BestFitMappingAttribute Class
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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)]
public sealed class BestFitMappingAttribute : Attribute


Initializes a new instance of the BestFitMappingAttribute class set to the value of the BestFitMapping property.


Gets the best-fit mapping behavior when converting Unicode characters to ANSI characters.


When implemented in a derived class, gets a unique identifier for this Attribute.(Inherited from Attribute.)


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)


Returns the hash code for this instance.(Inherited from Attribute.)


Gets the Type of the current instance.(Inherited from Object.)


When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)


When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)


Returns a string that represents the current object.(Inherited from Object.)


Enables or disables the throwing of an exception on an unmappable Unicode character that is converted to an ANSI '?' character.

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)

Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)

Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)


Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)

Provides access to properties and methods exposed by an object.(Inherited from 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 marshaler 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.


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.


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.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Windows Phone
Available since 8.1

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

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