Convert.ChangeType Method (Object, Type, IFormatProvider)

 
System_CAPS_noteNote

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Returns an object of the specified type whose value is equivalent to the specified object. A parameter supplies culture-specific formatting information.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

Public Shared Function ChangeType (
	value As Object,
	conversionType As Type,
	provider As IFormatProvider
) As Object

Parameters

value
Type: System.Object

An object that implements the IConvertible interface.

conversionType
Type: System.Type

The type of object to return.

provider
Type: System.IFormatProvider

An object that supplies culture-specific formatting information.

Return Value

Type: System.Object

An object whose type is conversionType and whose value is equivalent to value.

-or-

value, if the Type of value and conversionType are equal.

-or-

A null reference (Nothing in Visual Basic), if value is null and conversionType is not a value type.

Exception Condition
InvalidCastException

This conversion is not supported.

-or-

value is null and conversionType is a value type.

-or-

value does not implement the IConvertible interface.

FormatException

value is not in a format for conversionType recognized by provider.

OverflowException

value represents a number that is out of the range of conversionType.

ArgumentNullException

conversionType is null.

ChangeType is a general-purpose conversion method that converts the object specified by value to conversionType. The value parameter can be an object of any type, and conversionType can also be a Type object that represents any base or custom type. For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. The method requires that conversion of value to conversionType be supported.

The provider parameter is an IFormatProvider implementation that supplies formatting information for the conversion. Whether and how this parameter is used depends on the underlying IConvertible implementation. If value is a base data type, provider is used only for the following conversions:

If value is an application-defined type, its IConvertible implementation may use the provider parameter.

Notes to Callers:

The ChangeType(Object, Type, IFormatProvider) method can convert an enumeration value to another type. However, it cannot convert another type to an enumeration value, even if the source type is the underlying type of the enumeration. To convert a type to an enumeration value, use a casting operator (in C#) or a conversion function (in Visual Basic). The following example illustrates the conversion to and from a Continent enumeration value.

Public Enum Continent As Integer
   Africa = 0
   Antarctica = 1
   Asia = 2
   Australia = 3
   Europe = 4
   NorthAmerica = 5
   SouthAmerica = 6
End Enum

Module Example
   Public Sub Main()
      ' Convert a Continent to a Double.
      Dim cont As Continent = Continent.NorthAmerica
      Console.WriteLine("{0:N2}", 
                        Convert.ChangeType(cont, GetType(Double)))

      ' Convert a Double to a Continent.
      Dim number As Double = 6.0
      Try
         Console.WriteLine("{0}", 
                           Convert.ChangeType(number, GetType(Continent)))
      Catch e As InvalidCastException
         Console.WriteLine("Cannot convert a Double to a Continent")
      End Try

      Console.WriteLine("{0}", CType(number, Continent))   
   End Sub
End Module
' The example displays the following output:
'    5.00
'    Cannot convert a Double to a Continent
'    SouthAmerica

The Convert.ChangeType(Object, Type, IFormatProvider) method can convert a nullable type to another type. However, it cannot convert another type to a value of a nullable type, even if conversionType is the underlying type of the Nullable(Of T). To perform the conversion, you can use a casting operator (in C#) or a conversion function (in Visual Basic). The following example illustrates the conversion to and from a nullable type.

Module Example
   Public Sub Main()
      Dim intValue1 As Integer? = 12893
      Dim dValue1 As Double = CType(Convert.ChangeType(intValue1, GetType(Double), Nothing), Double)
      Console.WriteLine("{0} ({1})--> {2} ({3})", intValue1, intValue1.GetType().Name,
                        dValue1, dValue1.GetType().Name)


      Dim fValue1 As Single = 16.3478
      Dim intValue2 As Integer? = CType(fValue1, Integer) 
      Console.WriteLine("{0} ({1})--> {2} ({3})", fValue1, fValue1.GetType().Name,
                        intValue2, intValue2.GetType().Name)
   End Sub
End Module
' The example displays the following output:
'       12893 (Int32)--> 12893 (Double)
'       16.3478 (Single)--> 16 (Int32)

The following example defines a Temperature class that implements the IConvertible interface.

