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Mitigation: Custom IMessageFilter.PreFilterMessage Implementations
Learn about the custom IMessageFilter.PreFilterMessage implementation included in Windows Forms apps that target .NET Framework 4.6.1 and later.
03/30/2017
9cf47c5b-0bb2-45df-9437-61cd7e7c2f4d

Mitigation: Custom IMessageFilter.PreFilterMessage Implementations

In Windows Forms apps that target versions of the .NET Framework starting with the .NET Framework 4.6.1, a custom xref:System.Windows.Forms.IMessageFilter.PreFilterMessage%2A?displayProperty=nameWithType implementation can safely filter messages when the xref:System.Windows.Forms.Application.FilterMessage%2A?displayProperty=nameWithType method is called if the xref:System.Windows.Forms.IMessageFilter.PreFilterMessage%2A?displayProperty=nameWithType implementation:

  • Does one or both of the following:

    • Adds a message filter by calling the xref:System.Windows.Forms.Application.AddMessageFilter%2A method.

    • Removes a message filter by calling the xref:System.Windows.Forms.Application.RemoveMessageFilter%2A method. method.

  • And pumps messages by calling the xref:System.Windows.Forms.Application.DoEvents%2A?displayProperty=nameWithType method.

Impact

This change only affects Windows Forms apps that target versions of the .NET Framework starting with the .NET Framework 4.6.1.

For Windows Forms apps that target previous versions of the .NET Framework, such implementations in some cases throw an xref:System.IndexOutOfRangeException exception when the xref:System.Windows.Forms.Application.FilterMessage%2A?displayProperty=nameWithType method is called

Mitigation

If this change is undesirable, apps that target the .NET Framework 4.6.1 or a later version can opt out of it by adding the following configuration setting to the <runtime> section of the app’s configuration file:

<runtime>
    <AppContextSwitchOverrides value="Switch.System.Windows.Forms.DontSupportReentrantFilterMessage=true" />
</runtime>

In addition, apps that target previous versions of the .NET Framework but are running under the .NET Framework 4.6.1 or a later version can opt in to this behavior by adding the following configuration setting to the <runtime> section of the app’s configuration file:

<runtime>
    <AppContextSwitchOverrides value="Switch.System.Windows.Forms.DontSupportReentrantFilterMessage=false" />
</runtime>

See also