ParamArrayAttribute Class
Updated: December 2008
Indicates that the method will allow a variable number of arguments in its invocation. This class cannot be inherited.
Assembly: mscorlib (in mscorlib.dll)
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 illustrates both calling conventions.
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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. |
In performing overload resolution, compilers that support parameter arrays substitute a call to the method that includes a parameter array if the method that is actually called in code does not exist but has one fewer parameter than the method that includes the parameter array. For example, a call to the String.Split() instance method with no parameters (a method that does not actually exist in the String class) is resolved as a call to the String.Split(Char()) method. The compiler also passes an empty array of the required type to the method. This means that, when the method processes the elements in the parameter array, it must always be prepared to handle an array whose length is zero. 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", _ format)) End Select End Function Public Sub Display(<[ParamArray]()> formats() As String) If formats.Length = 0 Then Console.WriteLine(Me.ToString("G")) Else For Each format As String In formats Try Console.WriteLine(Me.ToString(format)) ' If there is an exception, do nothing. Catch End Try Next End If End Sub End Class
The following example illustrates three different calls to the Temperature.Display method. In the first call, the method is passed an array of format strings. In the second call, the method is passed four individual format strings as arguments. In the third call, 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:") temp1.Display(formats) Console.WriteLine() ' Call Display method with individual string arguments. Console.WriteLine("Calling Display with individual arguments:") temp1.Display("C", "F", "K", "G") Console.WriteLine() ' Call parameterless Display method. Console.WriteLine("Calling Display with an implicit parameter array:") temp1.Display() 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
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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