.net - Difference between @($null) and $null -

i have powershell code invoking .net method expects class passed parameter.

the method accepts null value parameter, , in c# you'd call follows:

var t = new foo(null) 

if translating powershell, 1 try:

$t = new-object foo($null) 

but returns following error

new-object : constructor not found. cannot find appropriate constructor type foo 

funilly enough, exact same error returned if call as:

$t = new-object foo 

so first question why? .net see $null absence of something, oppose type null .net? why 2 instructions above return same error?

now, found way around problem follows:

$t = new-object foo(@($null)) 

this works fine, but... why?

(@($null)).gettype() returns object[]

(@()).gettype() return object[], if run:

$t = new-object foo(@()) 

i still get:

new-object : constructor not found. cannot find appropriate constructor type foo 

so, second question: difference between @(), @($null) , $null?

also, , last question, else don't understand.

let's have .net method bar(string, foo)

if call:

$b = new-object bar("s", $null) 

this works fine, without having convert object[] or alike.

so third , last question is: why number of parameters affect whether needs passed array or not?

my guess if cast array call as

$b = new-object bar(@("s", $null)) 

meaning though method expects 2 parameters, i'm passing 1 array of objects , automatically maps each of array items onto parameters, still doesn't quite provide answer first 2 questions :)

edit

i know @() empty array , @($null) array 1 element happens null, that's not i'm asking.

however, if calling:

$t = new-object foo($null)

makes foo's constructor see no parameter being passed, why calling @() vs calling @($null) differ? surely @($null) should same @() in case, absence of parameter gets seen in .net method.

why $null mean , nothing @ all?

based on edit, have understanding of differences between $null , @($null).

unlike normal call .net method, calling constructor requires calling new-object cmdlet first. syntax new-object is:

new-object [-typename] <string> [[-argumentlist] <object[]>] [-property <idictionary>] 

let's think moment how new-object works. using reflection, new-object call typename.getconstructors() , find candidate constructors can accept n arguments n number of arguments in argumentlist.

if call new-object this:

new-object -typename foo 

then argumentlist has value $null. didn't pass $null, that's it's value anyway. if argumentlist $null, there no arguments. in example, that's obvious, didn't specify -argumentlist. if write:

new-object -typename foo -argumentlist $null 

this ends same end result - no argument list. argumentlist expects [object[]], , $null valid value [object[]], means there no arguments.

if write:

new-object -typename foo -argumentlist @($null) 

now passing [object[]] single value, value $null, , works expect.

this undoubtedly confusing, helps think of how might implement own c# function calls function , accepts param array. if param array null, received no arguments.

new-object can more confusing in cases. tripped overloaded constructors, 1 taking array, other taking multiple arguments. in c#, constructors like:

foo(object[] array);   // #1 foo(string s, int i);  // #2 

and incorrect attempt call constructor #1:

new-object -typename foo -argumentlist @($array) 

this calls constructor #2 because new-object expects [object[]] pass multiple arguments , want call constructor single array argument. correct powershell be:

new-object -typename foo -argumentlist (,$array) 

because of confusion, added new way call constructors in powershell v5. it's available preview (http://www.microsoft.com/en-us/download/details.aspx?id=42936), syntax looks static method call:

[foo]::new($name, $count) 

with new syntax, calling constructors no longer confusing, , side benefit, it's order of magnitude faster calling new-object.

edit: corrected example overloaded constructors.