Type literals can be used in several ways. They can be used as operators in a type cast, as part of a variable declaration, or as an object itself.
Casting
Casting involves the conversion of one type to another. Let's say you have a string that contains a number and you would like to convert the native type of that variable to a number. You can do so with the following cast:
PS C:\> $a = "123"
PS C:\> $a.GetType().Name
StringPS C:\> $i = [int]$a
PS C:\> $i.GetType().Name
Int32
Variable Declaration
Type literals can also be used to declare a variable of a specific type. Variables in PowerShell are dynamic in nature, meaning that they can change types depending on the context in which they are used. In some cases, you will want to be in more control of the type of your variables. By prefixing them with a type literal, PowerShell will honor your type request and enforce that type on the specified variable. In the above example, we casted a string to a integer with an explicit type literal cast. This could have been done directly on the input string "123". Or, you could have declared the variable $a to be of type int in it's variable declaration.
PS C:\> $a = [int]"123"
PS C:\> $a.GetType().Name
Int32
PS C:\> [int]$a = "123"
PS C:\> $a.GetType().Name
Int32
Object Representation and Static Member Access
In other situations, type literals can be used to access static methods in .NET classes. Static methods are methods that do not require an instance of the object to be created before you can call them. The System.DateTime class for instance has a static property Now() that will return the current date and time. By appending double colons "::" to the type literal and then adding the static method name and parameters, you call that method. In the following example, I'll pipe the type literal for DateTime into the Get-Member Cmdlet to list out all the static methods in the DateTime class:
PS C:\> [DateTime] | Get-Member -static
TypeName: System.DateTime
Name MemberType Definition
---- ---------- ----------
Compare Method static System.Int32 Compare(DateTime t1, DateTime t2)
DaysInMonth Method static System.Int32 DaysInMonth(Int32 year, Int32 month)
Equals Method static System.Boolean Equals(DateTime t1, DateTime t2),...
FromBinary Method static System.DateTime FromBinary(Int64 dateData)
FromFileTime Method static System.DateTime FromFileTime(Int64 fileTime)
FromFileTimeUtc Method static System.DateTime FromFileTimeUtc(Int64 fileTime)
FromOADate Method static System.DateTime FromOADate(Double d)
get_Now Method static System.DateTime get_Now()
get_Today Method static System.DateTime get_Today()
get_UtcNow Method static System.DateTime get_UtcNow()
IsLeapYear Method static System.Boolean IsLeapYear(Int32 year)
op_Addition Method static System.DateTime op_Addition(DateTime d, TimeSpan t)
op_Equality Method static System.Boolean op_Equality(DateTime d1, DateTime d2)
op_GreaterThan Method static System.Boolean op_GreaterThan(DateTime t1, DateTime t2)
op_GreaterThanOrEqual Method static System.Boolean op_GreaterThanOrEqual(DateTime t1,...
op_Inequality Method static System.Boolean op_Inequality(DateTime d1, DateTime d2)
op_LessThan Method static System.Boolean op_LessThan(DateTime t1, DateTime t2)
op_LessThanOrEqual Method static System.Boolean op_LessThanOrEqual(DateTime t1, DateTime t2)
op_Subtraction Method static System.DateTime op_Subtraction(DateTime d, TimeSpan t), ...
Parse Method static System.DateTime Parse(String s), static System.DateTime ...
ParseExact Method static System.DateTime ParseExact(String s, String format, ...
ReferenceEquals Method static System.Boolean ReferenceEquals(Object objA, Object objB)
SpecifyKind Method static System.DateTime SpecifyKind(DateTime value, DateTimeKind kind)
TryParse Method static System.Boolean TryParse(String s, DateTime& result), static...
TryParseExact Method static System.Boolean TryParseExact(String s, String format, ...
MaxValue Property static System.DateTime MaxValue {get;set;}
MinValue Property static System.DateTime MinValue {get;set;}
Now Property System.DateTime Now {get;}
Today Property System.DateTime Today {get;}
UtcNow Property System.DateTime UtcNow {get;}
PS C:\> [DateTime]::Now
Tuesday January 20, 2009 9:50:29 AM
Type Name Aliases
You may have noticed the native types in the above examples such as "String" and "Int32". PowerShell provides a set of type name aliases that are more in line with traditional programming languages that are aliases to the .NET framework types. You are free to use the type name aliases or the native types in your type literal usage, both are equivalent. The following table taken from Windows PowerShell in Action by Bruce Payette, lists out all the type name aliases and their native types.
| PowerShell Type Alias | | | Corresponding .NET Type |
|---|---|---|
| [int] | System.Int32 | |
| [int[]] | System.Int32[] | |
| [long] | System.Int64 | |
| [long[]] | System.Int64[] | |
| [string] | System.String | |
| [string[]] | System.String[] | |
| [char] | System.Char | |
| [char[]] | System.Char[] | |
| [bool] | System.Boolean | |
| [bool[]] | System.Boolean[] | |
| [byte] | System.Byte | |
| [byte[]] | System.Byte[] | |
| [double] | System.Double | |
| [double[]] | System.Double[] | |
| [decimal] | System.Decimal | |
| [decimal[]] | System.Decimal[] | |
| [float] | System.Single | |
| [single] | System.Single | |
| [regex] | System.Text.RegularExpression.Regex | |
| [array] | System.Array | |
| [xml] | System.Xml.XmlDocument | |
| [scriptblock] | System.Management.Automation.ScriptBlock | |
| [switch] | System.Management.Automation.SwitchParameter | |
| [hashtable] | System.Collections.Hashtable | |
| [psobject] | System.Management.Automation.PSObject | |
| [type] | System.Type | |
| [type[]] | System.Type[] | |