optional array Parameter in C#

Question

Possible Duplicate:
passing an empty array as default value of optional parameter in c#

I have a method that looks like below. Currently, the parameter tags is NOT optional

void MyMethod(string[] tags=null)
{
   tags= tags ?? new string[0];
   /*More codes*/
}

I want to make parameter tags optional, as per c# , to make a parameter optional you can set a default value in method signature. I tried the following hacks but none worked.

Code that didn't work - 1

void MyMethod(string[] tags=new string[0]){}

Code that didn't work - 2

void MyMethod(string[] tags={}){}

Please suggest what I am missing.

I have already seen this question:

Passing an empty array as default value of an optional parameter

Take a look here: http://stackoverflow.com/questions/3480382/passing-an-empty-array-as-default-value-of-optional-parameter-in-c-sharp — nowaq, Nov 21 '11 at 14:58
in this particular case, you might get away with C#3.0 compatible `void MyMethod(params string[] tags)` — sehe, Nov 21 '11 at 14:58
What's wrong with `tags = tags ?? new string[0];` anyway? And if you have already seen the question, the accepted answer clearly states that you *can't* set a default value with the C# 4 syntax, and that you can use the `??` operator instead as you are doing. — BoltClock, Nov 21 '11 at 14:58
What's wrong with `string[] tags=null` exactly? It does work. — H H, Nov 21 '11 at 15:01
@boltclock with `tags = tags ?? new string[0];` tags is not optional. — Rusi Nova, Nov 21 '11 at 15:01
@Rusi Nova: It is optional if you set a default value of `null`. — BoltClock, Nov 21 '11 at 15:02
@RusiNova, we might be using a different definition of optional. To be clear, as far as the language is concerned, `void Foo(string[] tags = null)` indeed makes supplying the argument *optional* for the caller. Whether or not the null is *usable* inside your method is a different matter. — Anthony Pegram, Nov 21 '11 at 15:03

score 49 · Accepted Answer · answered Nov 21 '11 at 14:58

49

The documentation for optional arguments says:

A default value must be one of the following types of expressions:

a constant expression;

an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;

an expression of the form default(ValType), where ValType is a value type.

Since new string[0] is neither a constant expression nor a new statement followed by a value type, it cannot be used as a default argument value.

The first code excerpt in your question is indeed a good workaround:

void MyMethod(string[] tags = null)
{
   tags = tags ?? new string[0];
   // Now do something with 'tags'...
}

answered Nov 21 '11 at 14:58

Frédéric Hamidi

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+1 And the workaround is suggested in the accepted answer to the duplicate question. – BoltClock Nov 21 '11 at 15:00
1

This is correct but if you use MyMethod(params Object[] parameterName) You can omit the params array and the compiler will pass an empty array for you. MyMethod() works just fine. – jwize Feb 15 '14 at 09:08
Upvoted for linking and in-lining the relevant authoritative documentation. "Teach a man to fish..." – aaronsnoswell Oct 19 '17 at 23:44

score 5 · Answer 2 · answered Nov 21 '11 at 15:02

not sure if i am getting it right this works.

  static void Main(string[] args)
        {
                TestMe();

        }


private static void TestMe(string[] param = null)
    {
        if (param == null)
        { 
            Console.WriteLine("its empty");
        }
    }

also the value of param has to be compile time constant

optional array Parameter in C#

2 Answers2

Linked