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网站建设优化东莞,厦门网络公司的网络平台,电子商务网站建设中的重要性,做详情页生成代码的网站引言最近因为一个触发器设置的结果总是不起效果的原因，进一步去了解[依赖属性的优先级](Dependency property value precedence - WPF .NET | Microsoft Learn)。在学习这个的过程中发现对SetCurrentValue一直以来的谬误。在WPF中依赖属性Dependency property的…

引言

最近因为一个触发器设置的结果总是不起效果的原因，进一步去了解[依赖属性的优先级](Dependency property value precedence - WPF .NET | Microsoft Learn)。在学习这个的过程中发现对SetCurrentValue一直以来的谬误。

在WPF中依赖属性Dependency property的三个方法SetValue 、SetCurrentValue、ClearValue。

SetCurrentValue 方法用于设置依赖属性的当前值，但不会覆盖该属性的值来源。这意味着，如果属性值是通过绑定、样式或触发器设置的，使用 SetCurrentValue 后，这些设置仍然有效。

然而这很容易让人以为SetValue 方法会使得数据绑定失效。就像先执行了ClearValue 一样。网上我看到的很多文章也这么说，这让我困惑了很久，但我实际操作下来，并非如此，实际上并不是。

为了验证这三个方法，我以设置按钮背景颜色为例，写了一个Demo。

其主要作用如下：

1. myButton绑定默认背景颜色的依赖属性

<Buttonx:Name="MyButton"Width="100"Height="50"Background="{Binding DefaultBackgroundColor,Mode=TwoWay,RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}"Content="MyButton" />

 public static readonly DependencyProperty DefaultBackgroundColorProperty =DependencyProperty.Register("DefaultBackgroundColor",typeof(Brush),typeof(MainWindow),new PropertyMetadata(Brushes.Pink,OnDefaultBackgroundColorChanged));private static void OnDefaultBackgroundColorChanged(DependencyObject d, DependencyPropertyChangedEventArgs e){MessageBox.Show("DefaultBackgroundColor changed");}public static readonly DependencyProperty DefaultForegroundColorProperty =DependencyProperty.Register("DefaultForegroundColor",typeof(Brush),typeof(MainWindow),new PropertyMetadata(Brushes.Gray,OnDefaultForegroundChanged));

2. 按钮1使用SetValue设置myButton的背景颜色属性，并判断绑定表达式是否为空

  private void SetValueChangeBackground_Click(object sender, RoutedEventArgs e){MyButton.SetValue(Button.BackgroundProperty, new SolidColorBrush(Colors.Green));IsBindingExpressionNull();}private void IsBindingExpressionNull()
{ if (MyButton.GetBindingExpression(Button.BackgroundProperty) == null){MessageBox.Show("BindingExpression is null");}
}

3. 按钮2使用SetCurrentValue设置myButton的背景颜色属性

 MyButton.SetCurrentValue(Button.BackgroundProperty, new SolidColorBrush(Colors.Orange));
IsBindingExpressionNull();

4. 按钮3 使用ClearValue 清楚背景颜色属性本地值

 // 清除本地值MyButton.ClearValue(Button.BackgroundProperty);
IsBindingExpressionNull();

5. 按钮4，修改依赖属性

DefaultBackgroundColor = new SolidColorBrush(Colors.LightGreen);

我使用.NET 8 做的Demo的现象如下：

ClearValue执行后，绑定表达式为Null，也就是说ClearValue之后，绑定的数据表达式会被清空
SetValue执行后，按钮颜色正常改变，绑定表达式不为Null。再次执行按钮4 修改依赖属性，按钮背景颜色也可以正常变化，也就是说SetValue之后数据表达式不会被清空，仍然有效。
SetCurrentValue与第2点现象完全一致。

源码

通过查看源码，发现SetCurrentValue 和SetValue都执行方法SetValueCommon，只是入参coerceWithCurrentValue不同，// SetValue时为false 和SetCurrentValue时为true。

并且推测当SetValue的value入参等于DependencyProperty.UnsetValue时，应当会和ClearValueCommon执行相同的方法。

以下是测试代码，实际测试结果也是如此，此时绑定表达式为Null。

private void SetValueChangeBackgroundUnsetValue_Click(object sender, RoutedEventArgs e){MyButton.SetValue(Button.BackgroundProperty, DependencyProperty.UnsetValue);  //查看源码发现，UnsetValue时才会是清除本地值，并且 ClearValueIsBindingExpressionNull();}

