C＃3.5参数的Optional和DefaultValue

I am using C# .net 3.5 to build an application. I have been working with optional parameter attributes in .net 4.0 with no problems. I did notice that with 3.5 there is the option (workaround) to add the following attributes to your method like so:

我正在使用C＃.net 3.5来构建应用程序。我一直在使用.net 4.0中的可选参数属性，没有任何问题。我注意到3.5使用选项（解决方法）将以下属性添加到您的方法中，如下所示：

    public static void MethodName(string name, [Optional][DefaultValue(null)]string placeHolder)
    {

    }

Even though I have added the attributes to the method, if I try and call it like so:

即使我已经将属性添加到方法中，如果我尝试这样调用它：

     MethodName("test");

The compiler will complain that it is looking for two parameters instead of one. Is it actually possible to do this using C# .net 3.5? Am I doing something wrong?

编译器会抱怨它正在寻找两个参数而不是一个。实际上是否可以使用C＃.net 3.5执行此操作？难道我做错了什么？

3 个解决方案

#1

Optional parameters are C# 4.0 language feature so it doesn't matter which framework you are targeting, but you have to compile it using VS 2010 or newer.

可选参数是C＃4.0语言功能，因此无论您要定位哪个框架都无关紧要，但您必须使用VS 2010或更新版本进行编译。

Use this syntax in VS 2010 or newer:

在VS 2010或更高版本中使用此语法：

public static void MethodName(string name, string placeHolder = null)
{
    // body
}

Or this in older one:

或者在旧版本中：

public static void MethodName(string name, string placeHolder)
{
    // body
}

public static void MethodName(string name)
{
    MethodName(name, null);
}

#2

The Optional attribute has been available since C# 1.0, and is used when interoperating with external code, it has no effect on method calls in your own code.

Optional属性自C＃1.0起可用，并且在与外部代码互操作时使用，它对您自己的代码中的方法调用没有影响。

As there is no optional parameters in C# 3, you can use overloading instead:

由于C＃3中没有可选参数，您可以使用重载：

public static void MethodName(string name, string placeHolder) {
  ...
}

public static void MethodName(string name) {
  MethodName(name, null);
}

(Side note: There is no C# version 3.5, that is a framework version.)

（旁注：没有C＃3.5版，这是一个框架版本。）

#3

Take a look at the following * thread: C# Optional Parameters in .net 3.5

看看下面的*线程：.net 3.5中的C＃可选参数

No use in copy pasting everything which has been said there, as the thread covers pretty much everything. Good luck.

没有使用复制粘贴已经说过的所有内容，因为该主题涵盖了几乎所有内容。祝你好运。

#1