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MarshalAsAttribute Class

Indicates how to marshal the data between managed and unmanaged code.

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

[AttributeUsageAttribute(AttributeTargets.Field|AttributeTargets.Parameter|AttributeTargets.ReturnValue, Inherited=false)] 
[ComVisibleAttribute(true)] 
public sealed class MarshalAsAttribute : Attribute
/** @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
Not applicable.

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.

NoteNote:

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();
System.Object
   System.Attribute
    System.Runtime.InteropServices.MarshalAsAttribute
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 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 Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0
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