PowerShell Winforms脚本首次调用ShowDialog报空值表达式错误如何解决？

问题根因 #

你代码的执行顺序逻辑错误是核心问题：

• $Form_HomePage.ShowDialog() 是阻塞方法，运行后会一直停留在主窗体的消息循环，直到你关闭主窗体，这行之后的 $Form_UserInformation 定义代码才会执行
• 首次运行时点「Start」按钮，$Form_UserInformation 还没被创建，属于空变量，调用方法就会抛出空引用错误
• 第二次运行脚本时，上一次运行时创建的 $Form_UserInformation 变量还残留在PowerShell会话中，所以不会报错；但打包为EXE后每次运行都是全新会话，没有残留变量，所以必现崩溃

修复方案 #

1. 调整代码顺序：所有窗体的创建逻辑全部放到 ShowDialog 调用之前，确保用到窗体变量时已经完成初始化
2. 调整点击事件逻辑：先隐藏主窗体，再显示第二个窗体，避免主窗体停留在前台
3. 补充依赖加载和窗体关闭逻辑：补全Winform依赖程序集加载，同时添加第二个窗体关闭后的交互逻辑

修正后完整可运行代码如下：

# 加载Winform依赖程序集，避免部分环境运行报错
Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing

# 提前创建所有窗体
# User Information Form
$Form_UserInformation = New-Object System.Windows.Forms.Form
$Form_UserInformation.text = "New User Script"
$Form_UserInformation.Size =  New-Object System.Drawing.Size(400,400)
$Form_UserInformation.FormBorderStyle = "FixedDialog"
$Form_UserInformation.TopMost = $false
$Form_UserInformation.MaximizeBox = $false
$Form_UserInformation.MinimizeBox = $true
$Form_UserInformation.ControlBox = "true"
$Form_UserInformation.StartPosition = "CenterScreen"
$Form_UserInformation.Font = "Segoe UI"
# 补充第二个窗体关闭逻辑：关闭后自动回到主窗体
$Form_UserInformation.Add_FormClosed({
    $Form_HomePage.Show()
})

# HomePage Form
$Form_HomePage = New-Object System.Windows.Forms.Form
$Form_HomePage.text = "New User Script"
$Form_HomePage.Size =  New-Object System.Drawing.Size(400,400)
$Form_HomePage.FormBorderStyle = "FixedDialog"
$Form_HomePage.TopMost = $false
$Form_HomePage.MaximizeBox = $false
$Form_HomePage.MinimizeBox = $true
$Form_HomePage.ControlBox = "true"
$Form_HomePage.StartPosition = "CenterScreen"
$Form_HomePage.Font = "Segoe UI"

# Title label
$label_HomeTitle = New-Object System.Windows.Forms.Label
$label_HomeTitle.Location = New-Object System.Drawing.Size(47,8)
$label_HomeTitle.Size = New-Object System.Drawing.Size(300,42)
$label_HomeTitle.Font = New-Object System.Drawing.Font("Segoe UI",24,[System.Drawing.FontStyle]::Regular)
$label_HomeTitle.TextAlign = "MiddleCenter"
$label_HomeTitle.Text = "Kuehne & Nagel"
$Form_HomePage.Controls.Add($label_HomeTitle)

# Subheading label
$label_HomeSubTitle = New-Object System.Windows.Forms.Label
$label_HomeSubTitle.Location = New-Object System.Drawing.Size(50,60)
$label_HomeSubTitle.Size = New-Object System.Drawing.Size(300,42)
$label_HomeSubTitle.Font = New-Object System.Drawing.Font("Segoe UI",16,[System.Drawing.FontStyle]::Regular)
$label_HomeSubTitle.TextAlign = "MiddleCenter"
$label_HomeSubTitle.Text = "New User Script"
$Form_HomePage.Controls.Add($label_HomeSubTitle)

# Next button
$button_HomeStart = New-Object System.Windows.Forms.Button
$button_HomeStart.Location = New-Object System.Drawing.Size(75,200)
$button_HomeStart.Size = New-Object System.Drawing.Size(240,32)
$button_HomeStart.TextAlign = "MiddleCenter"
$button_HomeStart.Text = "Start"
$button_HomeStart.Add_Click({
    # 先隐藏主窗体，再显示第二个页面
    $Form_HomePage.Hide()
    [void] $Form_UserInformation.ShowDialog()
})
$Form_HomePage.Controls.Add($button_HomeStart)

# About button
$button_About = New-Object System.Windows.Forms.Button
$button_About.Location = New-Object System.Drawing.Size(75,250)
$button_About.Size = New-Object System.Drawing.Size(240,32)
$button_About.TextAlign = "MiddleCenter"
$button_About.Text = "About"
$button_About.Add_Click({
    
})
$Form_HomePage.Controls.Add($button_About)

# Exit button
$button_HomeExit = New-Object System.Windows.Forms.Button
$button_HomeExit.Location = New-Object System.Drawing.Size(75,300)
$button_HomeExit.Size = New-Object System.Drawing.Size(240,32)
$button_HomeExit.TextAlign = "MiddleCenter"
$button_HomeExit.Text = "Exit"
$button_HomeExit.Add_Click({
    $Form_HomePage.Close()
})
$Form_HomePage.Controls.Add($button_HomeExit)

# Show Form
$Form_HomePage.Add_Shown({$Form_HomePage.Activate()})
[void] $Form_HomePage.ShowDialog()

内容的提问来源于stack exchange，提问作者Harry