optional array Parameter in C# [duplicate]

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

in this particular case, you might get away with C#3.0 compatible void MyMethod(params string[] tags) — sehe
– sehe, Commented Nov 21, 2011 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
– BoltClock, Commented Nov 21, 2011 at 14:58
@Rusi Nova: It is optional if you set a default value of null. — BoltClock
– BoltClock, Commented Nov 21, 2011 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
– Anthony Pegram, Commented Nov 21, 2011 at 15:03

Frédéric Hamidi · Accepted Answer · 2011-11-21 14:58:28Z

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'... }

+1 And the workaround is suggested in the accepted answer to the duplicate question.
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.
Upvoted for linking and in-lining the relevant authoritative documentation. "Teach a man to fish..."

np-hard · Accepted Answer · 2011-11-21 15:02:56Z

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

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