MarshalAsAttribute Class
Assembly: mscorlib (in mscorlib.dll)
'Declaration <AttributeUsageAttribute(AttributeTargets.Field Or AttributeTargets.Parameter Or AttributeTargets.ReturnValue, Inherited:=False)> _ <ComVisibleAttribute(True)> _ Public NotInheritable Class MarshalAsAttribute Inherits Attribute 'Usage Dim instance As MarshalAsAttribute
/** @attribute AttributeUsageAttribute(AttributeTargets.Field|AttributeTargets.Parameter|AttributeTargets.ReturnValue, Inherited=false) */ /** @attribute ComVisibleAttribute(true) */ public final class MarshalAsAttribute extends Attribute
AttributeUsageAttribute(AttributeTargets.Field|AttributeTargets.Parameter|AttributeTargets.ReturnValue, Inherited=false) ComVisibleAttribute(true) public final class MarshalAsAttribute extends Attribute
You can apply this attribute to parameters, fields, or return values.
This attribute is optional, as each data type has a default marshaling behavior. This attribute is only necessary when a given type can be marshaled to multiple types. For example, you can marshal a string to unmanaged code as either a LPStr, a LPWStr, a LPTStr, or a BStr. By default, the common language runtime marshals a string parameter as a BStr to COM methods. You can apply the MarshalAsAttribute attribute to an individual field or parameter to cause that particular string to be marshaled as a LPStr instead of a BStr. The Type Library Exporter (Tlbexp.exe) passes your marshaling preferences to the common language runtime.
Some parameters and return values have different default marshaling behavior when used with COM interop or platform invoke. By default, the runtime marshals a string parameter (and fields in a value type) as a LPStr to a platform invoke method or function. For additional information, see Default Marshaling Behavior.
In most cases, the attribute simply identifies the format of the unmanaged data using the UnmanagedType enumeration, as shown in the following C# signature:
void
MyMethod([MarshalAs(LPStr)] String s);
Some UnmanagedType enumeration members require additional information. For example, additional information is needed when the UnmanagedType is LPArray. For a complete description of how to use this attribute with arrays, see Default Marshaling for Arrays.
The Type Library Importer (Tlbimp.exe) also applies this attribute to parameters, fields, and return values to indicate that the data type in the input type library is not the default type for the corresponding managed data type. Tlbimp.exe always applies the MarshalAsAttribute to String and Object types for clarity, regardless of the type specified in the input type library.
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| The MarshalAsAttribute does not support marshaling of generic types. |
The following examples show the placement of the MarshalAsAttribute in managed source code as applied to parameters, field, and return values.
Imports System.Runtime.InteropServices Module Program 'Applied to a parameter. Public Sub M1(<MarshalAs(UnmanagedType.LPWStr)> ByVal msg As String) End Sub 'Applied to a field within a class. Class MsgText <MarshalAs(UnmanagedType.LPWStr)> Public msg As String End Class 'Applied to a a return value. Public Function M2() As <MarshalAs(UnmanagedType.LPWStr)> String End Function End Module //Applied to a parameter. public void M1 ([MarshalAs(UnmanagedType.LPWStr)]String msg); //Applied to a field within a class. class MsgText { [MarshalAs(UnmanagedType.LPWStr)] public String msg; } //Applied to a return value. [return: MarshalAs(UnmanagedType.LPWStr)] public String GetMessage();
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.
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