.net 为什么 PropertyInfo.SetValue 在此示例中不起作用以及如何使其起作用

发布于08月09日

以下代码似乎不起作用，尽管根据文档，带有装箱的部分应该可以起作用.我似乎找不到比Value Type Property setting的文章更多的关于这方面的信息

这道题也不同于Setting a property of a property in a struct?题，因为这道题与我正在try 使用的反思无关.

这些代码只是一个大得多的项目的一小部分(使用反射来完成一些事情)，但演示了这个问题.

本例的特殊之处在于，我们要设置的属性是值类型( struct )，而SetValue似乎没有很好地处理这一问题……

Imports System.Drawing

Module Module1
    Sub Main()
        Console.WriteLine("Starting test")
        Dim s As Size
        Console.WriteLine("Created size: {0}", s)
        s.Width = 5
        Console.WriteLine("Set Width: {0}", s)
        Dim pi = s.GetType().GetProperty("Height")
        Console.WriteLine("get property info: {0}", pi)
        pi.SetValue(s, 10, Nothing)
        Console.WriteLine("setting height with pi: {0}", s)
        Dim box As Object = s 'same result with  RuntimeHelpers.GetObjectValue(s)
        Console.WriteLine("boxing: {0}", box)
        pi.SetValue(box, 10, Nothing)
        Console.WriteLine("setting value: {0}", box)
        Console.WriteLine("")
        Console.WriteLine("End test")
        Console.ReadLine()
    End Sub
End Module

给出的结果是:

Starting test
Created size: {Width=0, Height=0}
Set Width: {Width=5, Height=0}
get property info: Int32 Height
setting height with pi: {Width=5, Height=0} 'expected 5,10
boxing: {Width=5, Height=0}
setting value: {Width=5, Height=0} 'expected 5,10

End test

同样的代码在C#中也可以(使用装箱)工作.

    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Starting test");
            Size s= new Size();
            Console.WriteLine("Created size: {0}", s);
            s.Width = 5;
            Console.WriteLine("Set Width: {0}", s);
            PropertyInfo pi = s.GetType().GetProperty("Height");
            Console.WriteLine("get property info: {0}", pi);
            Console.WriteLine("can write?: {0}", pi.CanWrite);
            pi.SetValue(s, 10, null);
            Console.WriteLine("setting height with pi: {0}", s);
            Object box = s; //same result with  RuntimeHelpers.GetObjectValue(s)
            Console.WriteLine("boxing: {0}", box);
            pi.SetValue(box, 10, null);
            Console.WriteLine("setting value: {0}", box);
            Console.WriteLine("");
            Console.WriteLine("End test");
            Console.ReadLine();
        }
    }

Starting test
Created size: {Width=0, Height=0}
Set Width: {Width=5, Height=0}
get property info: Int32 Height
can write?: True
setting height with pi: {Width=5, Height=0}
boxing: {Width=5, Height=0}
setting value: {Width=5, Height=10}

End test

如何在VB中实现这一点？

Dim heightProperty = GetType( Size ).GetProperty( "Height", BindingFlags.Public Or BindingFlags.Instance ) Dim localStruct As Size localStruct.Width = 5 localStruct.Dump( "Should have Width 5" ) ' This `SetValue` call below won't work because `vbc` will insert a hidden `PropertyInfo.SetValue(Object, Object)` call which will box `localStruct` such that you can't retrieve it after it's mutated, so effectively discarding it. heightProperty.SetValue( localStruct, 10 ) localStruct.Dump( ( "Should have Height 10, but has " + localStruct.ToString() ) ) Dim boxedAsObject As System.Object = localStruct LINQPad.Extensions.Dump( boxedAsObject, "boxed" ) ' interestingly, you can't use extension-method syntax on Object-typed boxed structs, weird. heightProperty.SetValue( boxedAsObject, 10) ' This won't work because `vbc` will insert a hidden `PropertyInfo.SetValue(Object, Object)` call which will reset `boxedAsObject`. LINQPad.Extensions.Dump( boxedAsObject, ( "Should have Height 10, but has " + boxedAsObject.ToString() ) ) ' https://www.pcreview.co.uk/threads/cannot-get-propertyinfo-setvalue-to-work-for-a-structure.1418755/ Dim boxedAsValueType As System.ValueType = localStruct heightProperty.SetValue( boxedAsValueType, 10 ) boxedAsValueType.Dump( "Will have Height = 10: " + boxedAsValueType.ToString() )

