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Dependency property security (WPF .NET)

  • Artikel

The accessibility of read-write dependency properties through the Windows Presentation Foundation (WPF) property system effectively makes them public properties. As a result, it's not possible to make security guarantees about read-write dependency property values. The WPF property system provides more security for read-only dependency properties so that you can restrict write access.

Access and security of property wrappers

A common language runtime (CLR) property wrapper is usually included in read-write dependency property implementations to simplify getting or setting property values. If included, the CLR property wrapper is a convenience method that implements the GetValue and SetValue static calls that interact with the underlying dependency property. Essentially, a CLR property wrapper exposes a dependency property as a CLR property backed by a dependency property rather than a private field.

Applying security mechanisms and restricting access to the CLR property wrapper might prevent usage of the convenience method, but those techniques won't prevent direct calls to GetValue or SetValue. In other words, a read-write dependency property is always accessible through the WPF property system. If you're implementing a read-write dependency property, avoid restricting access to the CLR property wrapper. Instead, declare the CLR property wrapper as a public member so callers are aware of the true access level of the dependency property.

Property system exposure of dependency properties

The WPF property system provides access to a read-write dependency property through its DependencyProperty identifier. The identifier is usable in GetValue and SetValue calls. Even if the static identifier field is non-public, several aspects of the property system will return a DependencyProperty as it exists on an instance of a class or derived class. For example, the GetLocalValueEnumerator method returns identifiers for dependency property instances with a locally set value. Also, you can override the OnPropertyChanged virtual method to receive event data that will report the DependencyProperty identifier for dependency properties that have changed value. To make callers aware of the true access level of a read-write dependency property, declare its identifier field as a public member.

Note

Although declaring a dependency property identifier field as private reduces the number of ways that a read-write dependency property is accessible, the property won't be private according to the CLR language definition.

Validation security

Applying a Demand to a ValidateValueCallback and expecting validation to fail on Demand failure, isn't an adequate security mechanism for restricting property value changes. Also, new value invalidation enforced through ValidateValueCallback can be suppressed by malicious callers, if those callers are operating within the application domain.

Access to read-only dependency properties

To restrict access, register your property as a read-only dependency property by calling the RegisterReadOnly method. The RegisterReadOnly method returns a DependencyPropertyKey, which you can assign to a non-public class field. For read-only dependency properties, the WPF property system will only provide write access to those who have a reference to the DependencyPropertyKey. To illustrate this behavior, the following test code:

  • Instantiates a class that implements both read-write and read-only dependency properties.
  • Assigns a private access modifier to each identifier.
  • Only implements get accessors.
  • Uses the GetLocalValueEnumerator method to access the underlying dependency properties through the WPF property system.
  • Calls GetValue and SetValue to test access to each dependency property value.
    /// <summary>
    ///  Test get/set access to dependency properties exposed through the WPF property system.
    /// </summary>
    public static void DependencyPropertyAccessTests()
    {
        // Instantiate a class that implements read-write and read-only dependency properties.
        Aquarium _aquarium = new();
        // Access each dependency property using the LocalValueEnumerator method.
        LocalValueEnumerator localValueEnumerator = _aquarium.GetLocalValueEnumerator();
        while (localValueEnumerator.MoveNext())
        {
            DependencyProperty dp = localValueEnumerator.Current.Property;
            string dpType = dp.ReadOnly ? "read-only" : "read-write";
            // Test read access.
            Debug.WriteLine($"Attempting to get a {dpType} dependency property value...");
            Debug.WriteLine($"Value ({dpType}): {(int)_aquarium.GetValue(dp)}");
            // Test write access.
            try
            {
                Debug.WriteLine($"Attempting to set a {dpType} dependency property value to 2...");
                _aquarium.SetValue(dp, 2);
            }
            catch (InvalidOperationException e)
            {
                Debug.WriteLine(e.Message);
            }
            finally
            {
                Debug.WriteLine($"Value ({dpType}): {(int)_aquarium.GetValue(dp)}");
            }
        }

        // Test output:

        // Attempting to get a read-write dependency property value...
        // Value (read-write): 1
        // Attempting to set a read-write dependency property value to 2...
        // Value (read-write): 2

