c# - 为什么使用对象时通用方法中的动态参数会引发空引用异常？

This question already has answers here:

Why is casting a dynamic of type object to object throwing a null reference exception?

(4个答案)


4年前关闭。




我想知道是否有人可以解释为什么在此代码中

public class SomeClass
{
    public T GenericMethod<T>(dynamic value)
    {
        return (T)value;
    }
}

“返回值”；调用时，语句抛出空引用异常:

new SomeClass().GenericMethod<object>(new object()); // throws System.NullReferenceException

可以通过以下方式按预期工作:

new SomeClass().GenericMethod<string>("SomeString"); // returns SomeString
new SomeClass().GenericMethod<object>("SomeString"); // returns SomeString

注意:
以下编译并运行正常

public class SomeOtherClass
{
    public T GenericMethod<T>(object value)
    {
        return (T)value;
    }
}

堆栈跟踪:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.CSharp.RuntimeBinder.ExpressionTreeCallRewriter.GenerateLambda(EXPRCALL pExpr)
   at Microsoft.CSharp.RuntimeBinder.Semantics.ExprVisitorBase.Visit(EXPR pExpr)
   at Microsoft.CSharp.RuntimeBinder.ExpressionTreeCallRewriter.Rewrite(TypeManager typeManager, EXPR pExpr, IEnumerable`1 listOfParameters)
   at Microsoft.CSharp.RuntimeBinder.RuntimeBinder.BindCore(DynamicMetaObjectBinder payload, IEnumerable`1 parameters, DynamicMetaObject[] args, DynamicMetaObject& deferredBinding)
   at Microsoft.CSharp.RuntimeBinder.RuntimeBinder.Bind(DynamicMetaObjectBinder payload, IEnumerable`1 parameters, DynamicMetaObject[] args, DynamicMetaObject& deferredBinding)
   at Microsoft.CSharp.RuntimeBinder.BinderHelper.Bind(DynamicMetaObjectBinder action, RuntimeBinder binder, IEnumerable`1 args, IEnumerable`1 arginfos, DynamicMetaObject onBindingError)
   at Microsoft.CSharp.RuntimeBinder.CSharpConvertBinder.FallbackConvert(DynamicMetaObject target, DynamicMetaObject errorSuggestion)
   at System.Dynamic.DynamicMetaObject.BindConvert(ConvertBinder binder)
   at System.Dynamic.DynamicMetaObjectBinder.Bind(Object[] args, ReadOnlyCollection`1 parameters, LabelTarget returnLabel)
   at System.Runtime.CompilerServices.CallSiteBinder.BindCore[T](CallSite`1 site, Object[] args)
   at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)

最佳答案

看来it's a bug于2012年首次报告，但显然仍未修复(截至2016年5月24日)