.NET Framework 中的型別轉換

發行項
08/12/2011

每個值都有相關聯的型別，該型別定義屬性，例如配置給值的空間量、能夠擁有的可能值範圍，以及提供的成員。許多值都可以表示成多種型別。例如，數值 4 就可以表示成整數值或浮點 (Floating-Point) 值。型別轉換會建立新型別的值，與舊型別的值相等，但是不一定會保留原始物件的識別 (或實際的值)。

.NET Framework 提供數個支援型別轉換的功能。這些需求包括下列各項：

Implicit 運算子，這個運算子定義型別之間可用的擴展轉換。如需詳細資訊，請參閱使用隱含運算子的隱含轉換一節。
Explicit 運算子，這個運算子定義型別之間可用的縮小轉換。如需詳細資訊，請參閱使用明確運算子的明確轉換一節。
IConvertible 介面，這個介面定義轉換至每一個基底 .NET Framework 資料型別的方式。如需詳細資訊，請參閱 IConvertible 介面一節。
Convert 類別，這個類別提供一組方法，這些方法實作 IConvertible 介面中的方法。如需詳細資訊，請參閱 Convert 類別一節。
TypeConverter 類別，這個類別是基底類別，可以擴充來支援從指定的型別轉換至其他任何型別。如需詳細資訊，請參閱 TypeConverter 類別一節。

使用隱含運算子的隱含轉換

擴展轉換包含從現有型別的值來建立新的值，這個型別具有比目標型別更嚴格的範圍或更受限制的成員清單。擴展轉換不可能導致資料遺失 (雖然可能導致精確度降低)。因為不可能遺失資料，編譯器可以隱含地 (或直接地) 處理轉換，而不需要使用明確的轉換方法或轉型運算子。

注意事項
雖然執行隱含轉換的程式碼可以呼叫轉換方法或使用轉型運算子，但支援隱含轉換的編譯器並不需要這些動作。

例如，Decimal 型別支援從 Byte、Char、Int16、Int32、Int64、SByte、UInt16、UInt32 和 UInt64 值的隱含轉換。下列範例說明其中一部分隱含轉換在指派值給 Decimal 變數時的情形。

Dim byteValue As Byte = 16
Dim shortValue As Short = -1024
Dim intValue As Integer = -1034000
Dim longValue As Long = CLng(1024^6)
Dim ulongValue As ULong = ULong.MaxValue

Dim decimalValue As Decimal

decimalValue = byteValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  byteValue.GetType().Name, decimalValue) 

decimalValue = shortValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  shortValue.GetType().Name, decimalValue) 

decimalValue = intValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  intValue.GetType().Name, decimalValue) 

decimalValue = longValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  longValue.GetType().Name, decimalValue) 

decimalValue = ulongValue
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.",
                  longValue.GetType().Name, decimalValue) 
' The example displays the following output:
'    After assigning a Byte value, the Decimal value is 16.
'    After assigning a Int16 value, the Decimal value is -1024.
'    After assigning a Int32 value, the Decimal value is -1034000.
'    After assigning a Int64 value, the Decimal value is 1152921504606846976.
'    After assigning a Int64 value, the Decimal value is 18446744073709551615.

byte byteValue = 16;
short shortValue = -1024;
int intValue = -1034000;
long longValue = 1152921504606846976;
ulong ulongValue = UInt64.MaxValue;

decimal decimalValue;

decimalValue = byteValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  byteValue.GetType().Name, decimalValue); 

decimalValue = shortValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  shortValue.GetType().Name, decimalValue); 

decimalValue = intValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  intValue.GetType().Name, decimalValue); 

decimalValue = longValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  longValue.GetType().Name, decimalValue); 

decimalValue = ulongValue;
Console.WriteLine("After assigning a {0} value, the Decimal value is {1}.", 
                  longValue.GetType().Name, decimalValue); 
// The example displays the following output:
//    After assigning a Byte value, the Decimal value is 16.
//    After assigning a Int16 value, the Decimal value is -1024.
//    After assigning a Int32 value, the Decimal value is -1034000.
//    After assigning a Int64 value, the Decimal value is 1152921504606846976.
//    After assigning a Int64 value, the Decimal value is 18446744073709551615.

如果特定的語言編譯器支援自訂運算子，您也可以在自己的自訂型別中定義隱含轉換。下列範例針對使用正負號和大小表示之名為 ByteWithSign 的帶正負號位元組資料型別，提供部分實作。它支援將 Byte 和 SByte 值隱含轉換為 ByteWithSign 值。

Public Structure ByteWithSign
   Private signValue As SByte 
   Private value As Byte

   Public Overloads Shared Widening Operator CType(value As SByte) As ByteWithSign
      Dim newValue As ByteWithSign
      newValue.signValue = CSByte(Math.Sign(value))
      newValue.value = CByte(Math.Abs(value))
      Return newValue
   End Operator  

   Public Overloads Shared Widening Operator CType(value As Byte) As ByteWithSign
      Dim NewValue As ByteWithSign
      newValue.signValue = 1
      newValue.value = value
      Return newValue
   End Operator

   Public Overrides Function ToString() As String 
      Return (signValue * value).ToString()
   End Function
End Structure

public struct ByteWithSign
{
   private SByte signValue; 
   private Byte value;

   public static implicit operator ByteWithSign(SByte value) 
   {
      ByteWithSign newValue;
      newValue.signValue = (SByte) Math.Sign(value);
      newValue.value = (byte) Math.Abs(value);
      return newValue;
   }  

   public static implicit operator ByteWithSign(Byte value)
   {
      ByteWithSign  newValue;
      newValue.signValue = 1;
      newValue.value = value;
      return newValue;
   }

   public override string ToString()
   { 
      return (signValue * value).ToString();
   }
}

