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BestFitMappingAttribute Class

Controls whether Unicode characters are converted to the closest matching ANSI characters.

Namespace: System.Runtime.InteropServices
Assembly: mscorlib (in mscorlib.dll)

[AttributeUsageAttribute(AttributeTargets.Assembly|AttributeTargets.Class|AttributeTargets.Struct|AttributeTargets.Interface, Inherited=false)] 
[ComVisibleAttribute(true)] 
public sealed class BestFitMappingAttribute : Attribute
/** @attribute AttributeUsageAttribute(AttributeTargets.Assembly|AttributeTargets.Class|AttributeTargets.Struct|AttributeTargets.Interface, Inherited=false) */ 
/** @attribute ComVisibleAttribute(true) */ 
public final class BestFitMappingAttribute extends Attribute
AttributeUsageAttribute(AttributeTargets.Assembly|AttributeTargets.Class|AttributeTargets.Struct|AttributeTargets.Interface, Inherited=false) 
ComVisibleAttribute(true) 
public final class BestFitMappingAttribute extends Attribute

You can apply this attribute to an assembly, interface, class, or structure.

By default, the common language runtime converts to ANSI characters any managed Unicode characters passed to an unmanaged method executing on Windows 98 or Windows Me. Best-fit mapping enables the Interop Marshaling to select a close-matching character when no exact match exists. For example, the marshaler converts the Unicode copyright character to 'c' for unmanaged methods that accept ANSI characters.

Caution noteCaution

Some characters lack a best-fit representation; these characters are called unmappable. Unmappable characters are usually converted to the default '?' ANSI character. Certain Unicode characters are converted to dangerous characters, such as the backslash '\' character, which can inadvertently change a path.

BestFitMappingAttribute provides two parameters to control aspects of best-fit mapping. You use the first parameter to toggle best-fit mapping on and off. The default value is true, which enables best-fit mapping on the assembly, interface, and class levels. An attribute applied to an interface or class overrides an assembly-level attribute. You can likewise enable or disable best-fit mapping for platform invoke calls by using the DllImportAttribute.BestFitMapping field. A value set by the platform invoke field overrides all levels of BestFitMappingAttribute.

You can use the second parameter to control throwing an exception on unmappable characters. The default value for the ThrowOnUnmappableChar field is false, which disables throwing an exception each time the runtime encounters a Unicode character that has to be converted to the '?' ANSI character. Even if best-fit mapping is true, unmappable characters generate an exception when the ThrowOnUnmappableChar field is true. To tighten security, you can toggle the first parameter to false and the second parameter to true. This combination of parameter settings turns best-fit mapping off, but enables the exception-throwing mechanism as a safety precaution.

Caution noteCaution

You cannot change the default values provided by BestFitMappingAttribute when passing a managed array whose elements are ANSI Chars or LPSTRs to an unmanaged safe array. Best-fit mapping is always enabled and no exception can be thrown. Be aware that this combination can compromise your security model.

The following example shows how to disable best fit mapping and to throw an exception on the conversion of Unicode characters to the '?' ANSI character. Setting BestFitMappingAttribute parameters in this manner provides an added measure of security.

[BestFitMapping(false, ThrowOnUnmappableChar = true)]
 interface IMyInterface1 
 {
     //Insert code here.
 }

System.Object
   System.Attribute
    System.Runtime.InteropServices.BestFitMappingAttribute

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

Windows 98, Windows 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

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

.NET Framework

Supported in: 2.0, 1.1

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