WPF 数据绑定：BindingOperations.SetBinding() 不使用依赖属性的值

Question

我有一个自己的依赖属性 Target.Height，使用 BindingOperations.SetBinding() 绑定到普通属性 Source.Height。更新 Target.Height 属性应该更新 Source.Height 属性。但不使用依赖属性的实际值，而是使用依赖属性的默认值。这是预期的行为吗？

感谢您的任何提示。我使用的代码：

public class Source
{
  private int m_height;
  public int Height
  {
    get { return m_height; }
    set { m_height = value; }
  }
}

public class Target : DependencyObject
{
  public static readonly DependencyProperty HeightProperty;

  static Target()
  {
    Target.HeightProperty =
      DependencyProperty.Register("Height", typeof(int), typeof(Target),
      new PropertyMetadata(666)); //the default value
  }

  public int Height
  {
    get { return (int)GetValue(Target.HeightProperty); }
    set { SetValue(Target.HeightProperty, value); }
  }
}



Source source = new Source();
Target target = new Target();

target.Height = 100;

Binding heightBinding = new Binding("Height");
heightBinding.Source = source;
heightBinding.Mode = BindingMode.OneWayToSource;

BindingOperations.SetBinding(target, Target.HeightProperty, heightBinding);

//target.Height and source.Height is now 666 instead of 100 ....

Answer 1

WPF 将 Binding 作为依赖属性的值。当您设置绑定时，您实际上是将当前的属性值替换为新的属性值。在 DependencyObject.SetValueCommon 的末尾，您可能会找到执行此操作的代码。在那里我们可以看到 WPF 获取一个默认值，然后使用表达式标记将其设置为当前属性值，然后附加 BindingExpression，它使用当前属性值（默认值）更新源。

this.SetEffectiveValue(entryIndex, dp, dp.GlobalIndex, metadata, expression, BaseValueSourceInternal.Local);
object defaultValue = metadata.GetDefaultValue(this, dp);
entryIndex = this.CheckEntryIndex(entryIndex, dp.GlobalIndex);
this.SetExpressionValue(entryIndex, defaultValue, expression);
DependencyObject.UpdateSourceDependentLists(this, dp, array, expression, true);
expression.MarkAttached();
expression.OnAttach(this, dp);
entryIndex = this.CheckEntryIndex(entryIndex, dp.GlobalIndex);
effectiveValueEntry = this.EvaluateExpression(entryIndex, dp, expression, metadata, valueEntry, this._effectiveValues[entryIndex.Index)]);
entryIndex = this.CheckEntryIndex(entryIndex, dp.GlobalIndex);