Declaring a variable s type in VB.NET

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Declaring a variable s type
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Figure 15.2 Using ForEach-Object to execute a method against each object contained within a variable
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each object. The new String objects produced by ToLower() will be placed into the pipeline and into the $computers variable. You can do something similar with properties, by using Select-Object. This will select the Length property of each object that I pipe to the cmdlet:
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PS C:\> $computers | select-object length Length -----9 7 9
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Because the property is numeric, PowerShell right-aligns the output.
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15.5 Declaring a variable s type
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So far, we ve just stuck objects into variables and let PowerShell figure out what kind of object was what. The fact is that PowerShell doesn t care what kinds of objects get put into the box. You, however, might care. For example, suppose you have a variable that you expect to contain a number. You plan to do some arithmetic with that number, and you ask a user to input that number. Here s an example, which you can type directly into the command line:
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PS C:\> $number = Read-Host "Enter a number" Enter a number: 100 PS C:\> $number = $number * 10 PS C:\> $number 100100100100100100100100100100
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TRY IT NOW I haven t showed you Read-Host yet I m saving it for the next
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chapter but its operation should be pretty obvious if you follow along with this example. What the heck 100 multiplied by 10 is 100100100100100100100100100100 What crazy New Math is that If you re sharp-eyed, you may have spotted what s happening. PowerShell didn t treat my input as a number; it treated it as a string. Instead of multiplying 100 by 10, PowerShell duplicated the string 100 ten times. So the result is the string 100, listed ten times in a row. Oops.
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Variables: a place to store your stuff
We can verify that the shell is in fact treating the input as a string:
PS C:\> $number = Read-Host "Enter a number" Enter a number: 100 PS C:\> $number | gm TypeName: System.String Name ---Clone CompareTo Contains MemberType ---------Method Method Method Definition ---------System.Object Clone() int CompareTo(System.Object valu... bool Contains(string value)
Yep, piping $number to Gm confirms that the shell sees it as a System.String, not a System.Int32. There are a couple of ways that we could choose to deal with this problem, and the easiest for me is the one we ll use right now. I m going to tell the shell that the $number variable should contain an integer, which will force the shell to try to convert any input to an actual number. I do that by specifying the desired data type, int, in square brackets immediately prior to the variable s first use:
PS C:\> [int]$number = Read-Host "Enter a number" Enter a number: 100 PS C:\> $number | gm Confirm that
Force variable to [int]
TypeName: System.Int32 Name ---CompareTo Equals GetHashCode GetType GetTypeCode ToString MemberType ---------Method Method Method Method Method Method
variable is Int32
Definition ---------int CompareTo(System.Object value), int CompareT... bool Equals(System.Object obj), bool Equals(int ... int GetHashCode() type GetType() System.TypeCode GetTypeCode() string ToString(), string ToString(string format...
PS C:\> $number = $number * 10 PS C:\> $number 1000
Variable was treated as number
Here, I ve used [int] to force $number to contain only integers B. After entering my input, I pipe $number to Gm to confirm that it is indeed an integer, not a string c. At the end, I can see that the variable was treated as a number and real multiplication took place d. Another benefit of this technique is that the shell will throw an error if it can t convert the input into a number, because $number is only capable of storing integers:
PS C:\> [int]$number = Read-Host "Enter a number" Enter a number: Hello Cannot convert value "Hello" to type "System.Int32". Error: "Input string was not in a correct format." At line:1 char:13 + [int]$number <<<< = Read-Host "Enter a number"
Declaring a variable s type
+ CategoryInfo : MetadataError: (:) [], ArgumentTransformati onMetadataException + FullyQualifiedErrorId : RuntimeException
That s a great way to help prevent problems later on down the line, because you re assured that $number will contain the exact type of data you expect it to. There are many different object types that you can use in place of [int], but these are some of the ones you ll use most commonly include:
[int] Integer numbers [single] and [double] Single-precision and double-precision floating num-
bers (numbers with a decimal portion) [string] A string of characters [char] Exactly one character (as in, [char]$c = 'X') [xml] An XML document; whatever string you assign to this will be parsed to make sure it contains valid XML markup (for example, [xml]$doc = Get-Content MyXML.xml) [adsi] An Active Directory Services Interface (ADSI) query; the shell will execute the query and place the resulting object or objects into the variable (such as [adsi]$user = "WinNT:\\MYDOMAIN\Administrator,user")
Specifying an object type for a variable is a great way to prevent certain tricky logic errors in more complex scripts. Once you specify the object type, PowerShell enforces it until you explicitly retype the variable:
putting string PS C:\> [int]$x = 5 Declares $x as integer into $x PS C:\> $x = 'Hello' Cannot convert value "Hello" to type "System.Int32". Error: "Input string was not in a correct format." At line:1 char:3 + $x <<<< = 'Hello' + CategoryInfo : MetadataError: (:) [], ArgumentTransformati onMetadataException + FullyQualifiedErrorId : RuntimeException Retypes $x
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