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OutAttribute Class

Indicates that data should be marshaled from callee back to caller.

Namespace:  System.Runtime.InteropServices
Assembly:  mscorlib (in mscorlib.dll)
[AttributeUsageAttribute(AttributeTargets.Parameter, Inherited = false)]
[ComVisibleAttribute(true)]
public sealed class OutAttribute : Attribute

You can apply this attribute to parameters.

The OutAttribute is optional. The attribute is supported for COM interop and platform invoke only. In the absence of explicit settings, the interop marshaler assumes rules based on the parameter type, whether the parameter is passed by reference or by value, and whether the type is blittable or non-blittable. For example, the StringBuilder class is always assumed to be In/Out and an array of strings passed by value is assumed to be In.

Out-only behavior is never a default marshaling behavior for parameters. You can apply the OutAttribute to value and reference types passed by reference to change In/Out behavior to Out-only behavior, which is equivalent to using the out keyword in C#. For example, arrays passed by value, marshaled as In-only parameters by default, can be changed to Out-only. However, the behavior does not always provide expected semantics when the types include all-blittable elements or fields because the interop marshaler uses pinning. If you do not care about passing data into the callee, Out-only marshaling can provide better performance for non-blittable types.

Combining the InAttribute and OutAttribute is particularly useful when applied to arrays and formatted, non-blittable types. Callers see the changes a callee makes to these types only when you apply both attributes. Since these types require copying during marshaling, you can use InAttribute and OutAttribute to reduce unnecessary copies.

For more information on the effect of OutAttribute on marshaling behavior, see Directional Attributes.

The following example shows how to apply the InAttribute and OutAttribute to a platform invoke prototype that passes an array as a parameter. The combination of directional attributes allows the caller to see the changes made by the callee.

using System.Runtime.InteropServices;
using System;


// Declare a class member for each structure element.
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public class OpenFileName
{
    public int structSize = 0;
    public string filter = null;
    public string file = null;
    // ...
}

public class LibWrap
{
    // Declare a managed prototype for the unmanaged function.
    [DllImport("Comdlg32.dll", CharSet = CharSet.Unicode)]
    public static extern bool GetOpenFileName([In, Out] OpenFileName ofn);
}

public class MainMethod
{
    static void Main()
    { }

}
System.Object
  System.Attribute
    System.Runtime.InteropServices.OutAttribute
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, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0
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