Various properties of PropertyMetadata, such as DefaultValue, are defined in the object model as read-write. This is so those properties can be adjusted after initialization of the PropertyMetadata object itself. However, once the metadata is applied to a dependency property as part of a call to Register, AddOwner, or OverrideMetadata, the property system will seal that metadata instance and the properties are now considered immutable. At the time of one of these calls, OnApply is called, and the value of this property is set to true.

The following example checks IsSealed prior to a set operation of a custom metadata property.

public Boolean SupportsMyFeature
{
    get { return _supportsMyFeature; }
    set { if (this.IsSealed != true) _supportsMyFeature = value; } //else may want to raise exception 
}
protected override void Merge(PropertyMetadata baseMetadata, DependencyProperty dp)
{
    base.Merge(baseMetadata, dp);
    MyCustomPropertyMetadata mcpm = baseMetadata as MyCustomPropertyMetadata;
    if (mcpm != null)
    {
        if (this.SupportsMyFeature == false)
        {//if not set, revert to base
            this.SupportsMyFeature = mcpm.SupportsMyFeature;
        }
    }
}

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

.NET Framework

Reference

Other Resources