如何用MOQ模拟OpenAIClient以返回Completions响应？

核心问题 #

Completions类及其内部成员的构造函数标记为internal，无法直接通过反序列化或手动实例化创建对象，导致Mock设置失败。

解决方案 #

方案1：通过反射创建Completions实例 #

利用反射绕过internal限制，手动从JSON提取值填充属性：

private async Task<Response<Completions>> CreateMockCompletionsResponse(string completionsJson)
{
    // 解析JSON到动态对象
    dynamic jsonObj = JsonConvert.DeserializeObject(completionsJson);

    // 反射创建Completions实例
    var completionsType = typeof(Completions);
    var completions = (Completions)Activator.CreateInstance(completionsType, true);

    // 设置顶层属性
    completionsType.GetProperty("Id")?.SetValue(completions, jsonObj.id);
    completionsType.GetProperty("Created")?.SetValue(completions, DateTimeOffset.FromUnixTimeSeconds(jsonObj.created));
    
    // 处理Choices集合
    var choicesType = typeof(CompletionChoice);
    var choicesList = new List<CompletionChoice>();
    foreach (var choiceJson in jsonObj.choices)
    {
        var choice = (CompletionChoice)Activator.CreateInstance(choicesType, true);
        choicesType.GetProperty("Text")?.SetValue(choice, choiceJson.text);
        choicesType.GetProperty("Index")?.SetValue(choice, choiceJson.index);
        choicesType.GetProperty("FinishReason")?.SetValue(choice, choiceJson.finish_reason);
        choicesList.Add(choice);
    }
    completionsType.GetProperty("Choices")?.SetValue(completions, choicesList);

    // 构建并返回Response
    var mockResponse = Mock.Of<Response>();
    return Response.FromValue(completions, mockResponse);
}

// 应用到Mock设置
client.Setup(x => x.GetCompletionsAsync(It.IsAny<string>(), It.IsAny<string>(), It.IsAny<CancellationToken>()))
      .ReturnsAsync(CreateMockCompletionsResponse(completionsJson));

方案2：直接Mock Completions及其内部对象 #

无需实例化真实类，用Moq模拟必要属性：

private async Task<Response<Completions>> CreateMockCompletionsResponse()
{
    // Mock CompletionChoice
    var mockChoice = new Mock<CompletionChoice>();
    mockChoice.Setup(c => c.Text).Returns("模拟的补全文本内容");
    mockChoice.Setup(c => c.Index).Returns(0);
    mockChoice.Setup(c => c.FinishReason).Returns("stop");

    // Mock Completions
    var mockCompletions = new Mock<Completions>();
    mockCompletions.Setup(c => c.Id).Returns("mock-completion-id-123");
    mockCompletions.Setup(c => c.Created).Returns(DateTimeOffset.Now);
    mockCompletions.Setup(c => c.Choices).Returns(new List<CompletionChoice> { mockChoice.Object });

    // 构建Response
    var mockResponse = Mock.Of<Response>();
    return Response.FromValue(mockCompletions.Object, mockResponse);
}

// 应用到Mock设置
client.Setup(x => x.GetCompletionsAsync(It.IsAny<string>(), It.IsAny<string>(), It.IsAny<CancellationToken>()))
      .ReturnsAsync(CreateMockCompletionsResponse());

方案3：DTO映射+反射复制（适合复杂JSON） #

先反序列化为自定义DTO，再通过反射将值复制到Completions实例：

1. 定义匹配结构的DTO：

public class CompletionsDto
{
    public string Id { get; set; }
    public long Created { get; set; }
    public List<CompletionChoiceDto> Choices { get; set; }
}

public class CompletionChoiceDto
{
    public string Text { get; set; }
    public int Index { get; set; }
    public string FinishReason { get; set; }
}

2. 映射并创建实例：

private async Task<Response<Completions>> CreateMockCompletionsResponse(string completionsJson)
{
    var dto = JsonConvert.DeserializeObject<CompletionsDto>(completionsJson);
    var completions = (Completions)Activator.CreateInstance(typeof(Completions), true);
    
    // 复制顶层属性
    typeof(CompletionsDto).GetProperties().ToList().ForEach(dtoProp =>
    {
        var targetProp = typeof(Completions).GetProperty(dtoProp.Name);
        if (targetProp == null) return;

        if (dtoProp.Name == "Created")
        {
            targetProp.SetValue(completions, DateTimeOffset.FromUnixTimeSeconds((long)dtoProp.GetValue(dto)));
        }
        else if (dtoProp.Name == "Choices" && dto.Choices != null)
        {
            var choices = dto.Choices.Select(dtoChoice =>
            {
                var choice = (CompletionChoice)Activator.CreateInstance(typeof(CompletionChoice), true);
                typeof(CompletionChoiceDto).GetProperties().ToList().ForEach(cDtoProp =>
                {
                    var cTargetProp = typeof(CompletionChoice).GetProperty(cDtoProp.Name);
                    cTargetProp?.SetValue(choice, cDtoProp.GetValue(dtoChoice));
                });
                return choice;
            }).ToList();
            targetProp.SetValue(completions, choices);
        }
        else
        {
            targetProp.SetValue(completions, dtoProp.GetValue(dto));
        }
    });

    var mockResponse = Mock.Of<Response>();
    return Response.FromValue(completions, mockResponse);
}

注意事项 #

• 反射操作需匹配目标类的属性名称和类型，API版本更新后需同步调整。
• Mock属性时只需设置测试场景用到的字段，无需覆盖全部属性。
• 反射创建的实例可能无法完全模拟真实响应的内部逻辑，需根据测试需求调整。

内容的提问来源于stack exchange，提问作者Punter Vicky