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DefaultParameterValueAttribute Class

.NET Framework 4

Sets the default value of a parameter when called from a language that supports default parameters. This class cannot be inherited.

System.Object
  System.Attribute
    System.Runtime.InteropServices.DefaultParameterValueAttribute

Namespace:  System.Runtime.InteropServices
Assembly:  System (in System.dll)
C#
C++
F#
VB
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[AttributeUsageAttribute(AttributeTargets.Parameter)]
public sealed class DefaultParameterValueAttribute : Attribute

The DefaultParameterValueAttribute type exposes the following members.

  Name Description
Public method DefaultParameterValueAttribute Initializes a new instance of the DefaultParameterValueAttribute class with the default value of a parameter.
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  Name Description
Public property TypeId When implemented in a derived class, gets a unique identifier for this Attribute. (Inherited from Attribute.)
Public property Value Gets the default value of a parameter.
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  Name Description
Public method Equals Infrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Protected method Finalize Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public method GetHashCode Returns the hash code for this instance. (Inherited from Attribute.)
Public method GetType Gets the Type of the current instance. (Inherited from Object.)
Public method IsDefaultAttribute When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from Attribute.)
Public method Match When overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Protected method MemberwiseClone Creates a shallow copy of the current Object. (Inherited from Object.)
Public method ToString Returns a string that represents the current object. (Inherited from Object.)
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  Name Description
Explicit interface implemetation Private method _Attribute.GetIDsOfNames Maps a set of names to a corresponding set of dispatch identifiers. (Inherited from Attribute.)
Explicit interface implemetation Private method _Attribute.GetTypeInfo Retrieves the type information for an object, which can be used to get the type information for an interface. (Inherited from Attribute.)
Explicit interface implemetation Private method _Attribute.GetTypeInfoCount Retrieves the number of type information interfaces that an object provides (either 0 or 1). (Inherited from Attribute.)
Explicit interface implemetation Private method _Attribute.Invoke Provides access to properties and methods exposed by an object. (Inherited from Attribute.)
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The DefaultParameterValueAttribute attribute allows you to specify a default parameter value in a language that does not otherwise support default parameters. After you apply this attribute to your code, languages that support default parameters can use the specified value as a default parameter.

Note that the DefaultParameterValueAttribute does not add support for default parameters to languages that do not support this feature. For example, if you use the DefaultParameterValueAttribute with a method written in C#, which does not support default parameters, you cannot use the default parameter when calling the method from C#. You can use the default parameter only when calling the method in a language such as C++ that does support the feature.

The DefaultParameterValueAttribute is particularly useful to specify default parameters for methods of a COM interop interface.

The following code example demonstrates how to apply the DefaultParameterValueAttribute attribute to a parameter of a method written in C#.

C#
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	static public void MethodWithDefaultParam([System.Runtime.InteropServices.DefaultParameterValue("DEFAULT_PARAM_VALUE")] string var)
	{
		Console.WriteLine("The passed value is: " + var);
	}

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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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Optional Parameters in C#
Note that C# 4.0 does support optional parameters.

This attribute can still be used to create optional parameters without the new syntax (or with older compilers). 
http://blog.slaks.net/2011/01/optional-parameters-in-c-4.html
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