"abc"}""",
+            JsonSerializer.Serialize((Ok "abc": Result<string, string>), o)
+        )
+
     type NamedAfterTypesA = NTA of int
 
     type NamedAfterTypesB = NTB of int * string