C＃3.5参数的Optional和DefaultValue

Question

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:

    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:

     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?

Answer 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.

Use this syntax in VS 2010 or newer:

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

Or this in older one:

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

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

Answer 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.

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

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.)

Answer 3

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

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