c# - 将类型 T 的泛型属性转换为已知的继承类型

Question

我收到以下编译器错误

Cannot convert type 'T' to 'ProjectReportingHandler'

为线

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;
    }
}

任何想法如何转换链接的泛型类型属性 result.Report到 ProjectReportingHandler ?

我会认为 where T : ReportingHandler会确保这是可能的:(

编辑:我似乎收到一些回复说我接受的答案不正确。它有效，我按如下方式实现它:

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;
}

为什么对一个有效的答案投反对票？ :(

Answer 1

where T : ReportingHandler是不足够的。
如果 T 会发生什么？是继承 ReportingHandler 的其他类型但不是 ProjectReportingHandler ?

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

如果您真的想按照现在的方式进行操作，则应首先转换为 ReportingHandler ， 像这样:

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

如果 T不继承 ProjectReportingHandler , projectReport将是 null .