然後，用戶端程式碼可以宣告 ByteWithSign 變數並指派 Byte 和 SByte 值給它，而不需要執行任何明確的轉換，也不需要使用任何轉型運算子，如下列範例所示。

Dim sbyteValue As SByte = -120
Dim value As ByteWithSign = sbyteValue
Console.WriteLine(value.ToString()) 
value = Byte.MaxValue
Console.WriteLine(value.ToString()) 
' The example displays the following output:
'       -120
'       255

SByte sbyteValue = -120;
ByteWithSign value = sbyteValue;
Console.WriteLine(value);
value = Byte.MaxValue;
Console.WriteLine(value);
// The example displays the following output:
//       -120
//       255

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使用明確運算子的明確轉換

縮小轉換包含從現有型別的值來建立新的值，這個型別具有比目標型別更廣的範圍或更大的成員清單。因為縮小轉換可能導致資料遺失，編譯器通常會要求必須透過呼叫轉換方法或轉型運算子來明確執行轉換。也就是說，開發人員程式碼中必須明確處理轉換。

注意事項
縮小轉換需要轉換方法或轉型運算子的主要目的是讓開發人員知道可能遺失資料，或可能發生 OverflowException，以便在程式碼中處理。不過，某些編譯器可能會放寬這項需求。例如，在 Visual Basic 中，如果 Option Strict 已關閉 (此為預設)，則 Visual Basic 編譯器會嘗試隱含地執行縮小轉換。

例如，UInt32、Int64 和 UInt64 資料型別的範圍超過 Int32 資料型別的範圍，如下表所示。

型別	與 Int32 的範圍比較
Int64	Int64.MaxValue 大於 Int32.MaxValue，而 Int64.MinValue 小於 (負值範圍大於) Int32.MinValue。
UInt32	UInt32.MaxValue 大於 Int32.MaxValue。
UInt64	UInt64.MaxValue 大於 Int32.MaxValue。

為了處理縮小轉換，.NET Framework 允許型別定義 Explicit 運算子。然後，個別語言編譯器就可以使用自己的語法來實作這個運算子，也可以呼叫 Convert 類型的成員來執行轉換 (如需 Convert 類別的詳細資訊，請參閱本主題稍後的 Convert 類別)。下列範例說明如何使用語言功能，處理從這些可能超出範圍的整數值至 Int32 值的明確轉換。

Dim number1 As Long = Integer.MaxValue + 20L
Dim number2 As UInteger = Integer.MaxValue - 1000
Dim number3 As ULong = Integer.MaxValue

Dim intNumber As Integer

Try
   intNumber = CInt(number1)
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                       number1.GetType().Name, intNumber)
Catch e As OverflowException
   If number1 > Integer.MaxValue Then
      Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                                        number1, Integer.MaxValue)
   Else
      Console.WriteLine("Conversion failed: {0} is less than {1}.\n", 
                                        number1, Integer.MinValue)
   End If
End Try

Try
   intNumber = CInt(number2)
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                       number2.GetType().Name, intNumber)
Catch e As OverflowException
   Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                                     number2, Integer.MaxValue)
End Try

Try
   intNumber = CInt(number3)
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                       number3.GetType().Name, intNumber)
Catch e As OverflowException
   Console.WriteLine("Conversion failed: {0} exceeds {1}.",
                                     number1, Integer.MaxValue)
End Try
' The example displays the following output:
'    Conversion failed: 2147483667 exceeds 2147483647.
'    After assigning a UInt32 value, the Integer value is 2147482647.
'    After assigning a UInt64 value, the Integer value is 2147483647.

long number1 = int.MaxValue + 20L;
uint number2 = int.MaxValue - 1000;
ulong number3 = int.MaxValue;

int intNumber;

try {
   intNumber = checked((int) number1);
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                     number1.GetType().Name, intNumber); 
}
catch (OverflowException) {
   if (number1 > int.MaxValue)
      Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                        number1, int.MaxValue);
   else
      Console.WriteLine("Conversion failed: {0} is less than {1}.", 
                        number1, int.MinValue);
}

try {
   intNumber = checked((int) number2);
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                     number2.GetType().Name, intNumber); 
}
catch (OverflowException) {
   Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                     number2, int.MaxValue);
}

try {
   intNumber = checked((int) number3);
   Console.WriteLine("After assigning a {0} value, the Integer value is {1}.", 
                     number3.GetType().Name, intNumber); 
}
catch (OverflowException) {
   Console.WriteLine("Conversion failed: {0} exceeds {1}.", 
                     number1, int.MaxValue);
}

// The example displays the following output:
//    Conversion failed: 2147483667 exceeds 2147483647.
//    After assigning a UInt32 value, the Integer value is 2147482647.
//    After assigning a UInt64 value, the Integer value is 2147483647.

明確轉換在不同的語言中可能會產生不同的結果，這些結果可能與對應的 Convert 方法所傳回的值不同。例如，如果 Double 值 12.63251 轉換為 Int32，則 Visual Basic CInt 方法與 .NET Framework Convert.ToInt32(Double) 方法都會四捨五入 Double 而傳回 13，但 C# (int) 運算子會截斷 Double 而傳回 12。同樣地，C# (int) 運算子不支援布林值對整數的轉換，但 Visual Basic CInt 方法會將 true 的值轉換為 -1。另一方面，Convert.ToInt32(Boolean) 方法會將 true 的值轉換為 1。

大多數的編譯器都允許明確轉換以已檢查或未檢查的方式執行。如果執行已檢查的轉換，當要轉換之型別的值不在目標型別的範圍內時，則會擲回 OverflowException。在同樣的狀況下執行未檢查的轉換時，轉換可能不會擲回例外狀況，但實際行為會變得不明確，且可能產生不正確的值。

