Question

我试图通过解析XML模板来执行在运行时动态创建的方法。我从＆＃34; DebugView＆＃34;得到了以下内容：在编译之前的表达式lambda：

.Lambda #Lambda1<Reporting.Core.Formatter+WriterDriver>(
    Reporting.Core.IReportWriter $writer,
    System.Object $data) {
    .Block(
        Reporting.Core.IReportWriter $writer,
        System.Object $data) {
        .Call System.Diagnostics.Trace.WriteLine("Initialize Driver");
        .Call System.Diagnostics.Trace.WriteLine($writer);
        .Block(Reporting.Core.IBannerWriter $bannerWriter) {
            $bannerWriter = .Call $writer.WriteBanner();
            .Call System.Diagnostics.Trace.WriteLine($bannerWriter);
            .Call $bannerWriter.WithLeftEntry(
                "Key1",
                "Value1");
            .Call $bannerWriter.WithLeftEntry(
                "Key 2",
                "Value 2");
            .Call $bannerWriter.WithRightEntry(
                "Another Key",
                "Another Value");
            .Call $bannerWriter.EndBanner()
        }
    }
}

并且这样称呼它，确保myReportWriter和myData之前不是null：

Action<IReportWriter, Object> Formatter = methodExpression.Compile();
Formatter(myReportWriter, myData)

~~在我的跟踪输出中，我甚至没有得到＆＃34;初始化驱动程序＆＃34;抛出空引用异常之前的输出。~~

仔细观察调试输出显示它实际上是进入＆＃34;初始化驱动程序＆＃34;并在第一个.Block内部的$ writer上为null添加另一个调试检查，显示(writer == null) == true。现在，问题是：为什么$ writer在传入方法调用时为null？

Answer 1

第一个.Block中的$ writer和$ data声明覆盖了.Lambda的相同内容。

更改表达式生成逻辑：

List<Expression> operations = new List<Expression>();
// populate operations...

var methodBody = Expression.Block(new[] {writer, data}, operations);
var lambda = Expression.Lambda<Action<IReportWrite, Object>>(methodBody, writer, data);
while (lambda.CanReduce) lambda.Reduce();

return lambda.Compile()

到：

List<Expression> operations = new List<Expression>();
// populate operations...

var methodBody = Expression.Block(operations);
var lambda = Expression.Lambda<Action<IReportWrite, Object>>(methodBody, writer, data);
while (lambda.CanReduce) lambda.Reduce();

return lambda.Compile()

这产生了一个工作表达式树，如下所示：

.Lambda #Lambda1<Reporting.Core.Formatter+WriterDriver>(
    Reporting.Core.IReportWriter $writer,
    System.Object $data) {
    .Block() {
        .Call System.Diagnostics.Trace.WriteLine("Initialize Driver");
        .Block(Reporting.Core.IBannerWriter $bannerWriter) {
            $bannerWriter = .Call $writer.WriteBanner();
            .Call System.Diagnostics.Trace.WriteLine($bannerWriter);
            .Call $bannerWriter.WithLeftEntry(
                "Key1",
                "Value1");
            .Call $bannerWriter.WithLeftEntry(
                "Key 2",
                "Value 2");
            .Call $bannerWriter.WithRightEntry(
                "Another Key",
                "Another Value");
            .Call $bannerWriter.EndBanner()
        }
    }
}

执行表达式委托会产生空引用异常

1 个答案: