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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)

[ComVisibleAttribute(true)]
[AttributeUsageAttribute(AttributeTargets.Field|AttributeTargets.Parameter|AttributeTargets.ReturnValue, Inherited = false)]
public sealed class MarshalAsAttribute : 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.

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.

using System;
using System.Text;
using System.Runtime.InteropServices;


class Program
{

//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 = "Hello World";
                }

//Applied to a return value.
[return: MarshalAs(UnmanagedType.LPWStr)]
    public String GetMessage()
    {
        return "Hello World";
    }


static void Main(string[] args)
    {  }
}

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 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

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