C# 可以利用反射给只读属性赋值吗?
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2022-05-02 12:45:20
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结论:可以
验证demo如下:
using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Linq; using System.Text; using System.Windows.Forms; namespace IconTest { public partial class Form2 : Form { public Form2() { InitializeComponent(); ReflectTest rt = new ReflectTest(); rt.GetType().GetProperty("ID").SetValue(rt, "Guid", null); MessageBox.Show(rt.ID); } } public class ReflectTest { private string id; [ReadOnly(true)] public string ID { get { return id; } set { id = value; } } } }
运行winform程序输出:
小注:
TypeDescriptor.GetProperties用来setvalue这没有作用:
TypeDescriptor.GetProperties(rt)["ID"].SetValue(rt, "Guid");
那么为什么TypeDescriptor.GetProperties用来setvalue没有效果呢?
将上面的代码拆成如下两句:
PropertyDescriptor prop = TypeDescriptor.GetProperties(rt)["ID"]; prop.SetValue(rt, "Guid");
单点跟踪进去,可以发现:
在获取到PropertyDescriptor这个抽象类的实例后,在调用SetValue方法的时候,是从其子类ReflectPropertyDescriptor调用的。
而具体的实现是在子类:ReflectPropertyDescriptor中,从微软源码中找到ReflectPropertyDescriptor及SetValue
public override void SetValue(object component, object value) { #if DEBUG if (PropDescUsageSwitch.TraceVerbose) { string compName = "(null)"; string valName = "(null)"; if (component != null) compName = component.ToString(); if (value != null) valName = value.ToString(); Debug.WriteLine("[" + Name + "]: SetValue(" + compName + ", " + valName + ")"); } #endif if (component != null) { ISite site = GetSite(component); IComponentChangeService changeService = null; object oldValue = null; object invokee = GetInvocationTarget(componentClass, component); Debug.Assert(!IsReadOnly, "SetValue attempted on read-only property [" + Name + "]"); if (!IsReadOnly) { // Announce that we are about to change this component // if (site != null) { changeService = (IComponentChangeService)site.GetService(typeof(IComponentChangeService)); Debug.Assert(!CompModSwitches.CommonDesignerServices.Enabled || changeService != null, "IComponentChangeService not found"); } // Make sure that it is ok to send the onchange events // if (changeService != null) { oldValue = SecurityUtils.MethodInfoInvoke(GetMethodValue, invokee, (object[])null); try { changeService.OnComponentChanging(component, this); } catch (CheckoutException coEx) { if (coEx == CheckoutException.Canceled) { return; } throw coEx; } } try { try { SecurityUtils.MethodInfoInvoke(SetMethodValue, invokee, new object[] { value }); OnValueChanged(invokee, EventArgs.Empty); } catch (Exception t) { // Give ourselves a chance to unwind properly before rethrowing the exception. // value = oldValue; // If there was a problem setting the controls property then we get: // ArgumentException (from properties set method) // ==> Becomes inner exception of TargetInvocationException // ==> caught here if (t is TargetInvocationException && t.InnerException != null) { // Propagate the original exception up throw t.InnerException; } else { throw t; } } } finally { // Now notify the change service that the change was successful. // if (changeService != null) { changeService.OnComponentChanged(component, this, oldValue, value); } } } } }
从代码中可以看出来,只读属性直接被跳过去了。。。。。。
那么PropertyInfo有没有什么限制呢?
PropertyInfo调用的SetValue如下所示:
在微软开源的代码中可以找到其具体实现如下:
[DebuggerStepThroughAttribute] [Diagnostics.DebuggerHidden] #if !FEATURE_CORECLR [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")] #endif public override void SetValue(Object obj, Object value, Object[] index) { SetValue(obj, value, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static, null, index, null); } [DebuggerStepThroughAttribute] [Diagnostics.DebuggerHidden] public override void SetValue(Object obj, Object value, BindingFlags invokeAttr, Binder binder, Object[] index, CultureInfo culture) { MethodInfo m = GetSetMethod(true); if (m == null) throw new ArgumentException(System.Environment.GetResourceString("Arg_SetMethNotFnd")); Object[] args = null; if (index != null) { args = new Object[index.Length + 1]; for(int i=0;i<index.Length;i++) args[i] = index[i]; args[index.Length] = value; } else { args = new Object[1]; args[0] = value; } m.Invoke(obj, invokeAttr, binder, args, culture); }
暂时没有看到PropertyInfo调用的SetValue有什么限制
PropertyInfo.GetSetMethod 方法 (Boolean)
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