JamesNK/Newtonsoft.Json · error · JsonSerializationException

Could not load assembly '{0}'.

Error message

Could not load assembly '{0}'.

What it means

During deserialization with TypeNameHandling, DefaultSerializationBinder.BindToType tries to load the assembly named in the JSON $type. If the assembly cannot be loaded (not found via Assembly.Load, partial-name load, or among currently loaded assemblies), it throws a JsonSerializationException (line 99). This is the binder's way of rejecting a $type that references an unavailable assembly.

Source

Thrown at Src/Newtonsoft.Json/Serialization/DefaultSerializationBinder.cs:99

                if (assembly == null)
                {
                    // will find assemblies loaded with Assembly.LoadFile outside of the main directory
                    Assembly[] loadedAssemblies = AppDomain.CurrentDomain.GetAssemblies();
                    foreach (Assembly a in loadedAssemblies)
                    {
                        // check for both full name or partial name match
                        if (a.FullName == assemblyName || a.GetName().Name == assemblyName)
                        {
                            assembly = a;
                            break;
                        }
                    }
                }
#endif

                if (assembly == null)
                {
                    throw new JsonSerializationException("Could not load assembly '{0}'.".FormatWith(CultureInfo.InvariantCulture, assemblyName));
                }

                Type? type = assembly.GetType(typeName);
                if (type == null)
                {
                    // if generic type, try manually parsing the type arguments for the case of dynamically loaded assemblies
                    // example generic typeName format: System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]
                    if (StringUtils.IndexOf(typeName, '`') >= 0)
                    {
                        try
                        {
                            type = GetGenericTypeFromTypeName(typeName, assembly);
                        }
                        catch (Exception ex)
                        {
                            throw new JsonSerializationException("Could not find type '{0}' in assembly '{1}'.".FormatWith(CultureInfo.InvariantCulture, typeName, assembly.FullName), ex);
                        }
                    }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Ensure the assembly named in $type is referenced and available at runtime (deploy the DLL, add the reference).
2. Use a custom SerializationBinder to remap assembly/type names to the ones available in your app.
3. Avoid TypeNameHandling for cross-boundary payloads, or strip $type and deserialize to a known concrete type.
4. Tighten TypeNameHandling scope (e.g. TypeNameHandling.None or Auto) so arbitrary external $type values are not honored.

Example fix

// before
var settings = new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.All };
var obj = JsonConvert.DeserializeObject(json, settings); // $type names a missing assembly -> throws

// after
var settings = new JsonSerializerSettings {
    TypeNameHandling = TypeNameHandling.Auto,
    SerializationBinder = new SafeBinder()
};
// SafeBinder.BindToType remaps known assembly names to currently loaded assemblies

public sealed class SafeBinder : DefaultSerializationBinder {
    public override Type BindToType(string assemblyName, string typeName) {
        foreach (var a in AppDomain.CurrentDomain.GetAssemblies())
            if (a.GetName().Name == assemblyName || a.FullName == assemblyName)
            {
                var t = a.GetType(typeName);
                if (t != null) return t;
            }
        throw new JsonSerializationException($"Refusing to deserialize unknown assembly '{assemblyName}'.");
    }
}
var settings = new JsonSerializerSettings {
    TypeNameHandling = TypeNameHandling.Auto,
    SerializationBinder = new SafeBinder()
};

static bool AssemblyIsLoaded(string assemblyName) =>
    AppDomain.CurrentDomain.GetAssemblies()
        .Any(a => a.GetName().Name == assemblyName || a.FullName == assemblyName);

try { return JsonConvert.DeserializeObject(json, type, settings); }
catch (JsonSerializationException ex) when (ex.Message.StartsWith("Could not load assembly"))
{
    // log, reject the payload, or prompt for the missing assembly reference
}

When it happens

Trigger: Deserializing JSON containing "$type": "Namespace.Type, Missing.Assembly" when that assembly is not referenced/loaded by the application.

Common situations: Type-name handling across service boundaries (the producer's assembly is not referenced by the consumer), deploying without a required assembly, or version/strong-name mismatches in the $type assembly name.

Related errors

• Could not find type '{0}' in assembly '{1}'.
• Unexpected BsonType value: {0}
• Unable to read beyond the end of the stream.
• Expected Bytes but got {0}.
• No object created.

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/f50a279189e1d2f8. Report an issue: GitHub.