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?

Answer 1

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.

Answer 2

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) .. 
}

Answer 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.

Answer 4

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.

Answer 5

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
} 

Answer 6

There's three easy solutions to this one:

1. Overload the method
2. Allow the method to accept 'null', and handle appropriately
3. Use .NET 4, which allows optional parameters

Answer 7

Create another method which calls the first?

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

Answer 8

Overload the function. rather than checking for conditional branch. Something like this:

public void SendCommand(String command,string strfilename)
{    
    if (command == "STOR " + 
        Path.GetFileName(uploadfilename)) //Uploading file to Server
    {
        //code
    }
    else if ............
}

public void SendCommand(String command)
{ 
        //code

}

Answer 9

You can do this in a few ways. If you're using .NET 4.0, you can use optional parameters on the method:

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

Otherwise, you can just create another method which calls the method you already have but passing the default parameters you want to be optional:

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

Answer 10

Optional parameters feature come in c# 4.0 here is the link. Otherwise you have to write overloaded function.

Answer 11

All of the answers given on this page are valid ways of accepting optional parameters, but in many cases, this is a sign that your method is trying to do too much.

In many cases you may be better off with the following unambiguous methods...

public void GetFileListFromServer()
{
    //Listing Files from Server.
    //code
}

public void UploadFileToServer(string strfilename)
{
    //Uploading file to Server
    //code
}

Answer 12

You can use params keyword :

private static void SendCommand(String command, params string[] filenames)
{

    if (command == "NLST *" ) //Listing Files from Server.
    {
        //code
    }

    if (command == "STOR ")
    {
        foreach (string fileName in filenames)
        {
            if (String.IsNullOrWhiteSpace(fileName))
            {
                // code
            }
        }
    }
}

and you can use it as :

static void Main(string[] args)
{
    SendCommand("NLST *");
    SendCommand("STOR ", "myfile1.txt", "myfile.txt");
}

Answer 13

Using [option] attribute of Runtime.InteropServices namespace has not yet been proposed here.

Check 4 Different ways to make Method Parameter Optional in C#