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DllImportAttribute.BestFitMapping Field

Enables or disables best-fit mapping behavior when converting Unicode characters to ANSI characters.

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

public bool BestFitMapping
public boolean BestFitMapping
public var BestFitMapping : boolean

If true, best-fit mapping behavior is enabled; otherwise, best-fit mapping is disabled. The BestFitMapping field is true by default. Settings for this field override the any level settings for the System.Runtime.InteropServices.BestFitMappingAttribute attribute.

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 provide 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.

Caution noteCaution

Certain Unicode characters are converted to dangerous characters, such as the backslash '\' character, which can inadvertently change a path. By setting the ThrowOnUnmappableChar field to true, you can signal the presence of an unmappable character to the caller by throwing an exception.

Caution noteCaution

You cannot change the default values provided by the BestFitMapping and ThrowOnUnmappableChar fields 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 is thrown. Be aware that this combination can compromise your security model.

In some cases, Visual Basic developers use the DllImportAttribute, instead of the Declare statement, to define a DLL function in managed code. Setting the BestFitMapping field is one of those cases. The following example shows how to apply the strictest character mapping security to a platform invoke method definitions by specifying the ANSI character set, disabling best fit mapping behavior, and throwing an exception on unmapped Unicode characters.

[DllImport("My.dll", CharSet = CharSet.Ansi, 
                       BestFitMapping = false
                       ThrowOnUnmappableChar = true)]

Windows 98, Windows 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, 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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