Question

我收到以下编译错误

无法将类型'T'转换为 'ProjectReportingHandler'

为行

var projectReport = (ProjectReportingHandler)result.Report;

尝试编译时：

public abstract class ReportingHandler
{
    // Report stuff
}

public abstract class ProjectReportingHandler: ReportingHandler
{
    // Report stuff
    // Project specific report stuff
}

public class ReportInstance<T>
    where T : ReportingHandler
{
    public T Report { get; private set; }
}

public class ReportLibraryEntry<T>
        where T : ReportingHandler
{
    public ReportInstance<T> Create()
    {
        ReportInstance<T> result = new ReportInstance<T>();

        if (result.Report is ProjectReportingHandler)
        {
            var projectReport = (ProjectReportingHandler)result.Report;
            // do stuff with project reports
        }

        return result;
    }
}

如何将链接的泛型类型属性result.Report强制转换为ProjectReportingHandler？

我原以为where T : ReportingHandler会确保这是可能的：（

编辑：我似乎得到了一些回复，说我接受的回答是不正确的。它有效，我按如下方式实现：

public ReportInstance<T> Create()
{
    ReportInstance<T> result = new ReportInstance<T>();

    ReportingHandler report = result.Report;
    if (report is ProjectReportingHandler)
    {
        var projectReport = (ProjectReportingHandler)report;
        // do stuff with project reports
    }

    return result;
}

为什么下来投票给一个有效的答案？：（

Answer 1

where T : ReportingHandler还不够。
如果T是继承ReportingHandler而非ProjectReportingHandler的其他类型会发生什么？

如果您确定T将始终从ProjectReportingHandler继承，则可以将约束更改为where T : ProjectReportingHandler。

如果你真的想按照你现在的方式去做，你应该先投射到ReportingHandler，如下所示：

var projectReport = ((ReportingHandler)result.Report) as ProjectReportingHandler;

如果T未继承ProjectReportingHandler，则projectReport将为null。

Answer 2

T：ReporingHandler不保证转换是可能的。仅仅因为ProjectReportingHandler是一个ReportingHandler并不意味着相反的情况。

Answer 3

如何使用as：

        ReportInstance<T> result = new ReportInstance<T>();
        var projectReport = result.Report as ProjectReportingHandler;
        if (projectReport != null)
        {
            //do stuff
        }

Answer 4

你可以通过首先将结果“作弊”。记录到一个对象，然后再到一个ProjectReportingHandler。

将类型T的泛型属性转换为已知的继承类型

4 个答案: