Fix empty Dictionary when passing JS object to CLR method accepting Dictionary<string, T>

[Copilot]

When a JavaScript object is passed to a CLR method expecting a concrete Dictionary<string, T> parameter, the resulting dictionary is always empty.

Root Cause

DefaultTypeConverter.TryConvert handles source types that are string-keyed generic dictionaries (e.g. ExpandoObject, which is what plain JS objects become via ToObject()) by iterating the target type's members and looking them up in the source. For Dictionary<string, object>, those members are Count, Keys, Values, etc. — none of which exist as keys in the source, so nothing is populated.

This only manifested with concrete dictionary classes. Interface targets like IDictionary<string, object> worked fine because ExpandoObject directly implements that interface, so the earlier type.IsInstanceOfType(value) check short-circuits correctly.

Fix

When both source and target are string-keyed generic dictionaries, iterate the source's keys and populate the target via IDictionary, converting each value to the target's declared value type.

// Before: empty — iterates Dictionary<string,object> members (Count, Keys…) not found in source
public void Method(Dictionary<string, object> values) { /* values is {} */ }

// After: correctly populated
engine.Evaluate("method({ value: 'world' })"); // values = { "value": "world" }

Changes

• DefaultTypeConverter.TryConvert: When source TypeDescriptor.IsStringKeyedGenericDictionary is true and the target type is also a string-keyed generic dictionary (detected via TypeDescriptor), populate the target dictionary from source keys rather than mapping to target class members. Falls back to the original member-mapping path for non-dictionary targets.
• InteropTests: Added Test3/Test4 methods and two new tests covering Dictionary<string, object> as a direct parameter and as an optional parameter with a null default.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Dictionary<string, object> in c# method always is empty</issue_title>
<issue_description>Hi, Im encountering an issue with Jint. I exposed this class to Jint:

using DragonEngine.Json;

namespace DragonEngine.Assets.Translation.Simple;

[JsonSerializableClass(AddToInstancePath = nameof(Content))]
public class Translatable : IJsonSerializable {
	[JsonSerializableProperty] public string Content { get; set; } = "";

	public Translatable(string content) {
		Content = content;
	}

	public string Translate(Dictionary<string, object>? variables = null) {
		return Tr.T(Content, variables);
	}
	public string T(Dictionary<string, object>? variables = null) => Translate(variables);

	public string TranslateOptional(Dictionary<string, object>? variables = null) {
		return Tr.TO(Content, variables);
	}
	public string TO(Dictionary<string, object>? variables = null) => TranslateOptional(variables);
}

When trying to call T() from Jint, the dictionary is always empty, even if it is correct in js. This seems to be consistent, since I already got the same problem on multiple other occations. When I accept an object, as an arg, and then I run my function that converts everything Jint into clr objects, it works fine.

Js code:

return Translatable.new('test.test2'):T({value = 'world'}

My conversion function:

	public override object? ConvertToClr(object? value) {
		// Handle null
		if (value == null) {
			return null!;
		}

		// Arrays are already correct, but recurse into them
		if (value is object[] array) {
			var result = new object[array.Length];
			for (int i = 0; i < array.Length; i++) {
				result[i] = ConvertToClr(array[i])!;
			}
			return result;
		}

		// JS objects become ExpandoObject
		if (value is ExpandoObject expando) {
			var dict = (IDictionary<string, object>)expando!;

			var result = new Dictionary<string, object>();
			foreach (var pair in dict) {
				result[pair.Key] = ConvertToClr(pair.Value)!;
			}
			return result;
		}

		// JsValue not yet converted
		if (value is JsValue jsValue) {
			return ConvertToClr(jsValue.ToObject()!);
		}

		// Primitive
		return value;
	}

Linux manjaro, net 8.0, c# 12.0, Jint 4.6.0

Btw, it would be nice if there was a built in ConvertToClr function</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Dictionary<string, object> in c# method always is empty #2311

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