如何在方法中放置可选参数？

Question

Is it possible to declarate optional parameters in methods?

In delphi,for example,I can do something like:

procedure Test(p1:integer;p2:integer;p3:integer = 2;p4:integer = 4)

When I call that function I can call it with four parameters or with two parameters:

Test(2,3); //p3 = 2,p4 = 4.
Test(2,3,4,5); //p3 = 4,p4 = 5;

How is that possible in C#?

Answer 1

I'm afraid this isn't possible in C# 1 to 3. However, the good news is that because it's been a much-demanded feature (though there are certainly some who would rather not see it), Microsoft have finally decided to add it to C# 4.

The C# 4 syntax goes as follows:

public static void SayHello(string s = "Hello World!")
{
    Console.WriteLine(s);
}

Usage:

SayHello(); // Prints "Hello World!"
SayHello("Hello."); // Prints "Hello."
SayHello(s: "Hello."); // Prints "Hello."

(The last example uses a named argument, which really isn't necessary in this case, but helps when you have multiple optional parameters.)

You can read more about that subject on this blog post .

Answer 2

You'll either have to wait for C# 4.0 , which supports optional parameters, or use standard overloading mechanisms:

void Test(int p1, int p2, int p3, int p4)
{ }

void Test(int p1, int p2)
{
    Test(p1, p2, 2, 4);
}

Answer 3

It will be possible in C# 4.0, but, untill that gets released, you can work around it by creating overloaded versions of your method:

public void MyMethod( int a, int b, int c )
{
    ...
}

public void MyMethod( int a, int b)
{
   MyMethod(a, b, 4);
}

Answer 4

You can use variable arguments

Use the params keyword.

void paramsExample(params int[] argsRest)
{

 if(argsRest.Length == 2) then...
 else if(argsRest.Length == 4) then...
 else error...

}

Answer 5

using System;
using System.Runtime.InteropServices;

   public static void SayHello([Optional][DefaultParameterValue("Hello Universe!")] string s)
   {
      Console.WriteLine(s);
   }

Done! :)

Answer 6

You can't do that yet. I think it's a feature of C# 4.0 .

You can use params as a work around, but that can only be used sequentially, not the way some languages treat default parameters.

Answer 7

You use overloads like this

Test(int p1, int p2)
Test(int p1, int p2, int p3)

You can have them call a private method like this

Test(int[] ps)

and then process the data.

Another way to handle this is to NOT use overloads/optional parameters, and instead name the methods according to what they are ment to do - it might seem like a bad tradeoff, but your code will probably get easier to read.