注意事項
在 C# 中，已檢查的轉換可以透過使用 checked 關鍵字搭配轉型運算子來執行，也可以指定 /checked+ 編譯器選項來執行。相反地，未檢查的轉換可使用 unchecked 關鍵字搭配轉型運算子來執行，或可以指定 /checked- 編譯器選項來執行。根據預設，明確轉換是未檢查的。在 Visual Basic 中，已檢查的轉換可藉由清除專案的 [進階編譯器設定] 對話方塊中的 [移除整數的溢位檢查] 核取方塊來執行，或可以指定 /removeintchecks- 編譯器選項來執行。相反地，未檢查的轉換可以藉由選取專案的 [進階編譯器設定] 對話方塊中的 [移除整數的溢位檢查] 核取方塊來執行，也可以指定 /removeintchecks+ 編譯器選項來執行。根據預設，明確轉換是檢查的。

在 C# 中，已檢查的轉換可以透過使用 checked 關鍵字搭配轉型運算子來執行，也可以指定 /checked+ 編譯器選項來執行。相反地，未檢查的轉換可使用 unchecked 關鍵字搭配轉型運算子來執行，或可以指定 /checked- 編譯器選項來執行。根據預設，明確轉換是未檢查的。在 Visual Basic 中，已檢查的轉換可藉由清除專案的 [進階編譯器設定] 對話方塊中的 [移除整數的溢位檢查] 核取方塊來執行，或可以指定 /removeintchecks- 編譯器選項來執行。相反地，未檢查的轉換可以藉由選取專案的 [進階編譯器設定] 對話方塊中的 [移除整數的溢位檢查] 核取方塊來執行，也可以指定 /removeintchecks+ 編譯器選項來執行。根據預設，明確轉換是檢查的。

下列 C# 範例使用 checked 和 unchecked 關鍵字，說明將超出 Byte 範圍的值轉換為 Byte 時的行為差異。已檢查的轉換會擲回例外狀況，但未檢查的轉換會指派 Byte.MaxValue 給 Byte 變數。

int largeValue = Int32.MaxValue;
byte newValue;

try {
   newValue = unchecked((byte) largeValue);
   Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.", 
                     largeValue.GetType().Name, largeValue,
                     newValue.GetType().Name, newValue);
}
catch (OverflowException) {
   Console.WriteLine("{0} is outside the range of the Byte data type.", 
                     largeValue);
}

try {
   newValue = checked((byte) largeValue);
   Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.", 
                     largeValue.GetType().Name, largeValue,
                     newValue.GetType().Name, newValue);
}
catch (OverflowException) {
   Console.WriteLine("{0} is outside the range of the Byte data type.", 
                     largeValue);
}
// The example displays the following output:
//    Converted the Int32 value 2147483647 to the Byte value 255.
//    2147483647 is outside the range of the Byte data type.

如果特定的語言編譯器支援自訂多載運算子，您也可以在自己的自訂型別中定義明確轉換。下列範例針對使用正負號和大小表示之名為 ByteWithSign 的帶正負號位元組資料型別，提供部分實作。它支援將 Int32 和 UInt32 值明確轉換為 ByteWithSign 值。

Public Structure ByteWithSign
   Private signValue As SByte 
   Private value As Byte

   Private Const MaxValue As Byte = Byte.MaxValue
   Private Const MinValue As Integer = -1 * Byte.MaxValue

   Public Overloads Shared Narrowing Operator CType(value As Integer) As ByteWithSign
      ' Check for overflow.
      If value > ByteWithSign.MaxValue Or value < ByteWithSign.MinValue Then
         Throw New OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", value))
      End If

      Dim newValue As ByteWithSign

      newValue.signValue = CSByte(Math.Sign(value))
      newValue.value = CByte(Math.Abs(value))
      Return newValue
   End Operator

   Public Overloads Shared Narrowing Operator CType(value As UInteger) As ByteWithSign
      If value > ByteWithSign.MaxValue Then 
         Throw New OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", value))
      End If

      Dim NewValue As ByteWithSign

      newValue.signValue = 1
      newValue.value = CByte(value)
      Return newValue
   End Operator

   Public Overrides Function ToString() As String 
      Return (signValue * value).ToString()
   End Function
End Structure

public struct ByteWithSign
{
   private SByte signValue; 
   private Byte value;

   private const byte MaxValue = byte.MaxValue;
   private const int MinValue = -1 * byte.MaxValue;

   public static explicit operator ByteWithSign(int value) 
   {
      // Check for overflow.
      if (value > ByteWithSign.MaxValue || value < ByteWithSign.MinValue)
         throw new OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", 
                                                   value));

      ByteWithSign newValue;
      newValue.signValue = (SByte) Math.Sign(value);
      newValue.value = (byte) Math.Abs(value);
      return newValue;
   }  

   public static explicit operator ByteWithSign(uint value)
   {
      if (value > ByteWithSign.MaxValue) 
         throw new OverflowException(String.Format("'{0}' is out of range of the ByteWithSign data type.", 
                                                   value));

      ByteWithSign newValue;
      newValue.signValue = 1;
      newValue.value = (byte) value;
      return newValue;
   }

   public override string ToString()
   { 
      return (signValue * value).ToString();
   }
}

然後，用戶端程式碼可以宣告 ByteWithSign 變數並指派 Int32 和 UInt32 值給它 (如果指派包含轉型運算子或轉換方法的話)，如下列範例所示。

Dim value As ByteWithSign

Try  
   Dim intValue As Integer = -120
   value = CType(intValue, ByteWithSign)
   Console.WriteLine(value) 
Catch e As OverflowException
   Console.WriteLine(e.Message)
End Try

Try
   Dim uintValue As UInteger = 1024
   value = CType(uintValue, ByteWithSign)
   Console.WriteLine(value) 
Catch e As OverflowException
   Console.WriteLine(e.Message)
End Try
' The example displays the following output:
'       -120
'       '1024' is out of range of the ByteWithSign data type.

