IFormatProvider.GetFormat(Type) 方法

public:
 System::Object ^ GetFormat(Type ^ formatType);

public object GetFormat(Type formatType);

abstract member GetFormat : Type -> obj

Public Function GetFormat (formatType As Type) As Object

參數

傳回

若IFormatProvider實作能提供該類型的物件，則為由 formatType指定的物件實例;否則，為 null。

範例

以下範例說明了實作 IFormatProvider 介面與 GetFormat 方法的類別的使用方式。 該 AcctNumberFormat 類別將 Int64 代表帳號的數值轉換為格式化的 12 位數帳號。 若參數指向實作的類別，則其GetFormat方法回ICustomFormatter傳自身的參考;否則，回GetFormat傳 null。formatType

public class AcctNumberFormat : IFormatProvider, ICustomFormatter
{
   private const int ACCT_LENGTH = 12;

   public object GetFormat(Type formatType)
   {
      if (formatType == typeof(ICustomFormatter))
         return this;
      else
         return null;
   }

   public string Format(string fmt, object arg, IFormatProvider formatProvider)
   {
      // Provide default formatting if arg is not an Int64.
      if (arg.GetType() != typeof(Int64))
         try {
            return HandleOtherFormats(fmt, arg);
         }
         catch (FormatException e) {
            throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);
         }

      // Provide default formatting for unsupported format strings.
      string ufmt = fmt.ToUpper(CultureInfo.InvariantCulture);
      if (!(ufmt == "H" || ufmt == "I"))
         try {
            return HandleOtherFormats(fmt, arg);
         }
         catch (FormatException e) {
            throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);
         }

      // Convert argument to a string.
      string result = arg.ToString();

      // If account number is less than 12 characters, pad with leading zeroes.
      if (result.Length < ACCT_LENGTH)
         result = result.PadLeft(ACCT_LENGTH, '0');
      // If account number is more than 12 characters, truncate to 12 characters.
      if (result.Length > ACCT_LENGTH)
         result = result.Substring(0, ACCT_LENGTH);

      if (ufmt == "I")                    // Integer-only format.
         return result;
      // Add hyphens for H format specifier.
      else                                         // Hyphenated format.
         return result.Substring(0, 5) + "-" + result.Substring(5, 3) + "-" + result.Substring(8);
   }

   private string HandleOtherFormats(string format, object arg)
   {
      if (arg is IFormattable)
         return ((IFormattable)arg).ToString(format, CultureInfo.CurrentCulture);
      else if (arg != null)
         return arg.ToString();
      else
         return String.Empty;
   }
}

open System
open System.Globalization

type AcctNumberFormat() =
    let [<Literal>] ACCT_LENGTH = 12

    interface IFormatProvider with
        member this.GetFormat(formatType: Type) =
            if formatType = typeof<ICustomFormatter> then
                this
            else
                null

    interface ICustomFormatter with
        member this.Format(fmt: string, arg: obj, formatProvider: IFormatProvider) =
            // Provide default formatting if arg is not an Int64.
            // Provide default formatting for unsupported format strings.
            let ufmt = fmt.ToUpper CultureInfo.InvariantCulture
            if arg.GetType() = typeof<Int64> && (ufmt = "H" || ufmt = "I") then
                // Convert argument to a string.
                let result = string arg

                let result =
                    // If account number is less than 12 characters, pad with leading zeroes.
                    if result.Length < ACCT_LENGTH then
                        result.PadLeft(ACCT_LENGTH, '0')
                    else result
                
                let result =
                    // If account number is more than 12 characters, truncate to 12 characters.
                    if result.Length > ACCT_LENGTH then
                        result.Substring(0, ACCT_LENGTH)
                    else result

                // Integer-only format.
                if ufmt = "I" then 
                    result
                // Add hyphens for H format specifier.
                else // Hyphenated format.
                    result.Substring(0, 5) + "-" + result.Substring(5, 3) + "-" + result.Substring(8)
            else
                try
                    this.HandleOtherFormats(fmt, arg)
                with :? FormatException as e ->
                    raise (FormatException($"The format of '{fmt}' is invalid.", e))
            
    member _.HandleOtherFormats(format: string, arg: obj) =
        match arg with
        | :? IFormattable as arg ->
            arg.ToString(format, CultureInfo.CurrentCulture)
        | null -> 
            string arg
        | _ -> 
            String.Empty