IL_0000 ldtoken Size IL_0005 call Type.GetTypeFromHandle (RuntimeTypeHandle) IL_000A ldstr "Height" IL_000F ldc.i4.s 14 // 20 IL_0011 call Type.GetProperty (String, BindingFlags) IL_0016 ldloca.s 00 // localStruct IL_0018 ldc.i4.5 IL_0019 call Size.set_Width (Int32) IL_001E ldloc.0 // localStruct IL_001F ldstr "Should have Width 5" IL_0024 call Extensions.Dump<Size> (Size, String) IL_0029 pop IL_002A dup IL_002B ldloc.0 // localStruct IL_002C box Size IL_0031 ldc.i4.s 0A // 10 IL_0033 box Int32 IL_0038 callvirt PropertyInfo.SetValue (Object, Object) IL_003D ldloc.0 // localStruct IL_003E ldstr "Should have Height 10, but has " IL_0043 ldloca.s 00 // localStruct IL_0045 constrained. Size IL_004B callvirt Object.ToString () IL_0050 call String.Concat (String, String) IL_0055 call Extensions.Dump<Size> (Size, String) IL_005A pop IL_005B ldloc.0 // localStruct IL_005C box Size IL_0061 stloc.1 // boxedAsObject IL_0062 ldloc.1 // boxedAsObject IL_0063 call RuntimeHelpers.GetObjectValue (Object) IL_0068 ldstr "boxed" IL_006D call Extensions.Dump<Object> (Object, String) IL_0072 pop IL_0073 dup IL_0074 ldloc.1 // boxedAsObject IL_0075 call RuntimeHelpers.GetObjectValue (Object) IL_007A ldc.i4.s 0A // 10 IL_007C box Int32 IL_0081 callvirt PropertyInfo.SetValue (Object, Object) IL_0086 ldloc.1 // boxedAsObject IL_0087 call RuntimeHelpers.GetObjectValue (Object) IL_008C ldstr "Should have Height 10, but has " IL_0091 ldloc.1 // boxedAsObject IL_0092 callvirt Object.ToString () IL_0097 call String.Concat (String, String) IL_009C call Extensions.Dump<Object> (Object, String) IL_00A1 pop IL_00A2 ldloc.0 // localStruct IL_00A3 box Size IL_00A8 stloc.2 // boxedAsValueType IL_00A9 ldloc.2 // boxedAsValueType IL_00AA ldc.i4.s 0A // 10 IL_00AC box Int32 IL_00B1 callvirt PropertyInfo.SetValue (Object, Object) IL_00B6 ldloc.2 // boxedAsValueType IL_00B7 ldstr "Will have Height = 10: " IL_00BC ldloc.2 // boxedAsValueType IL_00BD callvirt ValueType.ToString () IL_00C2 call String.Concat (String, String) IL_00C7 call Extensions.Dump<ValueType> (ValueType, String) IL_00CC pop IL_00CD ret

IL_0000 ldtoken Size IL_0005 call Type.GetTypeFromHandle (RuntimeTypeHandle) IL_000A ldstr "Height" IL_000F ldc.i4.s 14 // 20 IL_0011 call Type.GetProperty (String, BindingFlags) IL_0016 ldloca.s 00 // localStruct IL_0018 initobj Size IL_001E ldloca.s 00 // localStruct IL_0020 ldc.i4.5 IL_0021 call Size.set_Width (Int32) IL_0026 ldloc.0 // localStruct IL_0027 ldstr "Should have Width 5" IL_002C call Extensions.Dump<Size> (Size, String) IL_0031 pop IL_0032 dup IL_0033 ldloc.0 // localStruct IL_0034 box Size IL_0039 ldc.i4.s 0A // 10 IL_003B box Int32 IL_0040 callvirt PropertyInfo.SetValue (Object, Object) IL_0045 ldloc.0 // localStruct IL_0046 ldstr "Should have Height 10" IL_004B call Extensions.Dump<Size> (Size, String) IL_0050 pop IL_0051 ldloc.0 // localStruct IL_0052 box Size IL_0057 stloc.1 // boxedAsObject IL_0058 dup IL_0059 ldloc.1 // boxedAsObject IL_005A ldc.i4.s 0A // 10 IL_005C box Int32 IL_0061 callvirt PropertyInfo.SetValue (Object, Object) IL_0066 ldloc.1 // boxedAsObject IL_0067 ldstr "boxedAsObject" IL_006C call Extensions.Dump<Object> (Object, String) IL_0071 pop IL_0072 ldloc.0 // localStruct IL_0073 box Size IL_0078 stloc.2 // boxedAsValueType IL_0079 ldloc.2 // boxedAsValueType IL_007A ldc.i4.s 0A // 10 IL_007C box Int32 IL_0081 callvirt PropertyInfo.SetValue (Object, Object) IL_0086 ldloc.2 // boxedAsValueType IL_0087 ldstr "boxedAsValueType" IL_008C call Extensions.Dump<ValueType> (ValueType, String) IL_0091 pop IL_0092 ret

.net 为什么 PropertyInfo.SetValue 在此示例中不起作用以及如何使其起作用

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