ETS type converters

ETS uses two basic types of type converters when a call is made to the LanguagePrimitives.ConvertTo(System.Object, System.Type) method. When this method is called, PowerShell attempts to perform the type conversion using its standard PowerShell language converters or a custom converter. If PowerShell cannot perform the conversion, it throws an PSInvalidCastException exception.

Standard Windows PowerShell Language Converters

These standard conversions are checked before any custom conversions and cannot be overridden. The following table lists the type conversions performed by PowerShell when the ConvertTo(System.Object, System.Type) method is called. Be aware that references to the valueToConvert and resultType parameters refer to parameters of the ConvertTo(System.Object,System.Type) method.

From (valueToConvert)	To (resultType)	Returns
Null	String	""
Null	Char	'\0'
Null	Numeric	0 of the type specified in the resultType parameter.
Null	Boolean	Results of call to the IsTrue(System.Object)(Null) method.
Null	PSObject	New object of type PSObject.
Null	Non-value-type	Null.
Null	Nullable<T>	Null.
Derived Class	Base class	valueToConvert
Anything	Void	AutomationNull.Value
Anything	String	Calls ToString mechanism.
Anything	Boolean	IsTrue(System.Object) (valueToConvert)
Anything	PSObject	Results of call to the AsPSObject(System.Object) (valueToConvert) method.
Anything	Xml Document	Converts valueToConvert to string, then calls XMLDocument constructor.
Array	Array	Attempts to convert each element of the array.
Singleton	Array	Array[0] equals valueToConvert that is converted to the element type of the array.
IDictionary	Hash table	Results of call to Hashtable(valueToConvert).
String	Char[]	valueToConvert.ToCharArray
String	RegEx	Results of call to Regx(valueToConvert).
String	Type	Returns the appropriate type using the valueToConvert parameter to search RunspaceConfiguration.Assemblies.
String	Numeric	If valueToConvert is "", returns 0 of the resultType. Otherwise the culture "culture invariant" is used to produce a numeric value.
Integer	System.Enum	Converts the integer to the constant if the integer is defined by the enumeration. If the integer is not defined an PSInvalidCastException exception is thrown.

Custom conversions

If PowerShell cannot convert the type using a standard PowerShell language converter, it then checks for custom converters. PowerShell looks for several types of custom converters in the order described in this section. Be aware that references to the valueToConvert and resultType parameters refer to parameters of the ConvertTo(System.Object, System.Type) method. If a custom converter throws an exception, then no further attempt is made to convert the object and that exception is wrapped in a PSInvalidCastException exception which is then thrown.

PowerShell type converter

PowerShell type converters are used to convert a single type or a family of types, such as all types that derive from the System.Enum class. To create a PowerShell type converter you must implement an PSTypeConverter class and associate that implementation with the target class. There are two ways of associating the PowerShell type converter with its target class.

• Through the type configuration file
• By applying the TypeConverterAttribute attribute to the target class

PowerShell type converters, derived from the PSTypeConverter abstract class, provide methods for converting an object to a specific type or from a specific type. If the valueToConvert parameter contains an object that has a PowerShell Type converter associated with it, PowerShell calls the PSTypeConverter.ConvertTo(System.Object, System.Type,System.IFormatProvider, System.Boolean) method of the associated converter to convert the object to the type specified by the resultType parameter. If the resultType parameter references a type that has a PowerShell type converter associated with it, PowerShell calls the PSTypeConverter.ConvertFrom(System.Object,System.Type, System.IFormatProvider, System.Boolean) method of the associated converter to convert the object from the type specified by the resultType parameter.

System type converter

System type converters are used to convert a specific target class. This type of converter cannot be used to convert a family of classes. To create an system type converter you must implement an TypeConverter class and associate that implementation with the target class. There are two ways of associating the system type converter with its target class.

• Through the type configuration file
• By applying the TypeConverterAttribute attribute to the target class

Parse converter

If the valueToConvert parameter is a string, and the object type of the resultType parameter has a Parse method, then the Parse method is called to convert the string.

Constructor converter

If the object type of the resultType parameter has a constructor that has a single parameter that is the same type as the object of the valueToConvert parameter, then this constructor is called.

Implicit cast operator converter

If the valueToConvert parameter has an implicit cast operator that converts to resultType, then its cast operator is called. If the resultType parameter has an implicit cast operator that converts from valueToConvert, then its cast operator is called.

Explicit cast operator converter

If the valueToConvert parameter has an explicit cast operator that converts to resultType, then its cast operator is called. If the resultType parameter has an explicit cast operator that converts from valueToConvert, then its cast operator is called.