SystemTextJson: Add option to reject null string values

src/FsCodec.SystemTextJson/FsCodec.SystemTextJson.fsproj

@@ -8,6 +8,7 @@
     <Compile Include="Pickler.fs" />
     <Compile Include="UnionConverter.fs" />
     <Compile Include="TypeSafeEnumConverter.fs" />
+    <Compile Include="RejectNullStringConverter.fs" />
     <Compile Include="UnionOrTypeSafeEnumConverterFactory.fs" />
     <Compile Include="Options.fs" />
     <Compile Include="Serdes.fs" />

src/FsCodec.SystemTextJson/Options.fs

@@ -57,14 +57,19 @@ type Options private () =
             /// <summary>Apply <c>TypeSafeEnumConverter</c> if possible. Defaults to <c>false</c>.</summary>
             [<Optional; DefaultParameterValue(null)>] ?autoTypeSafeEnumToJsonString : bool,
             /// <summary>Apply <c>UnionConverter</c> for all Discriminated Unions, if <c>TypeSafeEnumConverter</c> not possible. Defaults to <c>false</c>.</summary>
-            [<Optional; DefaultParameterValue(null)>] ?autoUnionToJsonObject : bool) =
+            [<Optional; DefaultParameterValue(null)>] ?autoUnionToJsonObject : bool,
+            /// <summary>When set to <c>false</c> the codec will throw on <c>null</c> strings. Use <c>string option</c> to allow nulls.
+            [<Optional; DefaultParameterValue(null)>] ?allowNullStrings : bool) =
+
+        let defaultConverters: JsonConverter array =
+            if allowNullStrings = Some false then [| RejectNullStringConverter() |] else Array.empty
+        let converters = if converters = null then defaultConverters else Array.append converters defaultConverters
 
         Options.CreateDefault(
             converters =
                 (   match autoTypeSafeEnumToJsonString = Some true, autoUnionToJsonObject = Some true with
                     | tse, u when tse || u ->
-                        let converter : JsonConverter array = [| UnionOrTypeSafeEnumConverterFactory(typeSafeEnum = tse, union = u) |]
-                        if converters = null then converter else Array.append converters converter
+                        Array.append converters [| UnionOrTypeSafeEnumConverterFactory(typeSafeEnum = tse, union = u) |]
                     | _ -> converters),
             ?ignoreNulls = ignoreNulls,
             ?indent = indent,

src/FsCodec.SystemTextJson/RejectNullStringConverter.fs

@@ -0,0 +1,18 @@
+namespace FsCodec.SystemTextJson
+
+open System.Text.Json.Serialization
+
+type RejectNullStringConverter() =
+    inherit JsonConverter<string>()
+
+    override _.HandleNull = true
+    override _.CanConvert(t) = t = typeof<string>
+
+    override this.Read(reader, _typeToConvert, _options) =
+        let value = reader.GetString()
+        if value = null then nullArg "Expected string, got null. When allowNullStrings is false you must explicitly type optional strings as 'string option'"
+        value
+
+    override this.Write(writer, value, _options) =
+        if value = null then nullArg "Expected string, got null. When allowNullStrings is false you must explicitly type optional strings as 'string option'"
+        writer.WriteStringValue(value)

tests/FsCodec.NewtonsoftJson.Tests/FsCodec.NewtonsoftJson.Tests.fsproj

@@ -16,7 +16,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="FsCheck.Xunit" Version="2.14.3" />
+    <PackageReference Include="FsCheck.Xunit" Version="3.0.0-beta2" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="Serilog.Sinks.Console" Version="4.0.1" />

tests/FsCodec.NewtonsoftJson.Tests/StreamTests.fs

@@ -15,8 +15,8 @@ let serdes = Serdes Options.Default
 type Rec = { a : int; b : string; c : string }
 
 let [<Fact>] ``Can serialize/deserialize to stream`` () =
-    let value = { a = 10; b = "10"; c = null }
-    use stream = new MemoryStream()    
+    let value = { a = 10; b = "10"; c = "" }
+    use stream = new MemoryStream()
     serdes.SerializeToStream(value, stream)
     stream.Seek(0L, SeekOrigin.Begin) |> ignore
     let value' = serdes.DeserializeFromStream(stream)

