How can you use optional parameters in C#?

Surprised no one mentioned C# 4.0 optional parameters that work like this:

public void SomeMethod(int a, int b = 0)
{
   //some code
}

Edit: I know that at the time the question was asked, C# 4.0 didn't exist. But this question still ranks #1 in Google for "C# optional arguments" so I thought - this answer worth being here. Sorry.

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Another option is to use the params keyword

public void DoSomething(params object[] theObjects)
{
  foreach(object o in theObjects)
  {
    // Something with the Objects…
  }
}

Called like...

DoSomething(this, that, theOther);

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In C#, I would normally use multiple forms of the method:

void GetFooBar(int a) { int defaultBValue;  GetFooBar(a, defaultBValue); }
void GetFooBar(int a, int b)
{
 // whatever here
}

UPDATE: This mentioned above WAS the way that I did default values with C# 2.0. The projects I'm working on now are using C# 4.0 which now directly supports optional parameters. Here is an example I just used in my own code:

public EDIDocument ApplyEDIEnvelop(EDIVanInfo sender, 
                                   EDIVanInfo receiver, 
                                   EDIDocumentInfo info,
                                   EDIDocumentType type 
                                     = new EDIDocumentType(EDIDocTypes.X12_814),
                                   bool Production = false)
{
   // My code is here
}

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From this site:

http://www.tek-tips.com/viewthread.cfm?qid=1500861&page=1

C# does allow the use of the [Optional] attribute (from VB, though not functional in C#). So you can have a method like this:

using System.Runtime.InteropServices;
public void Foo(int a, int b, [Optional] int c)
{
  ...
}

In our API wrapper, we detect optional parameters (ParameterInfo p.IsOptional) and set a default value. The goal is to mark parameters as optional without resorting to kludges like having "optional" in the parameter name.

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You could use method overloading...

GetFooBar()
GetFooBar(int a)
GetFooBar(int a, int b)

It depends on the method signatures, the example I gave is missing the "int b" only method because it would have the same signature as the "int a" method.

You could use Nullable types...

GetFooBar(int? a, int? b)

You could then check, using a.HasValue, to see if a parameter has been set.

Another option would be to use a 'params' parameter.

GetFooBar(params object[] args)

If you wanted to go with named parameters would would need to create a type to handle them, although I think there is already something like this for web apps.

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You can use optional parameters in C# 4.0 without any worries. If we have a method like:

int MyMetod(int param1, int param2, int param3=10, int param4=20){....}

when you call the method, you can skip parameters like this:

int variab = MyMethod(param3:50; param1:10);

C# 4.0 implements a feature called "named parameters", you can actually pass parameters by their names, and of course you can pass parameters in whatever order you want :)

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An easy way which allows you to omit any parameters in any position, is taking advantage of nullable types as follows:

public void PrintValues(int? a = null, int? b = null, float? c = null, string s = "")
{
    if(a.HasValue)
        Console.Write(a);
    else
        Console.Write("-");

    if(b.HasValue)
        Console.Write(b);
    else
        Console.Write("-");

    if(c.HasValue)
        Console.Write(c);
    else
        Console.Write("-");

    if(string.IsNullOrEmpty(s)) // Different check for strings
        Console.Write(s);
    else
        Console.Write("-");
}

Strings are already nullable types so they don't need the ?.

Once you have this method, the following calls are all valid:

PrintValues (1, 2, 2.2f);
PrintValues (1, c: 1.2f);
PrintValues(b:100);
PrintValues (c: 1.2f, s: "hello");
PrintValues();

When you define a method that way you have the freedom to set just the parameters you want by naming them. See the following link for more information on named and optional parameters:

Named and Optional Arguments (C# Programming Guide) @ MSDN

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Hello Optional World

If you want the runtime to supply a default parameter value, you have to use reflection to make the call. Not as nice as the other suggestions for this question, but compatible with VB.NET.

using System;
using System.Runtime.InteropServices;
using System.Reflection;

namespace ConsoleApplication1
{
    class Class1
    {
        public static void sayHelloTo(
            [Optional,
            DefaultParameterValue("world")] string whom)
        {
            Console.WriteLine("Hello " + whom);
        }

        [STAThread]
        static void Main(string[] args)
        {
            MethodInfo mi = typeof(Class1).GetMethod("sayHelloTo");
            mi.Invoke(null, new Object[] { Missing.Value });
        }
    }
}

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I agree with stephenbayer. But since it is a webservice, it is easier for end-user to use just one form of the webmethod, than using multiple versions of the same method. I think in this situation Nullable Types are perfect for optional parameters.

public void Foo(int a, int b, int? c)
{
  if(c.HasValue)
  {
    // do something with a,b and c
  }
  else
  {
    // do something with a and b only
  }  
}

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optional parameters are for methods. if you need optional arguments for a class and you are:

using c# 4.0: use optional arguments in the constructor of the class, a solution i prefer, since it's closer to what is done with methods, so easier to remember. here's an example: class myClass { public myClass(int myInt = 1, string myString = "wow, this is cool: i can have a default string") { // do something here if needed } }

using c# versions previous to c#4.0: you should use constructor chaining (using the :this keyword), where simpler constructors lead to a "master constructor". example: class myClass { public myClass() { // this is the default constructor } public myClass(int myInt) : this(myInt, "whatever") { // do something here if needed } public myClass(string myString) : this(0, myString) { // do something here if needed } public myClass(int myInt, string myString) { // do something here if needed - this is the master constructor } }

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The typical way this is handled in C# as stephen mentioned is to overload the method. By creating multiple versions of the method with different parameters you effectively create optional parameters. In the forms with fewer parameters you would typically call the form of the method with all of the parameters setting your default values in the call to that method.

