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IncrementalHitTester Class

Dynamically performs hit testing on a Stroke.

Namespace:  System.Windows.Ink
Assembly:  PresentationCore (in PresentationCore.dll)

public abstract class IncrementalHitTester
This class is not typically used in XAML.

The IncrementalHitTester type exposes the following members.

Public propertyIsValidGets whether the IncrementalHitTester is hit testing.

Public methodAddPointAdds a Point to the IncrementalHitTester.
Public methodAddPoints(IEnumerable<Point>)Adds points to the IncrementalHitTester.
Public methodAddPoints(StylusPointCollection)Adds the specified StylusPoint objects to the IncrementalHitTester.
Protected methodAddPointsCoreInfrastructure. Adds points to the IncrementalHitTester.
Public methodEndHitTestingReleases resources used by the IncrementalHitTester.
Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

The IncrementalHitTester dynamically determines whether the user "hits" a Stroke. This class enables you to provide immediate feedback to the user in situations such as selecting and erasing ink.

There are two classes that inherit from IncrementalHitTester:

The following table lists where to learn how to manage digital ink on a custom control.

To do this…

See this article

Create a control that collects digital ink

Creating an Ink Input Control

Create a control that enables the user to select ink

How to: Select Ink from a Custom Control

Create a control that enables the user to point-erase ink

How to: Erase Ink on a Custom Control

The following example demonstrates how to add points to the IncrementalHitTester when the user inputs data with the stylus. The example includes a StrokeHit event handler that erases the part of the Stroke that the user intersects. To create a control that enables a user to erase ink, see How to: Erase Ink on a Custom Control.

        // Prepare to collect stylus packets. Get the 
        // IncrementalHitTester from the InkPresenter's 
        // StrokeCollection and subscribe to its StrokeHitChanged event.
        protected override void OnStylusDown(StylusDownEventArgs e)

            EllipseStylusShape eraserTip = new EllipseStylusShape(3, 3, 0);
            eraseTester = 
            eraseTester.StrokeHit += new StrokeHitEventHandler(eraseTester_StrokeHit);


        // Collect the StylusPackets as the stylus moves.
        protected override void OnStylusMove(StylusEventArgs e)
            if (eraseTester.IsValid)

        // Unsubscribe from the StrokeHitChanged event when the
        // user lifts the stylus.
        protected override void OnStylusUp(StylusEventArgs e)

            eraseTester.StrokeHit -= new

        // When the stylus intersects a stroke, erase that part of
        // the stroke.  When the stylus dissects a stoke, the 
        // Stroke.Erase method returns a StrokeCollection that contains
        // the two new strokes.
        void eraseTester_StrokeHit(object sender,
            StrokeHitEventArgs args)
            StrokeCollection eraseResult =
            StrokeCollection strokesToReplace = new StrokeCollection();

            // Replace the old stroke with the new one.
            if (eraseResult.Count > 0)
                presenter.Strokes.Replace(strokesToReplace, eraseResult);


.NET Framework

Supported in: 4, 3.5, 3.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.