ByteWithSign value;

try {
   int intValue = -120;
   value = (ByteWithSign) intValue;
   Console.WriteLine(value);
}
catch (OverflowException e) {
   Console.WriteLine(e.Message);
}

try {
   uint uintValue = 1024;
   value = (ByteWithSign) uintValue;
   Console.WriteLine(value);
}
catch (OverflowException e) {
    Console.WriteLine(e.Message);
}
// The example displays the following output:
//       -120
//       '1024' is out of range of the ByteWithSign data type.

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IConvertible 介面

為了支援從任何型別轉換至 Common Language Runtime 基底型別，.NET Framework 提供 IConvertible 介面。實作型別需要提供下列項目：

傳回實作型別之 TypeCode 的方法。
將實作型別轉換為每一個 Common Language Runtime 基底型別 (Boolean、Byte、DateTime、Decimal、Double 等) 的方法。
將實作型別的執行個體轉換為另一個指定之型別的通用轉換方法。不支援的轉換應該擲回 InvalidCastException。

每一個 Common Language Runtime 基底型別 (也就是 Boolean、Byte、Char、DateTime、Decimal、Double、Int16、Int32、Int64、SByte、Single、String、UInt16、UInt32 和 UInt64)，以及 DBNull 和 Enum 型別，都實作 IConvertible 介面。不過，這些都是明確的介面實作，且只能透過 IConvertible 介面變數來呼叫轉換方法，如下列範例所示。這個範例將 Int32 值轉換為其對等的 Char 值。

Dim codePoint As Integer = 1067
Dim iConv As IConvertible = codePoint
Dim ch As Char = iConv.ToChar(Nothing)
Console.WriteLine("Converted {0} to {1}.", codePoint, ch)

int codePoint = 1067;
IConvertible iConv = codePoint;
char ch = iConv.ToChar(null);
Console.WriteLine("Converted {0} to {1}.", codePoint, ch);

由於必須呼叫其介面上的轉換方法，而不是呼叫實作型別上的轉換方法，這種需求使得明確介面實作相當耗費資源。相反地，在 Common Language Runtime 基底型別之間轉換時，我們建議您呼叫 Convert 類別的適當成員。如需詳細資訊，請參閱下一節 Convert 類別。

注意事項
除了 .NET Framework 提供的 IConvertible 介面和 Convert 類別之外，個別語言可能還會提供執行轉換的方式。例如，C# 使用轉型 (Casting) 運算子、Visual Basic 使用編譯器實作的轉換函式，例如 CType、CInt 和 DirectCast。

在大多數情況下，IConvertible 介面的設計主要是支援在 .NET Framework 中的各基底型別之間轉換。不過，也可以使用自訂型別來實作介面，以支援從該型別轉換至其他自訂型別。如需詳細資訊，請參閱本主題稍後的使用 ChangeType 方法的自訂轉換一節。

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Convert 類別

雖然可以呼叫每一個基底類別的 IConvertible 介面實作來執行型別轉換，但建議的語言中立方式是呼叫 System.Convert 類別的方法，在不同的基底型別之間轉換。此外，也可以使用 Convert.ChangeType(Object, Type, IFormatProvider) 方法將指定的自訂型別轉換為另一種型別。

在基底型別之間轉換

Convert 類別提供在基底型別之間轉換的語言中立方式，且所有以 Common Language Runtime 為目標的語言都可以使用。它對於擴展和縮小轉換提供一組完整的方法，且對於不支援的轉換會擲回 InvalidCastException (例如，將 DateTime 值轉換為整數值)。縮小轉換是在已檢查的情況下執行，如果轉換失敗，則會擲回 OverflowException。

重要事項
因為 Convert 類別包含在每一個基底型別之間來回轉換的方法，所以不需要呼叫每一個基底型別的 IConvertible 明確介面實作。

下列範例說明如何使用 System.Convert 類別，在 .NET Framework 基底型別之間執行數個擴展和縮小轉換。

' Convert an Int32 value to a Decimal (a widening conversion).
Dim integralValue As Integer = 12534
Dim decimalValue As Decimal = Convert.ToDecimal(integralValue)
Console.WriteLine("Converted the {0} value {1} to the {2} value {3:N2}.", 
                  integralValue.GetType().Name,
                  integralValue,
                  decimalValue.GetType().Name,
                  decimalValue)

' Convert a Byte value to an Int32 value (a widening conversion).
Dim byteValue As Byte = Byte.MaxValue
Dim integralValue2 As Integer = Convert.ToInt32(byteValue)                                  
Console.WriteLine("Converted the {0} value {1} to " + 
                                  "the {2} value {3:G}.",
                                  byteValue.GetType().Name,
                                  byteValue,
                                  integralValue2.GetType().Name,
                                  integralValue2)

' Convert a Double value to an Int32 value (a narrowing conversion).
Dim doubleValue As Double = 16.32513e12
Try
   Dim longValue As Long = Convert.ToInt64(doubleValue)
   Console.WriteLine("Converted the {0} value {1:E} to " + 
                                     "the {2} value {3:N0}.",
                                     doubleValue.GetType().Name,
                                     doubleValue,
                                     longValue.GetType().Name,
                                     longValue)
Catch e As OverflowException
   Console.WriteLine("Unable to convert the {0:E} value {1}.",
                                     doubleValue.GetType().Name, doubleValue)
End Try                                                             

' Convert a signed byte to a byte (a narrowing conversion).     
Dim sbyteValue As SByte = -16
Try
   Dim byteValue2 As Byte = Convert.ToByte(sbyteValue)
   Console.WriteLine("Converted the {0} value {1} to " + 
                                     "the {2} value {3:G}.",
                                     sbyteValue.GetType().Name,
                                     sbyteValue,
                                     byteValue2.GetType().Name,
                                     byteValue2)
Catch e As OverflowException
   Console.WriteLine("Unable to convert the {0} value {1}.",
                                     sbyteValue.GetType().Name, sbyteValue)
End Try 
' The example displays the following output:
'       Converted the Int32 value 12534 to the Decimal value 12,534.00.
'       Converted the Byte value 255 to the Int32 value 255.
'       Converted the Double value 1.632513E+013 to the Int64 value 16,325,130,000,000.
'       Unable to convert the SByte value -16.