SetValueCommon

        /// <summary>///     The common code shared by all variants of SetValue/// </summary>// Takes metadata from caller because most of them have already retrieved it//  for their own purposes, avoiding the duplicate GetMetadata call.private void SetValueCommon(DependencyProperty  dp,object              value,PropertyMetadata    metadata,bool                coerceWithDeferredReference,bool                coerceWithCurrentValue, // SetValue时为false 和SetCurrentValue时为trueOperationType       operationType,bool                isInternal){if (IsSealed){throw new InvalidOperationException(SR.Get(SRID.SetOnReadOnlyObjectNotAllowed, this));}Expression newExpr = null;DependencySource[] newSources = null;EntryIndex entryIndex = LookupEntry(dp.GlobalIndex);// Treat Unset as a Clearif( value == DependencyProperty.UnsetValue ){Debug.Assert(!coerceWithCurrentValue, "Don't call SetCurrentValue with UnsetValue");// Parameters should have already been validated, so we call//  into the private method to avoid validating again.ClearValueCommon(entryIndex, dp, metadata);return;}// Validate the "value" against the DP.bool isDeferredReference = false;bool newValueHasExpressionMarker = (value == ExpressionInAlternativeStore);// First try to validate the value; only after this validation fails should we// do the more expensive checks (type checks) for the less common scenariosif (!newValueHasExpressionMarker){bool isValidValue = isInternal ? dp.IsValidValueInternal(value) : dp.IsValidValue(value);// for properties of type "object", we have to always check for expression & deferredreferenceif (!isValidValue || dp.IsObjectType){// 2nd most common is expressionnewExpr = value as Expression;if (newExpr != null){// For Expressions, perform additional validation// Make sure Expression is "attachable"if (!newExpr.Attachable){throw new ArgumentException(SR.Get(SRID.SharingNonSharableExpression));}// Check dispatchers of all Sources// CALLBACKnewSources = newExpr.GetSources();ValidateSources(this, newSources, newExpr);}else{// and least common is DeferredReferenceisDeferredReference = (value is DeferredReference);if (!isDeferredReference){if (!isValidValue){// it's not a valid value & it's not an expression, so throwthrow new ArgumentException(SR.Get(SRID.InvalidPropertyValue, value, dp.Name));}}}}}// Get old valueEffectiveValueEntry oldEntry;if (operationType == OperationType.ChangeMutableDefaultValue){oldEntry = new EffectiveValueEntry(dp, BaseValueSourceInternal.Default);oldEntry.Value = value;}else{oldEntry = GetValueEntry(entryIndex, dp, metadata, RequestFlags.RawEntry);}// if there's an expression in some other store, fetch it nowExpression currentExpr =(oldEntry.HasExpressionMarker)  ? _getExpressionCore(this, dp, metadata): (oldEntry.IsExpression)         ? (oldEntry.LocalValue as Expression):                                   null;// Allow expression to store value if new value is// not an Expression, if applicablebool handled = false;if ((currentExpr != null) && (newExpr == null)){// Resolve deferred references because we haven't modified// the expression code to work with DeferredReference yet.if (isDeferredReference){value = ((DeferredReference) value).GetValue(BaseValueSourceInternal.Local);}// CALLBACKhandled = currentExpr.SetValue(this, dp, value);entryIndex = CheckEntryIndex(entryIndex, dp.GlobalIndex);}// Create the new effective value entryEffectiveValueEntry newEntry;if (handled)                                                                                                                                                                                        ){// If expression handled set, then doneif (entryIndex.Found){newEntry = _effectiveValues[entryIndex.Index];}else{// the expression.SetValue resulted in this value being removed from the table;// use the default value.newEntry = EffectiveValueEntry.CreateDefaultValueEntry(dp, metadata.GetDefaultValue(this, dp));}coerceWithCurrentValue = false; // expression already handled the control-value}else{// allow a control-value to coerce an expression value, when the// expression didn't handle the valueif (coerceWithCurrentValue && currentExpr != null){currentExpr = null;}newEntry = new EffectiveValueEntry(dp, BaseValueSourceInternal.Local);// detach the old expression, if applicableif (currentExpr != null){// CALLBACKDependencySource[] currentSources = currentExpr.GetSources();UpdateSourceDependentLists(this, dp, currentSources, currentExpr, false);  // Remove// CALLBACKcurrentExpr.OnDetach(this, dp);currentExpr.MarkDetached();entryIndex = CheckEntryIndex(entryIndex, dp.GlobalIndex);}// attach the new expression, if applicableif (newExpr == null){// simple local value setnewEntry.HasExpressionMarker = newValueHasExpressionMarker;newEntry.Value = value;}else{Debug.Assert(!coerceWithCurrentValue, "Expression values not supported in SetCurrentValue");// First put the expression in the effectivevalueentry table for this object;// this allows the expression to update the value accordingly in OnAttachSetEffectiveValue(entryIndex, dp, dp.GlobalIndex, metadata, newExpr, BaseValueSourceInternal.Local);// Before the expression is attached it has default valueobject defaultValue = metadata.GetDefaultValue(this, dp);entryIndex = CheckEntryIndex(entryIndex, dp.GlobalIndex);SetExpressionValue(entryIndex, defaultValue, newExpr);UpdateSourceDependentLists(this, dp, newSources, newExpr, true);  // AddnewExpr.MarkAttached();// CALLBACKnewExpr.OnAttach(this, dp);// the attach may have added entries in the effective value table ...// so, update the entryIndex accordingly.entryIndex = CheckEntryIndex(entryIndex, dp.GlobalIndex);newEntry = EvaluateExpression(entryIndex,dp,newExpr,metadata,oldEntry,_effectiveValues[entryIndex.Index]);entryIndex = CheckEntryIndex(entryIndex, dp.GlobalIndex);}}UpdateEffectiveValue(entryIndex,dp,metadata,oldEntry,ref newEntry,coerceWithDeferredReference,coerceWithCurrentValue,operationType);}

