call C# method from powershell without supplying optional arguments

Question

I have a c# method I am loading from a dll with optional string arguments that default to null. For example

public void foo(string path, string new_name = null, bool save_now = true) { if(name == null) new_name = Path.GetFileNameWithoutExtension(path); ... if(save_now) Save(); }

I want to call this from within a powershell script and not supply a value for new_name but one for save_now. As per this seemingly very similar question I have tried

$default = [type]::Missing $obj.foo($path, $default, $false)

but this results in new_name being set as "System.Reflection.Missing" within the function.

Additionally I tried

$obj.foo($path, $null, $false)

but this results in new_name being set to the empty string, still not null. I could set the default to the empty string, but I was wondering if there was any good way to actually have the default value be used.

I'm pretty sure you could build something to do this with reflection. GetMethod returns a MethodInfo, which has a GetProperty method. It returns a ParameterInfo which has DefaultValue, HasDefaultValue, and IsOptional properties. So you could develop a PS function, say "InvokeWithNamedParameters" that emulates what VB supports. — Χpẘ
– Χpẘ, Commented Mar 2, 2016 at 3:43
Interesting idea. For me unfortunately the window to try this solution has passed, but maybe someone else with the same question will find this a viable technique. — irh
– irh, Commented Mar 2, 2016 at 23:45

Keith Hill · Accepted Answer · 2014-07-02 02:10:01Z

No can do in PowerShell. It doesn't support C#/VB optional parameters. It is the duty of the language calling the method to provide the default values when the programmer doesn't and PowerShell just doesn't do that.

StephenP · Accepted Answer · 2014-07-02 00:16:34Z

You can simply omit the optional parameters in the call. I modified your example to run it in PS. For example:

$c = @" public static class Bar { public static void foo(string path, string new_name = null, bool save_now = true) { System.Console.WriteLine(path); System.Console.WriteLine(new_name); System.Console.WriteLine(save_now); } } "@ add-type -TypeDefinition $c [Bar]::Foo("test",[System.Management.Automation.Language.NullString]::Value,$false)

This generates the following

test False

Test was passed explicitly, null is null and had no output, and the save_now evaluated to the default of True.

Thanks for the reply, but this still misses my question. I do want to supply a value for the last optional parameter, but not the middle one. Also, my C# is being imported from a dll, though I don't know if that changes things.
I see what you mean now. You can use [System.Management.Automation.Language.NullString]::Value in v3 which should do what you want. Powershell likes to cast $null to be an empty string. References msdn.microsoft.com/en-us/library/… and connect.microsoft.com/PowerShell/feedback/details/307821/…

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