// Convert an Int32 value to a Decimal (a widening conversion).
int integralValue = 12534;
decimal decimalValue = Convert.ToDecimal(integralValue);
Console.WriteLine("Converted the {0} value {1} to " +  
                                  "the {2} value {3:N2}.", 
                                  integralValue.GetType().Name, 
                                  integralValue, 
                                  decimalValue.GetType().Name, 
                                  decimalValue);
// Convert a Byte value to an Int32 value (a widening conversion).
byte byteValue = Byte.MaxValue;
int integralValue2 = Convert.ToInt32(byteValue);                                  
Console.WriteLine("Converted the {0} value {1} to " +  
                                  "the {2} value {3:G}.", 
                                  byteValue.GetType().Name, 
                                  byteValue, 
                                  integralValue2.GetType().Name, 
                                  integralValue2);

// Convert a Double value to an Int32 value (a narrowing conversion).
double doubleValue = 16.32513e12;
try {
   long longValue = Convert.ToInt64(doubleValue);
   Console.WriteLine("Converted the {0} value {1:E} to " +  
                                     "the {2} value {3:N0}.", 
                                     doubleValue.GetType().Name, 
                                     doubleValue, 
                                     longValue.GetType().Name, 
                                     longValue);
}
catch (OverflowException) {
   Console.WriteLine("Unable to convert the {0:E} value {1}.", 
                                     doubleValue.GetType().Name, doubleValue);
}

// Convert a signed byte to a byte (a narrowing conversion).     
sbyte sbyteValue = -16;
try {
   byte byteValue2 = Convert.ToByte(sbyteValue);
   Console.WriteLine("Converted the {0} value {1} to " +  
                                     "the {2} value {3:G}.", 
                                     sbyteValue.GetType().Name, 
                                     sbyteValue, 
                                     byteValue2.GetType().Name, 
                                     byteValue2);
}
catch (OverflowException) {
   Console.WriteLine("Unable to convert the {0} value {1}.", 
                                     sbyteValue.GetType().Name, sbyteValue);
}                                         
// The example displays the following output:
//       Converted the Int32 value 12534 to the Decimal value 12,534.00.
//       Converted the Byte value 255 to the Int32 value 255.
//       Converted the Double value 1.632513E+013 to the Int64 value 16,325,130,000,000.
//       Unable to convert the SByte value -16.

在某些情況下，尤其是在浮點值之間來回轉換時，即使沒有擲回 OverflowException，轉換也可能會降低精確度。下列範例說明這種精確度降低情況。在第一種情況下，Decimal 值在轉換為 Double 時會降低精確度 (有效位數較少)。在第二種情況下，Double 值會從 42.72 四捨五入為 43，以完成轉換。

Dim doubleValue As Double 

' Convert a Double to a Decimal.
Dim decimalValue As Decimal = 13956810.96702888123451471211d
doubleValue = Convert.ToDouble(decimalValue)
Console.WriteLine("{0} converted to {1}.", decimalValue, doubleValue)

doubleValue = 42.72
Try
   Dim integerValue As Integer = Convert.ToInt32(doubleValue)
   Console.WriteLine("{0} converted to {1}.",
                                     doubleValue, integerValue)
Catch e As OverflowException
   Console.WriteLine("Unable to convert {0} to an integer.",
                                     doubleValue)
End Try   
' The example displays the following output:
'       13956810.96702888123451471211 converted to 13956810.9670289.
'       42.72 converted to 43.

double doubleValue; 

// Convert a Double to a Decimal.
decimal decimalValue = 13956810.96702888123451471211m;
doubleValue = Convert.ToDouble(decimalValue);
Console.WriteLine("{0} converted to {1}.", decimalValue, doubleValue);

doubleValue = 42.72;
try {
   int integerValue = Convert.ToInt32(doubleValue);
   Console.WriteLine("{0} converted to {1}.", 
                                     doubleValue, integerValue);
}
catch (OverflowException) {      
   Console.WriteLine("Unable to convert {0} to an integer.", 
                                     doubleValue);
}   
// The example displays the following output:
//       13956810.96702888123451471211 converted to 13956810.9670289.
//       42.72 converted to 43.

關於同時列出 Convert 類別所支援之擴展和縮小轉換的表格，請參閱型別轉換表。

使用 ChangeType 方法的自訂轉換

Convert 類別除了支援轉換為每一個基底型別，也可以用於將自訂型別轉換為一個或多個預先定義的型別。這種轉換由 Convert.ChangeType(Object, Type, IFormatProvider) 方法執行，而這個方法會進一步包裝對於 value 參數之 IConvertible.ToType 方法的呼叫。這表示由 value 參數所表示的物件必須提供 IConvertible 介面的實作。

下列範例說明 IConvertible 介面可能的實作，這種實作允許 TemperatureCelsius 物件轉換為 TemperatureFahrenheit 物件，反之亦然。範例中定義一個基底類別 Temperature，這個類別實作 IConvertible 介面並覆寫 Object.ToString 方法。衍生的 TemperatureCelsius 和 TemperatureFahrenheit 類別分別覆寫基底類別的 ToType 和 ToString 方法。