tests/FsCodec.NewtonsoftJson.Tests/UnionConverterTests.fs

@@ -11,7 +11,7 @@ open FsCodec.NewtonsoftJson
 open Newtonsoft.Json
 #endif
 
-open FsCheck
+open FsCheck.FSharp
 open Swensen.Unquote.Assertions
 open System
 open System.IO
@@ -307,8 +307,8 @@ let render ignoreNulls = function
     | CaseZ (a, Some b) -> sprintf """{"case":"CaseZ","a":"%s","b":"%s"}""" (string a) (string b)
 
 type FsCheckGenerators =
-    static member CartId = Arb.generate |> Gen.map CartId |> Arb.fromGen
-    static member SkuId = Arb.generate |> Gen.map SkuId |> Arb.fromGen
+    static member CartId = ArbMap.defaults |> ArbMap.generate |> Gen.map CartId |> Arb.fromGen
+    static member SkuId = ArbMap.defaults |> ArbMap.generate |> Gen.map SkuId |> Arb.fromGen
 
 type DomainPropertyAttribute() =
     inherit FsCheck.Xunit.PropertyAttribute(QuietOnSuccess = true, Arbitrary=[| typeof<FsCheckGenerators> |])

tests/FsCodec.SystemTextJson.Tests/AutoUnionTests.fs

@@ -48,11 +48,7 @@ let [<FsCheck.Xunit.Property>] ``auto-encodes Unions and non-unions`` (x : Any)
 
     test <@ decoded = x @>
 
-(* 🙈 *)
-
-let (|ReplaceSomeNullWithNone|) value = TypeShape.Generic.map (function Some (null : string) -> None | x -> x) value
-
-let [<FsCheck.Xunit.Property>] ``Some null roundtripping hack for tests`` (ReplaceSomeNullWithNone (x : Any)) =
+let [<FsCheck.Xunit.Property>] ``It round trips`` (x: Any) =
     let encoded = serdes.Serialize x
     let decoded : Any = serdes.Deserialize encoded
     test <@ decoded = x @>

tests/FsCodec.SystemTextJson.Tests/FsCodec.SystemTextJson.Tests.fsproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="FsCheck.Xunit" Version="2.14.3" />
+    <PackageReference Include="FsCheck.Xunit" Version="3.0.0-beta2" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
     <PackageReference Include="Serilog.Sinks.Console" Version="4.0.1" />
     <PackageReference Include="Unquote" Version="6.1.0" />

tests/FsCodec.SystemTextJson.Tests/SerdesTests.fs

@@ -8,6 +8,7 @@ open Xunit
 type Record = { a : int }
 
 type RecordWithOption = { a : int; b : string option }
+type RecordWithString = { c : int; d : string }
 
 /// Characterization tests for OOTB JSON.NET
 /// The aim here is to characterize the gaps that we'll shim; we only want to do that as long as it's actually warranted
@@ -121,3 +122,27 @@ let [<Fact>] ``Switches off the HTML over-escaping mechanism`` () =
     test <@ ser = """{"a":1,"b":"\"+"}""" @>
     let des = serdes.Deserialize ser
     test <@ value = des @>
+
+let [<Fact>] ``Rejects null strings`` () =
+    let serdes = Serdes(Options.Create(allowNullStrings = false))
+    let value: string = null
+    raises <@ serdes.Serialize value @>
+
+    let value = [| "A"; null |]
+    raises <@ serdes.Serialize value @>
+
+    let value = [| Some "A"; None |]
+    let res = serdes.Serialize value
+    test <@ res = """["A",null]""" @>
+    let des = serdes.Deserialize res
+    test <@ des = value @>
+
+    let value = { c = 1; d = null }
+    raises <@ serdes.Serialize value @>
+
+    let value = { c = 1; d = "some string" }
+    let res = serdes.Serialize value
+    test <@ res = """{"c":1,"d":"some string"}""" @>
+    let des = serdes.Deserialize res
+    test <@ des = value @>
+