Enables or disables the throwing of an exception on an unmappable Unicode character that is converted to an ANSI "?" character.
Assemblies: mscorlib (in mscorlib.dll)
System.Runtime.InteropServices (in System.Runtime.InteropServices.dll)
true to indicate that an exception is thrown each time the interop marshaler converts an unmappable character; false to indicate that the field is disabled. This field is false 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 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.
Certain Unicode characters are converted to dangerous characters, such as the backslash '\' character, which can inadvertently change a path. By setting the field to true, you can signal the presence of an unmappable character to the caller by throwing an exception.
You cannot change the default values provided by the BestFitMapping and 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 to define a DLL function in managed code, instead of using the Declare statement. Setting the 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.