Public MustInherit Class Temperature
   Implements IConvertible

   Protected temp As Decimal

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

   Public Property Value As Decimal
      Get 
         Return Me.temp
      End Get
      Set
         Me.temp = Value
      End Set   
   End Property

   Public Overrides Function ToString() As String
      Return temp.ToString() & "º"
   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
      Throw New InvalidCastException(String.Format("Temperature-to-Boolean conversion is not supported."))
   End Function

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

   Public Function ToDouble(provider As IFormatProvider) As Double Implements IConvertible.ToDouble
      Return CDbl(temp)
   End Function

   Public Function ToInt16(provider As IFormatProvider) As Int16 Implements IConvertible.ToInt16
      If temp < Int16.MinValue Or temp > Int16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int16 data type.", temp))
      End If
      Return CShort(Math.Round(temp))
   End Function

   Public Function ToInt32(provider As IFormatProvider) As Int32 Implements IConvertible.ToInt32
      If temp < Int32.MinValue Or temp > Int32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int32 data type.", temp))
      End If
      Return CInt(Math.Round(temp))
   End Function

   Public Function ToInt64(provider As IFormatProvider) As Int64 Implements IConvertible.ToInt64
      If temp < Int64.MinValue Or temp > Int64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int64 data type.", temp))
      End If
      Return CLng(Math.Round(temp))
   End Function

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

   Public Function ToSingle(provider As IFormatProvider) As Single Implements IConvertible.ToSingle
      Return CSng(temp)
   End Function

   Public Overridable Overloads Function ToString(provider As IFormatProvider) As String Implements IConvertible.ToString
      Return temp.ToString(provider) & " °C"
   End Function

   ' If conversionType is a implemented by another IConvertible method, call it.
   Public Overridable Function ToType(conversionType As Type, provider As IFormatProvider) As Object Implements IConvertible.ToType
      Select Case Type.GetTypeCode(conversionType)
         Case TypeCode.Boolean
            Return Me.ToBoolean(provider)
         Case TypeCode.Byte
            Return Me.ToByte(provider)
         Case TypeCode.Char
            Return Me.ToChar(provider)   
         Case TypeCode.DateTime
            Return Me.ToDateTime(provider)
         Case TypeCode.Decimal
            Return Me.ToDecimal(provider)
         Case TypeCode.Double
            Return Me.ToDouble(provider)
         Case TypeCode.Empty
            Throw New NullReferenceException("The target type is null.")
         Case TypeCode.Int16
            Return Me.ToInt16(provider)
         Case TypeCode.Int32
            Return Me.ToInt32(provider)
         Case TypeCode.Int64
            Return Me.ToInt64(provider)
         Case TypeCode.Object
            ' Leave conversion of non-base types to derived classes.
            Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                           conversionType.Name))
         Case TypeCode.SByte
            Return Me.ToSByte(provider)
         Case TypeCode.Single
            Return Me.ToSingle(provider)
         Case TypeCode.String
            Return Me.ToString(provider)
         Case TypeCode.UInt16
            Return Me.ToUInt16(provider)
         Case TypeCode.UInt32
            Return Me.ToUInt32(provider)
         Case TypeCode.UInt64
            Return Me.ToUInt64(provider)
         Case Else
            Throw New InvalidCastException("Conversion not supported.")   
      End Select
   End Function

   Public Function ToUInt16(provider As IFormatProvider) As UInt16 Implements IConvertible.ToUInt16
      If temp < UInt16.MinValue Or temp > UInt16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt16 data type.", temp))
      End If
      Return CUShort(Math.Round(temp))
   End Function

   Public Function ToUInt32(provider As IFormatProvider) As UInt32 Implements IConvertible.ToUInt32
      If temp < UInt32.MinValue Or temp > UInt32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt32 data type.", temp))
      End If
      Return CUInt(Math.Round(temp))
   End Function

   Public Function ToUInt64(provider As IFormatProvider) As UInt64 Implements IConvertible.ToUInt64
      If temp < UInt64.MinValue Or temp > UInt64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt64 data type.", temp))
      End If
      Return CULng(Math.Round(temp))
   End Function
End Class

Public Class TemperatureCelsius : Inherits Temperature : Implements IConvertible
   Public Sub New(value As Decimal)
      MyBase.New(value)
   End Sub

   ' Override ToString methods.
   Public Overrides Function ToString() As String
      Return Me.ToString(Nothing)
   End Function

   Public Overrides Function ToString(provider As IFormatProvider ) As String
      Return temp.ToString(provider) + "°C" 
   End Function

  ' If conversionType is a implemented by another IConvertible method, call it.
   Public Overrides Function ToType(conversionType As Type, provider As IFormatProvider) As Object
      ' For non-objects, call base method.
      If Type.GetTypeCode(conversionType) <> TypeCode.Object Then
         Return MyBase.ToType(conversionType, provider)
      Else   
         If conversionType.Equals(GetType(TemperatureCelsius)) Then
            Return Me
         ElseIf conversionType.Equals(GetType(TemperatureFahrenheit))
            Return New TemperatureFahrenheit(CDec(Me.temp * 9 / 5 + 32))
         ' Unspecified object type: throw an InvalidCastException.
         Else
            Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _ 
                                           conversionType.Name))
         End If
      End If                                  
   End Function
End Class

Public Class TemperatureFahrenheit : Inherits Temperature : Implements IConvertible
   Public Sub New(value As Decimal)
      MyBase.New(value)
   End Sub

   ' Override ToString methods.
   Public Overrides Function ToString() As String
      Return Me.ToString(Nothing)
   End Function

   Public Overrides Function ToString(provider As IFormatProvider ) As String
      Return temp.ToString(provider) + "°F" 
   End Function