ClearValueCommon

/// <summary>///     The common code shared by all variants of ClearValue/// </summary>private void ClearValueCommon(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata){if (IsSealed){throw new InvalidOperationException(SR.Get(SRID.ClearOnReadOnlyObjectNotAllowed, this));}// Get old valueEffectiveValueEntry oldEntry = GetValueEntry(entryIndex,dp,metadata,RequestFlags.RawEntry);// Get current local value// (No need to go through read local callback, just checking// for presence of Expression)object current = oldEntry.LocalValue;// Get current expressionExpression currentExpr = (oldEntry.IsExpression) ? (current as Expression) : null;// Inform value expression of detachment, if applicableif (currentExpr != null){// CALLBACKDependencySource[] currentSources = currentExpr.GetSources();UpdateSourceDependentLists(this, dp, currentSources, currentExpr, false);  // Remove// CALLBACKcurrentExpr.OnDetach(this, dp);currentExpr.MarkDetached();entryIndex = CheckEntryIndex(entryIndex, dp.GlobalIndex);}// valuesource == Local && value == UnsetValue indicates that we are clearing the local valueEffectiveValueEntry newEntry = new EffectiveValueEntry(dp, BaseValueSourceInternal.Local);// Property is now invalidUpdateEffectiveValue(entryIndex,dp,metadata,oldEntry,ref newEntry,false /* coerceWithDeferredReference */,false /* coerceWithCurrentValue */,OperationType.Unknown);}

Mode对SetValue的影响

在wpf - 依赖属性SetValue（）和SetCurrentValue（）之间的区别是什么 - 堆栈溢出 — wpf - What’s the difference between Dependency Property SetValue() & SetCurrentValue() - Stack Overflow上看到和Mode还有关系，于是我又测试了一下：
在这里插入图片描述

将按钮的绑定方式改为OneWay,发现SetValue执行后，绑定为Null，绑定被销毁。

<Buttonx:Name="MyButton"Width="100"Height="50"Background="{Binding DefaultBackgroundColor,Mode=OneWay,RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}"Content="MyButton" />

总结

就我的测试Demo来看，

ClearValue会使得数据绑定失效。
SetCurrentValue和官方文档所述一致，不会覆盖该属性的值来源，如果属性值是通过绑定、样式或触发器设置的，使用 SetCurrentValue 后，这些设置仍然有效。
SetValue 设置的依赖属性为DependencyProperty.UnsetValue，会和ClearValue的表现一致。
SetValue在Mode=TwoWay时，和SetCurrentValue表现一致，数据绑定不会失效。
SetValue 在Mode=OneWay时。数据绑定也会失效。