        // Attempting to get a read-only dependency property value...
        // Value (read-only): 1
        // Attempting to set a read-only dependency property value to 2...
        // 'FishCountReadOnly' property was registered as read-only
        // and cannot be modified without an authorization key.
        // Value (read-only): 1
    }
}

public class Aquarium : DependencyObject
{
    public Aquarium()
    {
        // Assign locally-set values.
        SetValue(FishCountProperty, 1);
        SetValue(FishCountReadOnlyPropertyKey, 1);
    }

    // Failed attempt to restrict write-access by assigning the
    // DependencyProperty identifier to a non-public field.
    private static readonly DependencyProperty FishCountProperty =
        DependencyProperty.Register(
          name: "FishCount",
          propertyType: typeof(int),
          ownerType: typeof(Aquarium),
          typeMetadata: new PropertyMetadata());

    // Successful attempt to restrict write-access by assigning the
    // DependencyPropertyKey to a non-public field.
    private static readonly DependencyPropertyKey FishCountReadOnlyPropertyKey =
        DependencyProperty.RegisterReadOnly(
          name: "FishCountReadOnly",
          propertyType: typeof(int),
          ownerType: typeof(Aquarium),
          typeMetadata: new PropertyMetadata());

    // Declare public get accessors.
    public int FishCount => (int)GetValue(FishCountProperty);
    public int FishCountReadOnly => (int)GetValue(FishCountReadOnlyPropertyKey.DependencyProperty);
}

    ''' <summary>
    ''' ' Test get/set access to dependency properties exposed through the WPF property system.
    ''' </summary>
    Public Shared Sub DependencyPropertyAccessTests()
        ' Instantiate a class that implements read-write and read-only dependency properties.
        Dim _aquarium As New Aquarium()
        ' Access each dependency property using the LocalValueEnumerator method.
        Dim localValueEnumerator As LocalValueEnumerator = _aquarium.GetLocalValueEnumerator()
        While localValueEnumerator.MoveNext()
            Dim dp As DependencyProperty = localValueEnumerator.Current.[Property]
            Dim dpType As String = If(dp.[ReadOnly], "read-only", "read-write")
            ' Test read access.
            Debug.WriteLine($"Attempting to get a {dpType} dependency property value...")
            Debug.WriteLine($"Value ({dpType}): {CInt(_aquarium.GetValue(dp))}")
            ' Test write access.
            Try
                Debug.WriteLine($"Attempting to set a {dpType} dependency property value to 2...")
                _aquarium.SetValue(dp, 2)
            Catch e As InvalidOperationException
                Debug.WriteLine(e.Message)
            Finally
                Debug.WriteLine($"Value ({dpType}): {CInt(_aquarium.GetValue(dp))}")
            End Try
        End While

        ' Test output

        ' Attempting to get a read-write dependency property value...
        ' Value (read-write): 1
        ' Attempting to set a read-write dependency property value to 2...
        ' Value (read-write): 2

        ' Attempting to get a read-only dependency property value...
        ' Value (read-only): 1
        ' Attempting to set a read-only dependency property value to 2...
        ' 'FishCountReadOnly' property was registered as read-only
        ' and cannot be modified without an authorization key.
        ' Value (read-only): 1
    End Sub

End Class

Public Class Aquarium
    Inherits DependencyObject

    Public Sub New()
        ' Assign locally-set values.
        SetValue(FishCountProperty, 1)
        SetValue(FishCountReadOnlyPropertyKey, 1)
    End Sub

    ' Failed attempt to restrict write-access by assigning the
    ' DependencyProperty identifier to a non-public field.
    Private Shared ReadOnly FishCountProperty As DependencyProperty =
        DependencyProperty.Register(
            name:="FishCount",
            propertyType:=GetType(Integer),
            ownerType:=GetType(Aquarium),
            typeMetadata:=New PropertyMetadata())

    ' Successful attempt to restrict write-access by assigning the
    ' DependencyPropertyKey to a non-public field.
    Private Shared ReadOnly FishCountReadOnlyPropertyKey As DependencyPropertyKey =
        DependencyProperty.RegisterReadOnly(
            name:="FishCountReadOnly",
            propertyType:=GetType(Integer),
            ownerType:=GetType(Aquarium),
            typeMetadata:=New PropertyMetadata())

    ' Declare public get accessors.
    Public ReadOnly Property FishCount As Integer
        Get
            Return GetValue(FishCountProperty)
        End Get
    End Property

    Public ReadOnly Property FishCountReadOnly As Integer
        Get
            Return GetValue(FishCountReadOnlyPropertyKey.DependencyProperty)
        End Get
    End Property

End Class

See also