https://hake.hatenablog.com/entry/20170211/p1

簡単なGUI電卓を作成。
少々悩んだ点として、関数内でグローバル変数にアクセスする場合は、$varを$global:varとすること。でも、本当は引数経由でアクセスしたほうが良いのですが……。
あとボタンオブジェクトは、配列で処理するとソースがもっとすっきりするはず。

ソース

Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing

$number = "" # 被演算数値文字列
$op = "" # 演算記号文字列

function newButton {
    param(
        [Windows.Forms.Form]$form ,
        [string]$loc,
        [string]$size,
        [string]$caption
    )

    $btn = New-Object Windows.Forms.Button
    $btn.Location = $loc
    $btn.Size = $size
    $btn.Text = $caption
    $form.Controls.Add($btn)

    return $btn
}

function pushCLR{
    $txt.text  =  "0"
    $global:number = ""
    $global:op = ""
}

function pushNumber{
    param([string]$c) 
    if($txt.text -eq "0"){
        $txt.text  =  $c
    } else {
        $txt.text  +=  $c
    }    
}

function pushOp{
    param([string]$c) 

    if("+-*/".Contains($c)){
        $global:op = $c
        $global:number = $txt.Text
        $txt.Text = "0"
    }
}

function pushEq{
    if($global:op -eq ""){
        $global:number = $txt.Text
        $txt.Text = "0"
    } else {
        switch($global:op){
           "+" {$txt.Text = [double]$global:number + [double]$txt.Text; break}     
           "-" {$txt.Text = [double]$global:number - [double]$txt.Text; break}     
           "*" {$txt.Text = [double]$global:number * [double]$txt.Text; break}     
           "/" {$txt.Text = [double]$global:number / [double]$txt.Text; break}     
        }
    }
}

function pushDot{
    if(!$txt.text.Contains(".")){ $txt.text  +=  "." }
}




$font = New-Object Drawing.Font("ＭＳ ゴシック",12)

# Form
$form = New-Object Windows.Forms.Form 
$form.Text = "Calc"
$form.Size = "185,250" 
$form.Font = $font

# Text
$txt = New-Object Windows.Forms.TextBox
$txt.Location = "5,10"
$txt.Size = "160,20"
$txt.readonly = $true
$txt.TextAlign = [Windows.Forms.HorizontalAlignment]::Right
$form.Controls.Add($txt)

# Button
$btnC = newButton $form  "125,40" "40,30"  "C"
$btnC.Add_Click( {pushCLR} )
$btnC.Focus()

$btn7   = newButton $form    "5,70" "40,30"  "7"
$btn7.Add_Click( {pushNumber "7"} )
$btn8   = newButton $form   "45,70" "40,30"  "8"
$btn8.Add_Click( {pushNumber "8"} )
$btn9   = newButton $form   "85,70" "40,30"  "9"
$btn9.Add_Click( {pushNumber "9"} )
$btnDiv = newButton $form  "125,70" "40,30"  "/"
$btnDiv.Add_Click( {pushOp "/"} )

$btn4   = newButton $form    "5,100" "40,30"  "4"
$btn4.Add_Click( {pushNumber "4"} )
$btn5   = newButton $form   "45,100" "40,30"  "5"
$btn5.Add_Click( {pushNumber "5"} )
$btn6   = newButton $form   "85,100" "40,30"  "6"
$btn6.Add_Click( {pushNumber "6"} )
$btnMul = newButton $form  "125,100" "40,30"  "*"
$btnMul.Add_Click( {pushOp "*"} )

$btn1   = newButton $form    "5,130" "40,30"  "1"
$btn1.Add_Click( {pushNumber "1"} )
$btn2   = newButton $form   "45,130" "40,30"  "2"
$btn2.Add_Click( {pushNumber "2"} )
$btn3   = newButton $form   "85,130" "40,30"  "3"
$btn3.Add_Click( {pushNumber "3"} )
$btnSub = newButton $form  "125,130" "40,30"  "-"
$btnSub.Add_Click( {pushOp "-"} )

$btn0   = newButton $form    "5,160" "40,30"  "0"
$btn0.Add_Click( {pushNumber "0"} )
$btnDot = newButton $form   "45,160" "40,30"  "."
$btnDot.Add_Click( {pushDot} )
$btnEq  = newButton $form   "85,160" "40,30"  "="
$btnEq.Add_Click( {pushEq} )
$btnAdd = newButton $form  "125,160" "40,30"  "+"
$btnAdd.Add_Click( {pushOp "+"} )

# 表示
$form.ShowDialog()