C#中嵌套三元转if-else出现不可达代码问题求助

你遇到的问题核心是：对象初始化器的语法只支持属性赋值表达式，不能直接嵌入if-else执行语句，强行写入会导致语法错误或不可达代码提示。下面提供两种标准写法：

方法一：先计算属性值，再传入初始化器 #

先通过if-else算出NoOfWell的值，再把结果放到对象初始化器中赋值，逻辑清晰且符合语法要求：

public BrainSubseaJumperInputDTO FillInputDTO(ProjectInfoDTO projectInfoInput = null, BrainConceptsInputDTO conceptInput = null)
{
    var sumOfAllWells = conceptInput?.ConceptDCDetailsInputDTO
        .Sum(x => x.OilProducerWell + x.GasProducerWell + x.WaterInjectorWell + x.GasInjectorWell) ?? 0;
    
    // 提前用if-else计算NoOfWell的值
    int noOfWell;
    if (_brainConceptDCDetailsInput.HydrocacbornType == "Gas")
    {
        noOfWell = _brainConceptDCDetailsInput.GasFlowlineSize;
    }
    else if (_brainConceptDCDetailsInput.HydrocacbornType == "Oil")
    {
        noOfWell = _brainConceptDCDetailsInput.OilFlowlineSize;
    }
    else
    {
        noOfWell = 0;
    }
    
    return new BrainSubseaJumperInputDTO()
    {
        FlowLineSize = sumOfAllWells,
        NoOfWell = noOfWell
    };
}

方法二：先创建对象实例，再用if-else赋值 #

先初始化对象的已知属性，再通过if-else给NoOfWell单独赋值，这种写法更灵活，适合属性较多或逻辑复杂的场景：

public BrainSubseaJumperInputDTO FillInputDTO(ProjectInfoDTO projectInfoInput = null, BrainConceptsInputDTO conceptInput = null)
{
    var sumOfAllWells = conceptInput?.ConceptDCDetailsInputDTO
        .Sum(x => x.OilProducerWell + x.GasProducerWell + x.WaterInjectorWell + x.GasInjectorWell) ?? 0;
    
    // 先创建对象并初始化FlowLineSize
    var inputDto = new BrainSubseaJumperInputDTO
    {
        FlowLineSize = sumOfAllWells
    };
    
    // 用if-else给NoOfWell赋值
    if (_brainConceptDCDetailsInput.HydrocacbornType == "Gas")
    {
        inputDto.NoOfWell = _brainConceptDCDetailsInput.GasFlowlineSize;
    }
    else if (_brainConceptDCDetailsInput.HydrocacbornType == "Oil")
    {
        inputDto.NoOfWell = _brainConceptDCDetailsInput.OilFlowlineSize;
    }
    else
    {
        inputDto.NoOfWell = 0;
    }
    
    return inputDto;
}

额外推荐：用switch表达式简化判断（C# 8.0+） #

如果你的项目使用C# 8.0及以上版本，可以用switch表达式替代嵌套三元和if-else，语法更简洁易读：

public BrainSubseaJumperInputDTO FillInputDTO(ProjectInfoDTO projectInfoInput = null, BrainConceptsInputDTO conceptInput = null)
{
    var sumOfAllWells = conceptInput?.ConceptDCDetailsInputDTO
        .Sum(x => x.OilProducerWell + x.GasProducerWell + x.WaterInjectorWell + x.GasInjectorWell) ?? 0;
    
    var noOfWell = _brainConceptDCDetailsInput.HydrocacbornType switch
    {
        "Gas" => _brainConceptDCDetailsInput.GasFlowlineSize,
        "Oil" => _brainConceptDCDetailsInput.OilFlowlineSize,
        _ => 0
    };
    
    return new BrainSubseaJumperInputDTO
    {
        FlowLineSize = sumOfAllWells,
        NoOfWell = noOfWell
    };
}

内容的提问来源于stack exchange，提问作者andi azamuddin