How to pass optional parameters to a method in C#?

Question

How to pass optional parameters to a method in C#?

Suppose I created one method called SendCommand

public void SendCommand(string command,string strfileName)
{
            
    if (command == "NLST *" ) //Listing Files from Server.
    {
        //code
    }
    else if (command == "STOR " + Path.GetFileName(uploadfilename)) //Uploading file to Server
    {
        //code
    }
    else if ...
}

Now I want to call this method in main method like

SendCommand("STOR ", filename);
SendCommand("LIST"); // In this case i don't want to pass the second parameter

How to achieve that?

Solution 1

Use the params attribute:

public void SendCommand(String command, params string[] strfilename)
{
}

then you can call it like this:

SendCommand("cmd");
SendCommand("cmd", "a");
SendCommand("cmd", "b");

or if you use C# 4.0 you can use the new optional arguments feature:

public void SendCommand(String command, string strfilename=null)
{ 
   if (strfilename!=null) .. 
}

Solution 2

Pre .NET 4 you need to overload the method:

public void sendCommand(String command)
{
    sendCommand(command, null);
}

.NET 4 introduces support for default parameters, which allow you to do all this in one line.

public void SendCommand(String command, string strfilename = null)
{
  //method body as in question
}

---

By the way, in the question as you have written it you aren't calling the method in your first example either:

Sendcommand("STOR " + filename);

is still using a single parameter which is the concatenation of the two strings.

Solution 3

The obvious answer for this should be, don't do it that way.

You should either have a separate method for each command, or a command base class and a separate derived class for each command, with an Execute method.

It's bad design to have one method that handles every conceivable command.

You really don't want one Sendcommand() to handle every possible command.

Solution 4

Check C# 4.0 Optional Parameters.

Also make sure you are using .NET 4.

If you need to use older versions of .NET.

Method overloading is the solution :

public void SendCommand(String command)
{
    SendCommand(command, null);
    // or SendCommand(command, String.Empty);
} 

public void SendCommand(String command, String fileName)
{
    // your code here
} 

Solution 5

Folks,

I was looking at this thread, trying to solve a problem that doesn't actually exist, because C# "just passes through" a params array! Which I didn't know until I just tried it.

Here's an SSCCE:

using System;
using System.Diagnostics; // for Conditional compilation of method CONTENTS

namespace ConsoleApplication3
{
    public static class Log
    {
        [Conditional("DEBUG")] // active in Debug builds only (a no-op in Release builds)
        public static void Debug(string format, params object[] parms) {
            Console.WriteLine(format, parms); 
            // calls Console.WriteLine(string format, params object[] arg);
            // which I presume calls String.Format(string format, params object[] arg);
            // (Sweet! just not what I expected ;-)
        }
    }

    class Program //LogTest
    {
        static void Main(string[] args) {
            Log.Debug("args[0]={0} args[1]={1}", "one", "two");
            Console.Write("Press any key to continue . . .");
            Console.ReadKey();
        }

    }
}

Produces:

args[0]=one args[1]=two

Sweet!

But why? ... Well because (of course) the closest parameter-match to the heavily-overloaded Console.WriteLine method is (string format, params object[] arg) ... not (string format, object arg) as I was thinking.

I sort-of knew this had to be possible somehow, because (I presume) Console.WriteLine does it, I just somehow expected it to be HARD... and therefore think that the simplicity and "niceness" of this trick-of-the-language is note worthy.

CSharpLanguageDesigners.ToList().ForEach(dude=>dude.Kudos++);

Cheers. Keith.

PS: I wonder if VB.NET behaves the same way? I suppose it must.

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Comments

[Adam McKee]

possible duplicate of How can you use optional parameters in C#?

[Kieren Johnstone]

Please see my answer: don't use just one method, use one method per command or a separate command class for each command

[Flynn1179]

I thought about this, but in this case, I'm not sure having one overload calling the other with 'null' is necessary, it seems likely that one set of commands would require the parameter, and another set wouldn't, so each overload just handles a different set of commands.

[BoltClock]

It's overloading, not overriding :)

[Swapnil Gupta]

If I have 100 commands, I have to write 100 methods in that case ?

[Kieren Johnstone]

Yes. It would be better to have 100 classes actually - one file with 100 methods is too many. Both of those options are a hundred times better than one method that handles the execution of 100 commands! Can you imagine how long that method would be, how hard it would be to debug, how many optional parameters it would have to have: probably indented so much it will be hard to see which { or } is which. You definitely, definitely, definitely don't want one method to handle all commands. One per command or one class per command.

[Kieren Johnstone]

Adding this comment here where it can be seen - please don't implement a single method which does many completely different things. One method should roughly-speaking do one task. That means simple commands might be in separate methods - or if commands are complex, create a class for each command. This is for maintainability, readability, debuggability, and more. SendCommand should also be named appropriately; does it actually send a command somewhere? If it is required, it could be called ParseCommand - it parses it and leaves execution to other methods/classes.

[MPaul]

Also note that in the event that you have multiple optional arguments, you can use a colon to designate which optional parameter you're referring to. Ex.: SendCommand("cmd", strfilename:"abc" );