PowerShell And StringBuilder

问题

I am new in PowerShell but am familiar with .NET classes.

I am using System.Text.StringBuilder in PowerShell script. The script is that

Function MyStringFunc([String]$line) {
    $r = New-Object -TypeName "System.Collections.Generic.List``1[[System.String]]";
    $sb = New-Object -TypeName "System.Text.StringBuilder";

    foreach ($c in $line) {
        $sb.Append($c);
        $r.Add($sb.ToString());
    }

    return $r;
}

$line1 = "123";
$a = MyStringFunc $line1;
$a

I expected the result is

1
12
123

However the result is

                  Capacity                MaxCapacity                    Length
                  --------                -----------                    ------
                        16                 2147483647                         3
123

Did I do something wrong?

回答1:

Several of the methods on StringBuilder like Append IIRC, return the StringBuilder so you can call more StringBuilder methods. However the way PowerShell works is that it outputs all results (return values in the case of .NET method calls). In this case, cast the result to [void] to ignore the return value e.g.:

[void]$sb.Append($c)

Note that you don't need to end lines in ; in PowerShell. However if you put multiple commands on the same line then use ;' to separate those commands.

回答2:

Because you say your are new in PowerShell. My answer is a bit more on the way you are writting PowerShell.

PowerShell is a script language, it's used by non developper programmers such as administrators. You are a C# developper with all your knowledge, but you can write the thing you want to write more simply. In PowerShell it exists syntax to use Lists and Hastables, try the following :

$a = @()
$a += "Bonjour"
$a += "Salut"
$a
$a | get-member
Get-Member -InputObject $a

$rwc = @{}
$rwc += @{1="Blues"}
$rwc += @{2="Blacks"}
$rwc
$rwc | get-member
Get-Member -InputObject $rwc

Here are three functions doing the same thing, I know that stringbuilders are a bit more efficient in memory, but in this case who cares.

Function MyStringFunc([String]$line)
{ 
  $r = New-Object -TypeName "System.Collections.Generic.List``1[[System.String]]"; 
  $sb = New-Object -TypeName "System.Text.StringBuilder"; 

  foreach ($c in [Char[]]$line)
  { 
    $a = $sb.Append($c); 
    $r.Add($sb.ToString()); 
  }  
  return $r; 
} 

# A more simple way
Function MoreReadableStringFunction([String]$line)
{
  $r = @() # an untyped list

  foreach ($c in [Char[]]$line)
  { 
    $a += $c; 
    $r += $a;
  } 
  return $r;
}

# More simple but not so readable
Function MoreSimpleStringFunction([String]$line)
{
  $r = @() # an untyped list

  [Char[]]$line | % {$a += $_; $r += $a} 
  return $r;
}


Clear-Host
$line1 = "123";

$t1 = MyStringFunc $line1; 
$t1

$t2 = MoreReadableStringFunction $line1
$t2

$t3 = MoreSimpleStringFunction $line1
$t3

回答3:

foreach does not iterate through characters in a string, it treats it as a single item. So that we have to cast a string to [char[]] (or use $line.GetEnumerator()).
The expression $sb.Append($c) gets the builder instance. In PowerShell it gets written to the output. As far as it is unwanted we should suppress this output ($null = ... or ... > $null or ... | Out-Null).

The expected output is produced by this code:

Function MyStringFunc([String]$line) {
    $r = New-Object -TypeName "System.Collections.Generic.List``1[[System.String]]";
    $sb = New-Object -TypeName "System.Text.StringBuilder";

    foreach ($c in [char[]]$line) {
        $null = $sb.Append($c);
        $r.Add($sb.ToString());
    }

    return $r;
}

$line1 = "123";
$a = MyStringFunc $line1;
$a

来源：https://stackoverflow.com/questions/7801651/powershell-and-stringbuilder

标签

.net

powershell

stringbuilder