Imports System.Globalization

Public Class Temperature : Implements IConvertible
   Private m_Temp As Decimal

   Public Sub New(temperature As Decimal)
      Me.m_Temp = temperature
   End Sub

   Public ReadOnly Property Celsius() As Decimal
      Get
         Return Me.m_Temp
      End Get   
   End Property

   Public ReadOnly Property Kelvin() As Decimal
      Get
         Return Me.m_Temp + 273.15d   
      End Get
   End Property

   Public ReadOnly Property Fahrenheit() As Decimal
      Get
         Return Math.Round(CDec(Me.m_Temp * 9 / 5 + 32), 2)
      End Get      
   End Property

   Public Overrides Function ToString() As String
      Return m_Temp.ToString("N2") & "°C"
   End Function

   ' IConvertible implementations.
   Public Function GetTypeCode() As TypeCode _
                   Implements IConvertible.GetTypeCode
      Return TypeCode.Object
   End Function

   Public Function ToBoolean(provider As IFormatProvider) As Boolean _
                   Implements IConvertible.ToBoolean
      If m_Temp = 0 Then
         Return False
      Else
         Return True
      End If
   End Function 

   Public Function ToByte(provider As IFormatProvider) As Byte _
                   Implements IConvertible.ToByte
      If m_Temp < Byte.MinValue Or m_Temp > Byte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Byte type.", _ 
                                                   Me.m_Temp)) 
      Else
         Return Decimal.ToByte(Me.m_Temp)
      End If       
   End Function

   Public Function ToChar(provider As IFormatProvider) As Char _
                   Implements IConvertible.ToChar
      Throw New InvalidCastException("Temperature to Char conversion is not supported.")
   End Function 

   Public Function ToDateTime(provider As IFormatProvider) As Date _
                   Implements IConvertible.ToDateTime
      Throw New InvalidCastException("Temperature to DateTime conversion is not supported.")
   End Function

   Public Function ToDecimal(provider As IFormatProvider) As Decimal _
                   Implements IConvertible.ToDecimal
      Return Me.m_Temp
   End Function

   Public Function ToDouble(provider As IFormatProvider) As Double _
                   Implements IConvertible.ToDouble
      Return Decimal.ToDouble(Me.m_Temp)
   End Function   

   Public Function ToInt16(provider As IFormatProvider) As Int16 _
                   Implements IConvertible.ToInt16
      If Me.m_Temp < Int16.MinValue Or Me.m_Temp > Int16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int16 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt16(Me.m_Temp)   
      End If
   End Function

   Public Function ToInt32(provider As IFormatProvider) As Int32 _
                   Implements IConvertible.ToInt32
      If Me.m_Temp < Int32.MinValue Or Me.m_Temp > Int32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int32 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt32(Me.m_Temp)
      End If      
   End Function

   Public Function ToInt64(provider As IFormatProvider) As Int64 _
                   Implements IConvertible.ToInt64
      If Me.m_Temp < Int64.MinValue Or Me.m_Temp > Int64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int64 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt64(Me.m_Temp)
      End If      
   End Function

   Public Function ToSByte(provider As IFormatProvider) As SByte _
                   Implements IConvertible.ToSByte
      If Me.m_Temp < SByte.MinValue Or Me.m_Temp > SByte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the SByte type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToSByte(Me.m_Temp)
      End If      
   End Function

   Public Function ToSingle(provider As IFormatProvider) As Single _
                   Implements IConvertible.ToSingle
      Return Decimal.ToSingle(Me.m_Temp)
   End Function

   Public Overloads Function ToString(provider As IFormatProvider) As String _
                   Implements IConvertible.ToString
      Return m_Temp.ToString("N2", provider) & "°C"
   End Function

