如何正確調用powershell函數？

Question

我寫了一個函數將輸入的月份參數轉換爲某種格式。例如如果我將04傳遞給該函數並且該函數將返回「_APR _」。我寫的函數是象下面這樣：

function GetEnMonth() 
{ 
param([string] $month) 
switch ($month) 
{ 
    ($_ -eq "01"){$result = "_JAN_"} 
    ($_ -eq "02"){$result = "_FEB_"} 
    ($_ -eq "03"){$result = "_MAR_"} 
    ($_ -eq "04"){$result = "_APR_"} 
    ($_ -eq "05"){$result = "_MAY_"} 
    ($_ -eq "06"){$result = "_JUN_"} 
    ($_ -eq "07"){$result = "_JUL_"} 
    ($_ -eq "08"){$result = "_AUG_"} 
    ($_ -eq "09"){$result = "_SEP_"} 
    ($_ -eq "10"){$result = "_OCT_"} 
    ($_ -eq "11"){$result = "_NOV_"} 
    ($_ -eq "12"){$result = "_DEC_"} 
    default {$result ="_No_Result_"} 
} 
return [string]$result; 
} 

然後我用下面的命令來執行的函數來得到結果：

$mYear = $today.substring(0,4) 
$mMonth =$today.substring(4,2) 
$mDate = $today.substring(6,2) 
$monthInEn = GetEnMonth $mMonth 

好，結果始終是「_No_Result_」，爲什麼呢？下面是例外：

**** Exception type : System.Management.Automation.RuntimeException 
**** Exception message : You cannot call a method on a null-valued expression. 

有人能給我這個答案嗎？ 我已經搜索了很多Google，但沒有找到有用的解決方案。

Answer 1

你的switch敘述是錯誤試試這個：

function GetEnMonth() 
{ 
param([string] $month) 
switch ($month) 
{ 
    "01" {$result = "_JAN_"} 
    "02" {$result = "_FEB_"} 
    "03" {$result = "_MAR_"} 
    "04" {$result = "_APR_"} 
    "05" {$result = "_MAY_"} 
    "06" {$result = "_JUN_"} 
    "07" {$result = "_JUL_"} 
    "08" {$result = "_AUG_"} 
    "09" {$result = "_SEP_"} 
    "10" {$result = "_OCT_"} 
    "11" {$result = "_NOV_"} 
    "12" {$result = "_DEC_"} 
    default {$result ="_No_Result_"} 
} 
return [string]$result; 
} 

Answer 2

這應該工作：

function GetEnMonth 
{ 
param([string] $month) 
switch ($month) 
{ 
    "01"{$result = "_JAN_"} 
    "02"{$result = "_FEB_"} 
    "03"{$result = "_MAR_"} 
    "04"{$result = "_APR_"} 
    "05"{$result = "_MAY_"} 
    "06"{$result = "_JUN_"} 
    "07"{$result = "_JUL_"} 
    "08"{$result = "_AUG_"} 
    "09"{$result = "_SEP_"} 
    "10"{$result = "_OCT_"} 
    "11"{$result = "_NOV_"} 
    "12"{$result = "_DEC_"} 
    default {$result ="_No_Result_"} 
} 
return [string]$result; 
} 

Answer 3

這不是一個答案，但另一種方式做同在一行，並與文化param：

$month = 3 
$smonth=[string]::format([System.Globalization.CultureInfo]"en-US", "_{0:MMM}_",[datetime]("$month/01")) 
$smonth 
_MAR_ 

Answer 4

這是我對此的看法。我將參數類型更改爲Int，使用這個參數可以獲得諸如「03」或3的過程。我還添加了break語句以使交換機工作更快。

function GetEnMonth 
{ 
    param([int] $month) 

    switch ($month) 
    { 
     1 {"_JAN_"; break} 
     2 {"_FEB_"; break} 
     3 {"_MAR_"; break} 
     4 {"_APR_"; break} 
     5 {"_MAY_"; break} 
     6 {"_JUN_"; break} 
     7 {"_JUL_"; break} 
     8 {"_AUG_"; break} 
     9 {"_SEP_"; break} 
     10 {"_OCT_"; break} 
     11 {"_NOV_"; break} 
     12 {"_DEC_"; break} 
     default {"_No_Result_"} 
    } 
}