OutAttribute Class

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

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

<ComVisibleAttribute(True)> _
<AttributeUsageAttribute(AttributeTargets.Parameter, Inherited:=False)> _
Public NotInheritable Class OutAttribute
	Inherits Attribute
Dim instance As OutAttribute

/** @attribute ComVisibleAttribute(true) */ 
/** @attribute AttributeUsageAttribute(AttributeTargets.Parameter, Inherited=false) */ 
public final class OutAttribute extends Attribute
AttributeUsageAttribute(AttributeTargets.Parameter, Inherited=false) 
public final class OutAttribute extends 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.

' Declare a class member for each structure element.
< StructLayout( LayoutKind.Sequential, CharSet:=CharSet.Auto )> _
Public Class OpenFileName

   Public structSize As Integer = 0
   Public filter As String = Nothing
   Public file As String = Nothing
   ' ...
End Class 'OpenFileName

Public Class LibWrap
   ' Declare managed prototype for the unmanaged function.
   Declare Auto Function GetOpenFileName Lib "Comdlg32.dll" ( _
      <[In], Out> ByVal ofn As OpenFileName ) As Boolean
End Class 'LibWrap

// Declare a class member for each structure element.
/** @attribute StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)
public class OpenFileName
    public int structSize = 0;
    public String filter = null;
    public String file = null;
    // ...
} //OpenFileName

public class LibWrap
    // Declare a managed prototype for the unmanaged function.
    /** @attribute DllImport("Comdlg32.dll", CharSet = CharSet.Auto)
    public static native boolean GetOpenFileName(
        /** @attribute In()
            @attribute Out()
        OpenFileName ofn);
} //LibWrap


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 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.

.NET Framework

Supported in: 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

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