Powershell给我“您不能在空值表达式上调用方法。”但工作原理不一致。 到底是怎么回事？

Question

TL:DR I have three nearly identical PowerShell pieces of code with different results.

I am a PowerShell newbie using it to make changes to an XML file. In the file below, I am trying to change the two android:color fields and the android:src field. (BTW, the following is for illustration purposes only. It is the result of my attempts to understand what is happening and how to get it to work)

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
<foo>
    <color android:color="#FFFFFF" />
</foo>
<item>
    <color android:color="#000000" />
</item>
<item>
    <bitmap android:src="@drawable/square150x150logo" android:tileMode="disabled" android:gravity="center" />
</item>
</layer-list>

The following is the snippet of code that I am using to make the desired changes:

Write-Host "Start test"

$xdoc."layer-list".foo.color.SetAttribute(
"color", 
"http://schemas.android.com/apk/res/android",
"FirstValue")
if(-not $?){
   Write-Host "one failed"
}

$xdoc."layer-list".item.color.SetAttribute(
"color", 
"http://schemas.android.com/apk/res/android",
"SecondValue")
if(-not $?){
    Write-Host "two failed"
}
$xdoc."layer-list".item.bitmap.SetAttribute(
"src", 
"http://schemas.android.com/apk/res/android",
"@drawable/Splash_150.png")
if(-not $?){
   Write-Host "three failed"
}

$xdoc.Save($SplashPath)
Write-Host "splashscreen.xml data updated"

This is the file after PowerShell has done its thing:

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
<foo>
    <color android:color="FirstValue" />
</foo>
<item>
    <color android:color="SecondValue" />
</item>
<item>
    <bitmap android:src="@drawable/square150x150logo" android:tileMode="disabled" android:gravity="center" />
</item>
</layer-list>

So, the first and second desired changes have been made. But the output from PowerShell indicates that both the second and third changes failed because of "You cannot call a method on a null-valued expression."

Here is the output from PowerShell:

1>  Start test
1>  FirstValue
1>  You cannot call a method on a null-valued expression.
1>  At C:\Users\will\Documents\XF\myApp\SetConfig.ps1:105 char:6
1>  +      $xdoc."layer-list".item.color.SetAttribute(
1>  +      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1>      + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
1>      + FullyQualifiedErrorId : InvokeMethodOnNull
1>   
1>  two failed
1>  You cannot call a method on a null-valued expression.
1>  At C:\Users\will\Documents\XF\myApp\SetConfig.ps1:113 char:7
1>  +       $xdoc."layer-list".item.bitmap.SetAttribute(
1>  +       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1>      + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
1>      + FullyQualifiedErrorId : InvokeMethodOnNull
1>   
1>  three failed
1>  splashscreen.xml data updated
1>  Ending Powershell

So, why did the second one work but PowerShell gave an error message? How can I get the third one to work? Thanks!

Answer 1

As per the XML you have two "item" XML elements. So I think you might need to be reference individual element using the array notation instead. See if the below code snippet works.

Write-Host "Start test"
$xdoc."layer-list".foo.color.SetAttribute(
"color", 
"http://schemas.android.com/apk/res/android",
"FirstValue")
if(-not $?){
   Write-Host "one failed"
}

$xdoc."layer-list".item[0].color.SetAttribute(
"color", 
"http://schemas.android.com/apk/res/android",
"SecondValue")
if(-not $?){
    Write-Host "two failed"
}
$xdoc."layer-list".item[1].bitmap.SetAttribute(
"src", 
"http://schemas.android.com/apk/res/android",
"@drawable/Splash_150.png")
if(-not $?){
   Write-Host "three failed"
}

$xdoc.Save($SplashPath)
Write-Host "splashscreen.xml data updated"