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ReadOnlyArrayAttribute Class

.NET Framework 4.6 and 4.5

When applied to an array parameter in a Windows Runtime component, specifies that the contents of the array that is passed to that parameter are used only for input. The caller expects the array to be unchanged by the call. See the Remarks section for important information about callers that are written using managed code.

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


[AttributeUsageAttribute(AttributeTargets.Parameter, Inherited = false, 
	AllowMultiple = false)]
public sealed class ReadOnlyArrayAttribute : Attribute


Initializes a new instance of the ReadOnlyArrayAttribute class.


When implemented in a derived class, gets a unique identifier for this Attribute.(Inherited from Attribute.)


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)


Returns the hash code for this instance.(Inherited from Attribute.)


Gets the Type of the current instance.(Inherited from Object.)


When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)


When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)


Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)

Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)

Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)


Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)

Provides access to properties and methods exposed by an object.(Inherited from Attribute.)

If an array parameter in your Windows Runtime component is passed by value (ByVal in Visual Basic), you must apply one of the following attributes to it:

  • Apply the ReadOnlyArrayAttribute attribute if you intend the contents of the array to be used for input only.

  • Apply the WriteOnlyArrayAttribute attribute if you intend the contents of the array to be used for output only (that is, the method sets the contents of the array but does not read them).

Applying both attributes to a parameter causes an error. For more information, including the standard pattern for making changes to an array, see Passing arrays to a Windows Runtime component in the Windows Dev Center.


Parameters that have the ReadOnlyArrayAttributeattribute behave differently depending on whether the caller is written in native code or managed code. If the caller is native code (JavaScript or Visual C++ component extensions), the array is copied when the call crosses the application binary interface (ABI) boundary. Elements are converted if necessary. Therefore, any accidental changes the method makes to an input-only array are not visible to the caller.

If the caller is managed code, the array is not copied. The original array is available to the called method, as it would be in any method call in the .NET Framework. Array contents are mutable in .NET Framework code, so any changes the method makes to the array are visible to the caller. This is important to remember because it affects unit tests written for a Windows Runtime component. If the tests are written in managed code, the contents of an array will appear to be mutable during testing.

Applying this attribute to a parameter that has the InAttribute or OutAttribute attribute causes an error when the module is exported. Applying the attribute to an out parameter also causes an error.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 4.5
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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