WriteOnlyArrayAttribute Class

WriteOnlyArrayAttribute Class

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

When applied to an array parameter in a Windows Runtime component, specifies that the contents of an array that is passed to that parameter are used only for output. The caller does not guarantee that the contents are initialized, and the called method should not read the contents. 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 WriteOnlyArrayAttribute : Attribute

The WriteOnlyArrayAttribute type exposes the following members.

Public methodWriteOnlyArrayAttributeInitializes a new instance of the WriteOnlyArrayAttribute class.

Public methodEqualsInfrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeReturns the hash code for this instance. (Inherited from Attribute.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodMatchWhen overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

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.

Important noteImportant Note:

Parameters that have the WriteOnlyArrayAttribute attribute behave differently depending on whether the caller is written in native code or managed code. If the caller is native code (JavaScript or C++/CX), the called method can't make any assumptions about the contents of the original array. For example, the array the method receives might not be initialized, or might contain default values. The method is expected to set the values of all the elements in the array.

If the caller is managed code, the caller's original array is passed to the called method, as it would be in any method call in the .NET Framework. Array contents are mutable in managed code, so the method can selectively read and change those values. 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, and the results are likely to be different if the method is called from native code later.

Applying this attribute to an out parameter or to a parameter that has the InAttribute attribute causes an error when the module is exported. Applying the attribute to a parameter that has the OutAttribute attribute causes an error unless the parameter also has the Visual Basic ByRef modifier. In that case, the attribute is redundant but allowed.

Windows Phone OS

Supported in: 8.1, 8.0

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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