   Public Overrides Function ToType(conversionType As Type, provider As IFormatProvider) As Object
      ' For non-objects, call base methood.
      If Type.GetTypeCode(conversionType) <> TypeCode.Object Then
         Return MyBase.ToType(conversionType, provider)
      Else   
         ' Handle conversion between derived classes.
         If conversionType.Equals(GetType(TemperatureFahrenheit)) Then 
            Return Me
         ElseIf conversionType.Equals(GetType(TemperatureCelsius))
            Return New TemperatureCelsius(CDec((MyBase.temp - 32) * 5 / 9))
         ' Unspecified object type: throw an InvalidCastException.
         Else
            Throw New InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", _
                                           conversionType.Name))
         End If
      End If                                  
   End Function
End Class

using System;

public abstract class Temperature : IConvertible
{
   protected decimal temp;

   public Temperature(decimal temperature)
   {
      this.temp = temperature;
   }

   public decimal Value
   { 
      get { return this.temp; } 
      set { this.temp = Value; }        
   }

   public override string ToString()
   {
      return temp.ToString(null as IFormatProvider) + "º";
   }

   // IConvertible implementations.
   public TypeCode GetTypeCode() { 
      return TypeCode.Object;
   }   

   public bool ToBoolean(IFormatProvider provider) {
      throw new InvalidCastException(String.Format("Temperature-to-Boolean conversion is not supported."));
   }

   public byte ToByte(IFormatProvider provider) {
      if (temp < Byte.MinValue || temp > Byte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Byte data type.", temp));
      else
         return (byte) temp;
   }

   public char ToChar(IFormatProvider provider) {
      throw new InvalidCastException("Temperature-to-Char conversion is not supported.");
   }

   public DateTime ToDateTime(IFormatProvider provider) {
      throw new InvalidCastException("Temperature-to-DateTime conversion is not supported.");
   }

   public decimal ToDecimal(IFormatProvider provider) {
      return temp;
   }

   public double ToDouble(IFormatProvider provider) {
      return (double) temp;
   }

   public short ToInt16(IFormatProvider provider) {
      if (temp < Int16.MinValue || temp > Int16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int16 data type.", temp));
      else
         return (short) Math.Round(temp);
   }

   public int ToInt32(IFormatProvider provider) {
      if (temp < Int32.MinValue || temp > Int32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int32 data type.", temp));
      else
         return (int) Math.Round(temp);
   }

   public long ToInt64(IFormatProvider provider) {
      if (temp < Int64.MinValue || temp > Int64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int64 data type.", temp));
      else
         return (long) Math.Round(temp);
   }

   public sbyte ToSByte(IFormatProvider provider) {
      if (temp < SByte.MinValue || temp > SByte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the SByte data type.", temp));
      else
         return (sbyte) temp;
   }

   public float ToSingle(IFormatProvider provider) {
      return (float) temp;
   }

   public virtual string ToString(IFormatProvider provider) {
      return temp.ToString(provider) + "°";
   }

   // If conversionType is implemented by another IConvertible method, call it.
   public virtual object ToType(Type conversionType, IFormatProvider provider) {
      switch (Type.GetTypeCode(conversionType))
      {
         case TypeCode.Boolean:
            return this.ToBoolean(provider);
         case TypeCode.Byte:
            return this.ToByte(provider);
         case TypeCode.Char:
            return this.ToChar(provider);
         case TypeCode.DateTime:
            return this.ToDateTime(provider);
         case TypeCode.Decimal:
            return this.ToDecimal(provider);
         case TypeCode.Double:
            return this.ToDouble(provider);
         case TypeCode.Empty:
            throw new NullReferenceException("The target type is null.");
         case TypeCode.Int16:
            return this.ToInt16(provider);
         case TypeCode.Int32:
            return this.ToInt32(provider);
         case TypeCode.Int64:
            return this.ToInt64(provider);
         case TypeCode.Object:
            // Leave conversion of non-base types to derived classes.
            throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", 
                                           conversionType.Name));
         case TypeCode.SByte:
            return this.ToSByte(provider);
         case TypeCode.Single:
            return this.ToSingle(provider);
         case TypeCode.String:
            IConvertible iconv = this;
            return iconv.ToString(provider);
         case TypeCode.UInt16:
            return this.ToUInt16(provider);
         case TypeCode.UInt32:
            return this.ToUInt32(provider);
         case TypeCode.UInt64:
            return this.ToUInt64(provider);
         default:
            throw new InvalidCastException("Conversion not supported.");
      }
   }

   public ushort ToUInt16(IFormatProvider provider) {
      if (temp < UInt16.MinValue || temp > UInt16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt16 data type.", temp));
      else
         return (ushort) Math.Round(temp);
   }

   public uint ToUInt32(IFormatProvider provider) {
      if (temp < UInt32.MinValue || temp > UInt32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt32 data type.", temp));
      else
         return (uint) Math.Round(temp);
   }

   public ulong ToUInt64(IFormatProvider provider) {
      if (temp < UInt64.MinValue || temp > UInt64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt64 data type.", temp));
      else
         return (ulong) Math.Round(temp);
   }
}

public class TemperatureCelsius : Temperature, IConvertible
{
   public TemperatureCelsius(decimal value) : base(value)
   {
   }

   // Override ToString methods.
   public override string ToString()
   {
      return this.ToString(null);
   }

   public override string ToString(IFormatProvider provider)
   {
      return temp.ToString(provider) + "°C"; 
   }

   // If conversionType is a implemented by another IConvertible method, call it.
   public override object ToType(Type conversionType, IFormatProvider provider) {
      // For non-objects, call base method.
      if (Type.GetTypeCode(conversionType) != TypeCode.Object) {
         return base.ToType(conversionType, provider);
      }   
      else
      {   
         if (conversionType.Equals(typeof(TemperatureCelsius)))
            return this;
         else if (conversionType.Equals(typeof(TemperatureFahrenheit)))
            return new TemperatureFahrenheit((decimal) this.temp * 9 / 5 + 32);
         else
            throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", 
                                           conversionType.Name));
      }
   }
}

public class TemperatureFahrenheit : Temperature, IConvertible 
{
   public TemperatureFahrenheit(decimal value) : base(value)
   {
   }

