访问默认参数值

某些语言（如 C++ 托管扩展和 Microsoft Visual Basic .NET）支持将默认值赋给参数。例如，以下示例是一个合法的 Visual Basic .NET 声明，此声明将默认值赋给两个参数。

[Visual Basic]

Public Sub MyMethod (a as Integer, _

                     Optional b as Double = 1.2, _

                     Optional c as Integer=1)

您可以使用参数属性来分配默认的参数值。

通过确切指定哪些参数是默认值或略去尾部的默认参数，可以声明参数的默认值。例如，以下所有示例都是对 MyMethod 的有效调用。

[Visual Basic]

MyMethod(10, 55.3, 12)

MyMethod(10, 1.3) ' c == 1

MyMethod(11) ' b == 1.2, c == 1

[C#]

MyMethod (10, 55.3, 12);

MyMethod (10, 1.3); // c == 1

MyMethod (11); // b == 1.2, c == 1

要使用反射检索参数的默认值，请获取该参数的 ParameterInfo 对象，然后使用 ParameterInfo.DefaultValue 属性检索默认值。如果不存在默认值，该属性将返回 Value.DBNull。

以下示例向控制台显示 MyMethod 的默认值。

[Visual Basic]

Dim m As MethodInfo = t.GetMethod("MyMethod")

Dim ps As ParameterInfo() = m.GetParameters()

Dim i As Integer

For i = 0 To ps.Length - 1

    Console.WriteLine("Default Value == {0}", ps(i).DefaultValue)

Next i

[C#]

MethodInfo m = t.GetMethod ("MyMethod");

ParameterInfo[] ps = m.GetParameters();

for (int i = 0; i < ps.Length; i++) {

    Console.WriteLine("Default Value == {0}", ps[i].DefaultValue);

}

要调用包含具有默认值的参数的方法，请使用 Type.Missing 作为 InvokeMember 方法的参数值。这样，晚期绑定服务就能够为指定的参数值使用默认值。如果为不带默认值的参数传递 Type.Missing，则将引发 ArgumentException。有一点务必要注意，并非所有编译器的绑定机制都会遵守 Type.Missing 的这些规则。有些联编程序可能不支持此功能，或者可能以不同的方式来处理 Type.Missing。当使用 Type.Missing 时，默认值不必是结尾的参数值。

C# 语言不支持默认参数。

以下 Visual Basic .NET 示例显示如何调用具有默认参数的方法。

[Visual Basic]

Option Strict Off

Imports System

Imports System.Reflection

Public Class OptionalArg

    Public Sub MyMethod (a As Integer, Optional b As Double = 1.2, Optional c As Integer=1)

        Console.WriteLine("a = " & a & " b = " & b & " c = " & c)

    End Sub

End Class

Module Module1

    Sub Main()

        Dim o As New OptionalArg

        Dim t As Type

        t = GetType(OptionalArg)

        Dim Param As Object()= {10, 20, 30}

        t.InvokeMember("MyMethod", _

                        BindingFlags.Public Or _

                        BindingFlags.Instance Or _

                        BindingFlags.InvokeMethod Or _

                        BindingFlags.OptionalParamBinding, _

                        Nothing, _

                        o, _

                        New Object() {10, 55.3, 12})

        t.InvokeMember("MyMethod", _

                        BindingFlags.Public Or _

                        BindingFlags.Instance Or _

                        BindingFlags.InvokeMethod Or _

                        BindingFlags.OptionalParamBinding, _

                        Nothing, _

                        o, _

                        New Object() {10, 1.3, Type.Missing})

        t.InvokeMember("MyMethod", _

                        BindingFlags.Public Or _

                        BindingFlags.Instance Or _

                        BindingFlags.InvokeMethod Or _

                        BindingFlags.OptionalParamBinding, _

                        Nothing, _

                        o, _

                        New Object() {10, Type.Missing, Type.Missing})

    End Sub

End Module

当使用上述方法时，即使调用方未指定任何值，仍会考虑尾部的默认参数。这是调用具有默认参数的方法时最常用的方式。

如果是使用 MethodBase.Invoke 来调用方法，则需要显式指定哪些参数是默认值，指定的方法是为所有没有值的参数传递一个包含 Type.Missing 的对象数组。

Default parameters should not be used

Cause: An externally visible type contains an externally visible method that uses a default parameter.

Rule Description

Methods that use default parameters are allowed under the Common Language Specification (CLS); however, the CLS allows compilers to ignore the values assigned to these parameters. Code written for compilers that ignore default parameter values must explicitly provide arguments for each default parameter. To maintain the behavior that you want across programming languages, methods that use default parameters should be replaced with method overloads that provide the default parameters.

The C# compiler ignores the values of default parameters as does the Managed C++ compiler when accessing managed code. The Visual Basic compiler supports methods with default parameters using the Optional keyword.

How to Fix Violations

To fix a violation of this rule, replace the method that uses default parameters with method overloads that supply the default parameters.

When to Exclude Messages

Do not exclude a message from this rule.

Example Code

The following example shows a method that uses default parameters, and the overloaded methods that provide an equivalent functionality.

[Visual Basic]

Imports System

<Assembly: CLSCompliant(True)>

Namespace DesignLibrary

    Public Class DefaultVersusOverloaded

        Sub DefaultParameters(Optional parameter1 As Integer = 1, _

                              Optional parameter2 As Integer = 5)

            ' ...

            Console.WriteLine("{0} : {1}", parameter1, parameter2)

        End Sub

        Sub OverloadedMethod()

            OverloadedMethod(1, 5)

        End Sub

        Sub OverloadedMethod(parameter1 As Integer)

            OverloadedMethod(parameter1, 5)

        End Sub

        Sub OverloadedMethod(parameter1 As Integer, parameter2 As Integer)

            ' ...

            Console.WriteLine("{0} : {1}", parameter1, parameter2)

        End Sub

    End Class

End Namespace

Related Rules

Replace repetitive arguments with params array

See Also

Method Usage Guidelines | Common Language Specification