The type or namespace name 'Newtonsoft' could not be found (PS 5.1)

I'm trying to use Newtonsoft.Json library:

[Reflection.Assembly]::LoadFile( 'C:\Users\user\Documents\WindowsPowerShell\Modules\newtonsoft.json.0.2.201\libs\Newtonsoft.Json.dll' )

Add-Type -TypeDefinition @"
using Newtonsoft.Json;

class BigIntegerConverter: JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(Org.BouncyCastle.Math.BigInteger));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        System.Numerics.BigInteger big = (System.Numerics.BigInteger)reader.Value;
        return new Org.BouncyCastle.Math.BigInteger(big.ToString());
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteRawValue(value.ToString());
    }
}
"@

and getting the next error:

> The type or namespace name 'Newtonsoft' could not be found (are you missing a using directive or an assembly reference?)

Why it can't find Newtonsoft tho it loaded? I can call it in PS code via [Newtonsoft.Json.JsonConverter], but not in type definitions!

Thanks!

<!-- language-all: sh -->

Assuming you load the Newtonsoft.Json.dll and BouncyCastle.Cryptography.dll assemblies before invoking your script - e.g., with Add-Type -LiteralPath 'C:\Users\user\Documents\WindowsPowerShell\Modules\newtonsoft.json\1.0.2.201\libs\Newtonsoft.Json.dll' - you can define your class using PowerShell code, i.e. a PowerShell class definition, as follows:

# Note: Assumes that Newtonsoft.Json.dll and BouncyCastle.Cryptography.dll
#       were loaded *before* running this script.
class BigIntegerConverter : Newtonsoft.Json.JsonConverter {
    [bool] CanConvert([Type] $objectType){
        return $objectType -is [Org.BouncyCastle.Math.BigInteger]
    }

    [object] ReadJson([Newtonsoft.Json.JsonReader] $reader, [Type] $objectType, [object] $existingValue, [Newtonsoft.Json.JsonSerializer] $serializer) {
        [bigint] $big = $reader.Value
        return [Org.BouncyCastle.Math.BigInteger]::new($big.ToString())
    }

    WriteJson([Newtonsoft.Json.JsonWriter] $writer, [object] $value, [Newtonsoft.Json.JsonSerializer] $serializer) {
        $writer.WriteRawValue($value.ToString())
    }
}

The - unfortunate - requirement that any types referenced in a PowerShell class have to have been loaded before the script at hand is parsed is discussed in this answer.

As for what you tried:

Mathias is correct in that you normally only need to make sure that the assemblies needed for compiling the C# code passed to Add-Type be passed to -ReferencedAssemblies.

For reasons unknown to me, with Newtonsoft.Json.dll specifically, the code below additionally requires loading the Newtonsoft.Json.dll assembly beforehand too.

• Note: The code below works in Windows PowerShell only - unlike the PowerShell class implementation above, which works in both editions.
  • However, it can be made to work in PowerShell (Core) 7+, with extra effort:

    • Use the .NET Standard 2.0 implementation of NewtonSoft.Json.dll (all implementations are part of the same Newtonsoft.Json NuGet package)

      • In the -ReferencedAssemblies argument:
        • Add netstandard
        • Replace System.Numerics with System.Runtime.Numerics
      • You then do not load Newtonsoft.Json.dll beforehand with Add-Type -LiteralPath.

    • Alternatively, use the .NET (Core) implementation of the assembly:

      • Follow the same steps as above, except for adding netstandard
      • Add -IgnoreWarnings -WarningAction Ignore to the Add-Type call, to ignore and silence warnings regarding a version mismatch between PowerShell's .NET runtime version and the one targeted by Newtonsoft.Json.dll

    • If you accidentally use a .NET Framework implementation, you'll get errors about not finding core types such as Object, presumably because they're looked for in .NET Framework assemblies.

$jsonAssembly = 'C:\Users\user\Documents\WindowsPowerShell\Modules\newtonsoft.json.0.2.201\libs\Newtonsoft.Json.dll'
# Specify the path to BouncyCastle.Cryptography.dll here.
$bouncyCastleAssembly = 'C:\path\to\BouncyCastle.Cryptography.dll'

# !! Inexplicably, this is needed too, possibly also for BouncyCastle.Cryptography.dll
Add-Type -LiteralPath $jsonAssembly

Add-Type -ReferencedAssemblies $jsonAssembly, $bouncyCastleAssembly, System.Numerics -TypeDefinition @'
using System;
using System.Numerics;
using Newtonsoft.Json;

public class BigIntegerConverter: JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(Org.BouncyCastle.Math.BigInteger));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        BigInteger big = (BigInteger)reader.Value;
        return new Org.BouncyCastle.Math.BigInteger(big.ToString());
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteRawValue(value.ToString());
    }
'@

Also note:

• The addition of using System; to enable namespace-free references to class Type

• The addition of System.Numerics to -ReferencedAssemblies as well as using System.Numerics; to allow namespace-free references to BigInteger

• Adding public to the BigInterConverter class definition to ensure that the resulting class is public.