Import Attribute. Best Fit Mapping Field
Enables or disables best-fit mapping behavior when converting Unicode characters to ANSI characters.
public: bool BestFitMapping;
public bool BestFitMapping;
val mutable BestFitMapping : bool
Public BestFitMapping As Boolean
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)] int SomeFuncion2(int parm);
[DllImport("My.dll", CharSet = CharSet.Ansi, BestFitMapping = false, ThrowOnUnmappableChar = true)] internal static extern int SomeFuncion2(int parm);
<DllImport("My.dll", CharSet:=CharSet.Ansi, BestFitMapping:=False, ThrowOnUnmappableChar:=True)> Friend Shared Function SomeFuncion2(parm As Integer) As Integer End Function
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 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 ThrowOnUnmappableChar 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 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.