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Using overloads or using C# 4.0 or above

 private void GetVal(string sName, int sRoll)
 {
   if (sRoll > 0)
   {
    // do some work
   }
 }

 private void GetVal(string sName)
 {
    GetVal("testing", 0);
 }

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You can overload your method. One method contains one parameter GetFooBar(int a) and the other contain both parameters, GetFooBar(int a, int b)

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You can use default.

public void OptionalParameters(int requerid, int optinal = default){}

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For a larger number of optional parameters, a single parameter of Dictionary<string,Object> could be used with the ContainsKey method. I like this approach because it allows me to pass a List<T> or a T individually without having to create a whole other method (nice if parameters are to be used as filters, for example).

Example (new Dictionary<string,Object>() would be passed if no optional parameters are desired):

public bool Method(string ParamA, Dictionary<string,Object> AddlParams) {
    if(ParamA == "Alpha" && (AddlParams.ContainsKey("foo") || AddlParams.ContainsKey("bar"))) {
        return true;
    } else {
        return false;
    }
}

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Instead of default parameters, why not just construct a dictionary class from the querystring passed .. an implementation that is almost identical to the way asp.net forms work with querystrings.

i.e. Request.QueryString["a"]

This will decouple the leaf class from the factory / boilerplate code.

You also might want to check out Web Services with ASP.NET. Web services are a web api generated automatically via attributes on C# classes.

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A little late to the party, but I was looking for the answer to this question and ultimately figured out yet another way to do this. Declare the data types for the optional args of your web method to be type XmlNode. If the optional arg is omitted this will be set to null, and if it's present you can get is string value by calling arg.Value, i.e.,

[WebMethod]
public string Foo(string arg1, XmlNode optarg2)
{
    string arg2 = "";
    if (optarg2 != null)
    {
        arg2 = optarg2.Value;
    }
    ... etc
}

What's also decent about this approach is the .NET generated home page for the ws still shows the argument list (though you do lose the handy text entry boxes for testing).

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I have a web service to write that takes 7 parameters. Each is an optional query attribute to a sql statement wrapped by this web service. So two workarounds to non-optional params come to mind... both pretty poor:

method1(param1, param2, param 3, param 4, param 5, param 6, param7) method1(param1, param2, param3, param 4, param5, param 6) method 1(param1, param2, param3, param4, param5, param7)... start to see the picture. This way lies madness. Way too many combinations.

Now for a simpler way that looks awkward but should work: method1(param1, bool useParam1, param2, bool useParam2, etc...)

That's one method call, values for all parameters are required, and it will handle each case inside it. It's also clear how to use it from the interface.

It's a hack, but it will work.

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I had to do this in a VB.Net 2.0 Web Service. I ended up specifying the parameters as strings, then converting them to whatever I needed. An optional parameter was specified with an empty string. Not the cleanest solution, but it worked. Just be careful that you catch all the exceptions that can occur.

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For just in case if someone wants to pass a callback (or delegate) as an optional parameter, can do it this way.

Optional Callback parameter:

public static bool IsOnlyOneElement(this IList lst, Action callbackOnTrue = (Action)((null)), Action callbackOnFalse = (Action)((null)))
{
    var isOnlyOne = lst.Count == 1;
    if (isOnlyOne && callbackOnTrue != null) callbackOnTrue();
    if (!isOnlyOne && callbackOnFalse != null) callbackOnFalse();
    return isOnlyOne;
}

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optional parameters are nothing but default parameters! i suggest you give both of them default parameters. GetFooBar(int a=0, int b=0) if you don't have any overloaded method, will result in a=0, b=0 if you don't pass any values,if you pass 1 value, will result in, passed value for a, 0 and if you pass 2 values 1st will be assigned to a and second to b.

hope that answers your question.

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In the case when default values aren't available the way to add an optional parameter is to use .NET OptionalAttribute class - https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.optionalattribute?view=netframework-4.8

Example of the code is below:

namespace OptionalParameterWithOptionalAttribute
{
    class Program
    {
        static void Main(string[] args)
        {
            //Calling the helper method Hello only with required parameters
            Hello("Vardenis", "Pavardenis");
            //Calling the helper method Hello with required and optional parameters
            Hello("Vardenis", "Pavardenis", "Palanga");
        }
        public static void Hello(string firstName, string secondName, 
            [System.Runtime.InteropServices.OptionalAttribute] string  fromCity)
        {
            string result = firstName + " " + secondName;
            if (fromCity != null)
            {
                result += " from " + fromCity;
            }
            Console.WriteLine("Hello " + result);
        }

    }
}

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You can try this too
Type 1
public void YourMethod(int a=0, int b = 0) { //some code }

Type 2
public void YourMethod(int? a, int? b) { //some code }

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