   // Override ToString methods.
   public override string ToString()
   {
      return this.ToString(null);
   }

   public override string ToString(IFormatProvider provider)
   {
      return temp.ToString(provider) + "°F"; 
   }

   public override object ToType(Type conversionType, IFormatProvider provider)
   { 
      // For non-objects, call base methood.
      if (Type.GetTypeCode(conversionType) != TypeCode.Object) {
         return base.ToType(conversionType, provider);
      }   
      else
      {   
         // Handle conversion between derived classes.
         if (conversionType.Equals(typeof(TemperatureFahrenheit))) 
            return this;
         else if (conversionType.Equals(typeof(TemperatureCelsius)))
            return new TemperatureCelsius((decimal) (this.temp - 32) * 5 / 9);
         // Unspecified object type: throw an InvalidCastException.
         else
            throw new InvalidCastException(String.Format("Cannot convert from Temperature to {0}.", 
                                           conversionType.Name));
      }                                 
   }
}

下列範例說明對這些 IConvertible 實作的多個呼叫，以將 TemperatureCelsius 物件轉換為 TemperatureFahrenheit 物件，反之亦然。

Dim tempC1 As New TemperatureCelsius(0)
Dim tempF1 As TemperatureFahrenheit = CType(Convert.ChangeType(tempC1, GetType(TemperatureFahrenheit), Nothing), TemperatureFahrenheit)
Console.WriteLine("{0} equals {1}.", tempC1, tempF1) 
Dim tempC2 As TemperatureCelsius = CType(Convert.ChangeType(tempC1, GetType(TemperatureCelsius), Nothing), TemperatureCelsius)
Console.WriteLine("{0} equals {1}.", tempC1, tempC2) 
Dim tempF2 As New TemperatureFahrenheit(212)
Dim tempC3 As TEmperatureCelsius = CType(Convert.ChangeType(tempF2, GEtType(TemperatureCelsius), Nothing), TemperatureCelsius)
Console.WriteLine("{0} equals {1}.", tempF2, tempC3) 
Dim tempF3 As TemperatureFahrenheit = CType(Convert.ChangeType(tempF2, GetType(TemperatureFahrenheit), Nothing), TemperatureFahrenheit)
Console.WriteLine("{0} equals {1}.", tempF2, tempF3)
' The example displays the following output:
'       0°C equals 32°F.
'       0°C equals 0°C.
'       212°F equals 100°C.
'       212°F equals 212°F.

TemperatureCelsius tempC1 = new TemperatureCelsius(0);
TemperatureFahrenheit tempF1 = (TemperatureFahrenheit) Convert.ChangeType(tempC1, typeof(TemperatureFahrenheit), null);
Console.WriteLine("{0} equals {1}.", tempC1, tempF1);
TemperatureCelsius tempC2 = (TemperatureCelsius) Convert.ChangeType(tempC1, typeof(TemperatureCelsius), null);
Console.WriteLine("{0} equals {1}.", tempC1, tempC2);
TemperatureFahrenheit tempF2 = new TemperatureFahrenheit(212);
TemperatureCelsius tempC3 = (TemperatureCelsius) Convert.ChangeType(tempF2, typeof(TemperatureCelsius), null);
Console.WriteLine("{0} equals {1}.", tempF2, tempC3);
TemperatureFahrenheit tempF3 = (TemperatureFahrenheit) Convert.ChangeType(tempF2, typeof(TemperatureFahrenheit), null);
Console.WriteLine("{0} equals {1}.", tempF2, tempF3);
// The example displays the following output:
//       0°C equals 32°F.
//       0°C equals 0°C.
//       212°F equals 100°C.
//       212°F equals 212°F.

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TypeConverter 類別

.NET Framework 也可讓您延伸 System.ComponentModel.TypeConverter 類別，並透過 System.ComponentModel.TypeConverterAttribute 屬性來建立型別轉換子與型別之間的關聯，以定義自訂型別的型別轉換子。下表特別強調這種作法與實作自訂型別之 IConvertible 介面的差異。

注意事項
只要自訂型別有為其定義的型別轉換子，即會提供設計階段支援給自訂型別。

使用 TypeConverter 的轉換	使用 IConvertible 的轉換
是從 TypeConverter 衍生另外一個類別，以針對自訂型別來實作。透過套用 TypeConverterAttribute 屬性，將這個衍生類別與自訂型別建立關聯。	是由自訂型別所實作來執行轉換。該型別的使用者在該型別上叫用 IConvertible 轉換方法。
可以在設計階段和在執行階段兩處使用。	只能在執行階段使用。
使用反映，因此，速度比 IConvertible 所啟用的轉換還要慢。	不使用反映。
允許在自訂型別與其他資料型別之間的雙向型別轉換。例如，針對 MyType 所定義的 TypeConverter 允許從 MyType 轉換為 String，也允許從 String 轉換為 MyType。	允許從自訂型別轉換為其他資料型別，但不允許從其他資料型別轉換為自訂型別。

如需使用型別轉換子執行轉換的詳細資訊，請參閱 System.ComponentModel.TypeConverter。

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請參閱

參考

System.Convert

IConvertible

概念

型別轉換表

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.NET Framework 中的型別轉換

使用隱含運算子的隱含轉換

使用明確運算子的明確轉換

IConvertible 介面

Convert 類別

在基底型別之間轉換

使用 ChangeType 方法的自訂轉換

TypeConverter 類別

請參閱

參考

概念

其他資源