添加输出流时，布尔函数会更改返回的数据类型

Question

I'm having some trouble understanding the inner workings on powershell. Take this example. I have a boolean function (Test-MyCode), that works fine, as long as the Write-Output cmdlet is left out. Once the Write-Output cmdlet is added, the return type changes to a Object[] array.

.GetType() can be used to view the datatype.

Why is this?

function Test-MyCode
{
    if( 2 -gt 0)
    {
        Write-Output "This function will return false"
        return $false
    }
    else
    {
        Write-Output "could return true if condition is changed"
        return $true
    }
}

function Invoke-MyCode
{
    Write-Output "this is main"
    Write-Output "do stuff"
    # Test-MyCode is configured to return false... yet its true
    if (Test-MyCode)
    {
        Write-Output "yep, no longer boolean"
    }

}

Invoke-MyCode

Answer 1

From Get-Help Write-Output :

NAME
    Write-Output

SYNOPSIS
    Sends the specified objects to the next command in the pipeline. If the
    command is the last command in the pipeline, the objects are displayed in
    the console.

I think your main confusion stems from the fact that PowerShell functions are not statically bound to return a certain type .

The return keyword doesn't work exactly like in C# , but roughly means:

1. Call Write-Ouput on the results of the expression after the return keyword
2. Exit the current scope

See theabout_Return ,about_Functions andabout_Functions_OutputTypeAttribute help files for more information

---

In the simple example above, I would be attracted to creating a new custom object that contains both the string and the result, but the applicability of this "solution" may vary:

function Test-MyCode
{
    if( 2 -gt 0)
    {
        New-Object psobject -Property @{
            Message = "This function will return false"
            Result  = $false
        }
    }
    else
    {
        New-Object psobject -Property @{
            Message = "could return true if condition is changed"
            Result  = $true
        }
    }
}

function Invoke-MyCode
{
    Write-Host "this is main"
    Write-Host "do stuff"
    # Test-MyCode is configured to return false... yet its true
    if (($codetest = Test-MyCode).Result)
    {
        Write-Host $codetest.Message
    }
}

Invoke-MyCode

Notice how Write-Output is implied when I just "dump" an object onto the pipeline with New-Object