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ParamArrayAttribute Class

Indicates that a method will allow a variable number of arguments in its invocation. This class cannot be inherited.


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

<AttributeUsageAttribute(AttributeTargets.Parameter, Inherited := True, AllowMultiple := False)> _
<ComVisibleAttribute(True)> _
Public NotInheritable Class ParamArrayAttribute _
	Inherits Attribute

The ParamArrayAttribute type exposes the following members.

Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryParamArrayAttributeInitializes a new instance of the ParamArrayAttribute class with default properties.

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

Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryEqualsInfrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetHashCodeReturns the hash code for this instance. (Inherited from Attribute.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodIsDefaultAttributeWhen overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from Attribute.)
Public methodSupported by the XNA FrameworkMatchWhen overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibraryToStringReturns a string that represents the current object. (Inherited from Object.)

Explicit interface implemetationPrivate method_Attribute.GetIDsOfNamesMaps a set of names to a corresponding set of dispatch identifiers. (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.GetTypeInfoRetrieves the type information for an object, which can be used to get the type information for an interface. (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.GetTypeInfoCountRetrieves the number of type information interfaces that an object provides (either 0 or 1). (Inherited from Attribute.)
Explicit interface implemetationPrivate method_Attribute.InvokeProvides access to properties and methods exposed by an object. (Inherited from Attribute.)

The ParamArrayAttribute indicates that a method parameter is a parameter array. A parameter array allows the specification of an unknown number of arguments. A parameter array must be the last parameter in a formal parameter list, and it must be a single-dimension array. When the method is called, a parameter array permits arguments to a method to be specified in either of two ways:

  • As a single expression of a type that is implicitly convertible to the parameter array type. The parameter array functions as a value parameter.

  • As zero or more arguments, where each argument is an expression of a type that is implictly convertible to the type of the parameter array element.

The example in the next section illustrates both calling conventions.


Typically, the ParamArrayAttribute is not used directly in code. Instead, individual language keywords, such as ParamArray in Visual Basic and params in C#, are used as wrappers for the ParamArrayAttribute class. Some languages, such as C#, may even require the use of the language keyword and prohibit the use of ParamArrayAttribute.

During overload resolution, when compilers that support parameter arrays encounter a method overload that does not exist but has one fewer parameter than an overload that includes a parameter array, they will replace the method with the overload that includes the parameter array. For example, a call to the String.Split() instance method (which does not exist in the String class) is resolved as a call to the String.Split(Char()) method. The compiler will also pass an empty array of the required type to the method. This means that the method must always be prepared to handle an array whose length is zero when it processes the elements in the parameter array. The example provides an illustration.

For more information about using attributes, see Extending Metadata Using Attributes.

The following example defines a Temperature class that includes a Display method, which is intended to display one or more formatted temperature values. The method has a single parameter, formats, which is defined as a parameter array.

Public Class Temperature 
   Private temp As Decimal 

   Public Sub New(temperature As Decimal)
      Me.temp = temperature
   End Sub 

   Public Overrides Function ToString() As String 
      Return ToString("C")
   End Function 

   Public Overloads Function ToString(format As String) As String 
      If String.IsNullOrEmpty(format) Then format = "G" 

      Select Case format
         Case "G", "C" 
            Return temp.ToString("N") + "  °C" 
         Case "F" 
            Return (9 * temp / 5 + 32).ToString("N") + "  °F" 
         Case "K"  
            Return (temp + 273.15d).ToString("N") + "  °K"  
         Case Else 
            Throw New FormatException(String.Format("The '{0}' format specifier is not supported", _
      End Select                                                          
   End Function          

   Public Sub Display(<[ParamArray]()> formats() As String)
      If formats.Length = 0 Then
         For Each format As String In formats
            ' If there is an exception, do nothing. 
            End Try    
      End If 
   End Sub 
End Class

The following example illustrates three different calls to the Temperature.Display method. In the first, the method is passed an array of format strings. In the second, the method is passed four individual format strings as arguments. In the third, the method is called with no arguments. As the output from the example illustrates, the Visual Basic and C# compilers translate this into a call to the Display method with an empty string array.

Public Module Example
   Public Sub Main()
      Dim temp1 As New Temperature(100)
      Dim formats() As String = { "C", "G", "F", "K" } 

      ' Call Display method with a string array.
      Console.WriteLine("Calling Display with a string array:")

      ' Call Display method with individual string arguments.
      Console.WriteLine("Calling Display with individual arguments:")
      temp1.Display("C", "F", "K", "G")

      ' Call parameterless Display method.
      Console.WriteLine("Calling Display with an implicit parameter array:")
   End Sub 
End Module 
' The example displays the following output: 
'       Calling Display with a string array: 
'       100.00  °C 
'       100.00  °C 
'       212.00  °F 
'       373.15  °K 
'       Calling Display with individual arguments: 
'       100.00  °C 
'       212.00  °F 
'       373.15  °K 
'       100.00  °C 
'       Calling Display with an implicit parameter array: 
'       100.00  °C

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library

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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