Nested Ternary Implicit type Conversion issue: 无法确定条件表达式的类型，因为它们之间没有隐式转换

Question

我过去经常在 C# 中使用嵌套三元组。在我引入了一个事件处理程序之后，作为赋值操作，出现了错误："Type of conditional expression cannot be determined because there is no implicit conversion between 'void' and 'void'"

这是我正在编写的代码的两个片段，上面一个是函数式的，下面一个给出了类型转换错误：

   AppTableConfigCollection collection = 
            tableType == TableType.Attribute
                ? _attributeTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.ATTRIBUTE) 
                    : tableType == TableType.Core
                        ? _coreTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.CORE)
                        : tableType == TableType.Domain
                            ?_domainTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.DOMAIN)
                            : tableType == TableType.Configuration
                                ?_configTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.CONFIG)
                                    :_offlineTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.OFFLINE);





        PropertyChangedEventHandler property =
            tableType == TableType.Attribute
                ? _attributeTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                : tableType == TableType.Core
                    ? _coreTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                    : tableType == TableType.Domain
                        ? _domainTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                        : tableType == TableType.Configuration
                            ? _configTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                                : _offlineTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);

感谢您提供的任何帮助！

Answer 1

首先，一定要听听评论者的意见。 Switch/case 看起来确实会产生更易读、更可维护的代码。

其次，您在第二个代码片段中尝试做的是将您的属性更改处理程序设置为订阅处理程序的结果，这是无效的。

具体来说，_attributeTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged) 不是 return 值——它 return 是 void。所以，你的任务是不可能的。

您必须分步完成您想做的事情。

例如：

PropertyChangedEventHandler handler;
switch (tableType)
{
    case TableType.Attribute:
        handler = new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
        _attributeTableConfigs.PropertyChanged += handler;
        break;
    case TableType.Core:
        handler = new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
        _coreTableConfigs.PropertyChanged += handler;
        break;
    case TableType.Domain:
        handler = new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
        _domainTableConfigs.PropertyChanged += handler;
        break;
    case TableType.Configuration:
        handler = new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
        _configTableConfigs.PropertyChanged += handler;
        break;
    default:
        handler = new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
        _offlineTableConfigs.PropertyChanged += handler;
        break;
}

或者，如果处理程序始终相同：

PropertyChangedEventHandler handler = appTableConfigs_PropertyChanged;
switch (tableType)
{
    case TableType.Attribute:
        _attributeTableConfigs.PropertyChanged += handler;
        break;
    case TableType.Core:
        _coreTableConfigs.PropertyChanged += handler;
        break;
    case TableType.Domain:
        _domainTableConfigs.PropertyChanged += handler;
        break;
    case TableType.Configuration:
        _configTableConfigs.PropertyChanged += handler;
        break;
    default:
        _offlineTableConfigs.PropertyChanged += handler;
        break;
}

Answer 2

我建议使用 switch 语句，因为它更易于阅读和更新

switch (tableType)
        {
            case TableType.Attribute:
                _attributeTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.ATTRIBUTE);
                _attributeTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
                break;
            case TableType.Core:
                _coreTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.CORE);
                _coreTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                break;
            case TableType.Domain:
                _domainTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.DOMAIN);
                _domainTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                break;
            case TableType.Configuration:
                _configTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.CONFIG);
                _configTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged)
                break;
            default:
                _offlineTableConfigs = new AppTableConfigCollection(_db, AppTableConfigCollection.OFFLIINE);
                _offlineTableConfigs.PropertyChanged += new PropertyChangedEventHandler(appTableConfigs_PropertyChanged);
                break;
        }

Nested Ternary Implicit type Conversion issue: 无法确定条件表达式的类型，因为它们之间没有隐式转换

Nested Ternary Implicit type Conversion issue: Type of conditional expression cannot be determined because there is no implicit conversion between

c#

ternary-operator