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Invoking an executable from PowerShell with a dynamic number of parameters

Paul Stovell

Calling an executable from PowerShell is easy - most of the time, you just put an & in front. To illustrate, let’s take this C# executable:

static void Main(string[] args)
{
    for (int i = 0; i < args.Length; i++)
    {
        Console.WriteLine("[" + i + "] = '" + args[i] + "'");
    }
}

If we call it like this:

& .\Argsy.exe arg1 "argument 2"

We get:

[0] = 'arg1'
[1] = 'argument 2'

PowerShell variables can also be passed to arguments:

$myvariable = "argument 2"
& .\Argsy.exe arg1 $myvariable

# Output:
[0] = 'arg1'
[1] = 'argument 2'

Note that the value of $myvariable contained a space, but PowerShell was smart enough to pass the whole value as a single argument.

This gets tricky when you want to conditionally or dynamically add arguments. For example, you might be tempted to try this:

$args = ""
$environments = @("My Environment", "Production")
foreach ($environment in $environments) 
{
    $args += "--environment "
    $args += $environment + " "
}

& .\Argsy.exe $args

However, you’ll be disappointed with the output:

[0] = '--environment My Environment --environment Production '

The right way

The way to do this instead is to create an array. You can still use the += syntax in PowerShell to build the array:

$args = @() # Empty array
$environments = @("My Environment", "Production")
foreach ($environment in $environments) 
{
    $args += "--environment"
    $args += $environment
}
& .\Argsy.exe $args

Which outputs what we’d expect:

[0] = '--environment'
[1] = 'My Environment'
[2] = '--environment'
[3] = 'Production'

You can also mix regular strings with arrays:

& .\Argsy.exe arg1 "argument 2" $args

# Output:
[0] = 'arg1'
[1] = 'argument 2'
[2] = '--environment'
[3] = 'MyEnvironment'
[4] = '--environment'
[5] = 'Production'

Edge case

There’s is a very odd edge case to what I said above about passing a single string with all the arguments. Take this example, which is similar to the one above:

$args = "--project Foo --environment My Environment --environment Production"
& .\Argsy.exe $args

# Output: 
[0] = '--project Foo --environment My Environment --environment Production'

To make it work as intended, just put a quote around the first argument, and the behavior changes completely! (The backticks are PowerShell’s escape characters)

$args = "`"--project`" Foo --environment My Environment --environment Production"
& .\Argsy.exe $args

# Output: 
[0] = '--project'
[1] = 'Foo'
[2] = '--environment'
[3] = 'My'
[4] = 'Environment'
[5] = '--environment'
[6] = 'Production'

The behavior doesn’t change if the first argument isn’t quoted:

$args = "--project `"Foo`" --environment My Environment --environment Production"
& .\Argsy.exe $args

# Output: 
[0] = '--project Foo --environment My Environment --environment Production'

Ahh, PowerShell. Always full of surprises!

Paul Stovell

Founder and CEO. Paul started Octopus with his wife Sonia in 2012. In his spare time, he loves working on carpentry and DIY projects.

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