   Public Function ToType(conversionType As Type, provider As IFormatProvider) As Object _
                   Implements IConvertible.ToType
      Select Case Type.GetTypeCode(conversionType)
         Case TypeCode.Boolean 
            Return Me.ToBoolean(Nothing)
         Case TypeCode.Byte
            Return Me.ToByte(Nothing)
         Case TypeCode.Char
            Return Me.ToChar(Nothing)
         Case TypeCode.DateTime
            Return Me.ToDateTime(Nothing)
         Case TypeCode.Decimal
            Return Me.ToDecimal(Nothing)
         Case TypeCode.Double
            Return Me.ToDouble(Nothing)
         Case TypeCode.Int16
            Return Me.ToInt16(Nothing)
         Case TypeCode.Int32
            Return Me.ToInt32(Nothing)
         Case TypeCode.Int64
            Return Me.ToInt64(Nothing)
         Case TypeCode.Object
            If GetType(Temperature).Equals(conversionType) Then
               Return Me
            Else
               Throw New InvalidCastException(String.Format("Conversion to a {0} is not supported.", _
                                                            conversionType.Name))
            End If 
         Case TypeCode.SByte
            Return Me.ToSByte(Nothing)
         Case TypeCode.Single
            Return Me.ToSingle(Nothing)
         Case TypeCode.String
            Return Me.ToString(provider)
         Case TypeCode.UInt16
            Return Me.ToUInt16(Nothing)
         Case TypeCode.UInt32
            Return Me.ToUInt32(Nothing)
         Case TypeCode.UInt64
            Return Me.ToUInt64(Nothing)   
         Case Else
            Throw New InvalidCastException(String.Format("Conversion to {0} is not supported.", conversionType.Name))   
      End Select
   End Function

   Public Function ToUInt16(provider As IFormatProvider) As UInt16 _
                   Implements IConvertible.ToUInt16
      If Me.m_Temp < UInt16.MinValue Or Me.m_Temp > UInt16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt16 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt16(Me.m_Temp)
      End If   
   End Function

   Public Function ToUInt32(provider As IFormatProvider) As UInt32 _
                   Implements IConvertible.ToUInt32
      If Me.m_Temp < UInt32.MinValue Or Me.m_Temp > UInt32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt32 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt32(Me.m_Temp)
      End If   
   End Function

   Public Function ToUInt64(provider As IFormatProvider) As UInt64 _
                   Implements IConvertible.ToUInt64
      If Me.m_Temp < UInt64.MinValue Or Me.m_Temp > UInt64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt64 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt64(Me.m_temp)
      End If   
   End Function
End Class

The following example creates an instance of the Temperature class and calls the ChangeType(Object, Type, IFormatProvider) method to convert it to the basic numeric types supported by the .NET Framework and to a String. It illustrates that the ChangeType method wraps a call to the source type's IConvertible implementation.

Module Example
   Public Sub Main()
      Dim cool As New Temperature(5)
      Dim targetTypes() As Type = { GetType(SByte), GetType(Int16), GetType(Int32), _
                                    GetType(Int64), GetType(Byte), GetType(UInt16), _
                                    GetType(UInt32), GetType(UInt64), GetType(Decimal), _
                                    GetType(Single), GetType(Double), GetType(String) }
      Dim provider As New CultureInfo("fr-FR")

      For Each targetType As Type In targetTypes
         Try
            Dim value As Object = Convert.ChangeType(cool, targetType, provider)
            Console.WriteLine("Converted {0} {1} to {2} {3}.", _
                              cool.GetType().Name, cool.ToString(), _
                              targetType.Name, value)
         Catch e As InvalidCastException
            Console.WriteLine("Unsupported {0} --> {1} conversion.", _
                              cool.GetType().Name, targetType.Name)
         Catch e As OverflowException
            Console.WriteLine("{0} is out of range of the {1} type.", _
                              cool, targetType.Name)
         End Try                     
      Next
   End Sub
End Module
' The example displays the following output:
'       Converted Temperature 5.00°C to SByte 5.
'       Converted Temperature 5.00°C to Int16 5.
'       Converted Temperature 5.00°C to Int32 5.
'       Converted Temperature 5.00°C to Int64 5.
'       Converted Temperature 5.00°C to Byte 5.
'       Converted Temperature 5.00°C to UInt16 5.
'       Converted Temperature 5.00°C to UInt32 5.
'       Converted Temperature 5.00°C to UInt64 5.
'       Converted Temperature 5.00°C to Decimal 5.
'       Converted Temperature 5.00°C to Single 5.
'       Converted Temperature 5.00°C to Double 5.
'       Converted Temperature 5.00°C to String 5,00°C.

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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