Public Class AcctNumberFormat : Implements IFormatProvider, ICustomFormatter

   Private Const ACCT_LENGTH As Integer = 12
   
   Public Function GetFormat(formatType As Type) As Object _
                   Implements IFormatProvider.GetFormat
      If formatType Is GetType(ICustomFormatter) Then
         Return Me
      Else
         Return Nothing
      End If
   End Function
   
   Public Function Format(fmt As String, arg As Object, formatProvider As IFormatProvider) As String _
                          Implements ICustomFormatter.Format

      ' Provide default formatting if arg is not an Int64.
       If Not TypeOf arg Is Int64 Then
         Try 
            Return HandleOtherFormats(fmt, arg) 
         Catch e As FormatException 
            Throw New FormatException(String.Format("The format of '{0}' is invalid.", fmt), e)
         End Try
       End If   
                     
      ' Provider default formatting for unsupported format strings.
      Dim ufmt As String = fmt.ToUpper(CultureInfo.InvariantCulture)
      If Not (ufmt = "H" Or ufmt = "I") Then
         Try
            Return HandleOtherFormats(fmt, arg)
         Catch e As FormatException 
            Throw New FormatException(String.Format("The format of '{0}' is invalid.", fmt), e)
         End Try
      End If   

      ' Convert argument to a string.
      Dim result As String = arg.ToString()
      
      ' If account number is less than 12 characters, pad with leading zeroes.
      If result.Length < ACCT_LENGTH Then result = result.PadLeft(ACCT_LENGTH, "0"c)
      ' If account number is more than 12 characters, truncate to 12 characters.
      If result.Length > ACCT_LENGTH Then result = Left(result, ACCT_LENGTH)   
      
      If ufmt = "I"                              ' Integer-only format.
         Return result
      ' Add hyphens for H format specifier.
      Else                                       ' Hypenated format.
         Return Left(result, 5) & "-" & Mid(result, 6, 3) & "-" & Right(result, 4)
      End If   
   End Function   

   Private Function HandleOtherFormats(fmt As String, arg As Object) As String
      If TypeOf arg Is IFormattable Then
         Return DirectCast(arg, IFormattable).ToString(fmt, CultureInfo.CurrentCulture)
      ElseIf arg IsNot Nothing Then
         Return arg.ToString()
      Else
         Return String.Empty
      End If
   End Function
End Class

該類別的實例 AcctNumberFormat 可作為參數傳遞給提供格式化或解析服務的方法。 例如，以下程式碼將一個 AcctNumberFormat 類別傳給 String.Format(IFormatProvider, String, Object[]) 產生格式化的 12 位數帳號的方法。

using System;
using System.Globalization;

public enum DaysOfWeek { Monday=1, Tuesday=2 };

public class TestFormatting
{
   public static void Main()
   {
      long acctNumber;
      double balance;
      DaysOfWeek wday;
      string output;

      acctNumber = 104254567890;
      balance = 16.34;
      wday = DaysOfWeek.Monday;

      output = String.Format(new AcctNumberFormat(),
                             "On {2}, the balance of account {0:H} was {1:C2}.",
                             acctNumber, balance, wday);
      Console.WriteLine(output);

      wday = DaysOfWeek.Tuesday;
      output = String.Format(new AcctNumberFormat(),
                             "On {2}, the balance of account {0:I} was {1:C2}.",
                             acctNumber, balance, wday);
      Console.WriteLine(output);
   }
}
// The example displays the following output:
//       On Monday, the balance of account 10425-456-7890 was $16.34.
//       On Tuesday, the balance of account 104254567890 was $16.34.

open System
open System.Globalization

type DaysOfWeek = Monday = 1 | Tuesday = 2

[<EntryPoint>]
let main _ =
    let acctNumber = 104254567890L
    let balance = 16.34
    let wday = DaysOfWeek.Monday

    let output = 
        String.Format(AcctNumberFormat(), 
                      "On {2}, the balance of account {0:H} was {1:C2}.", 
                      acctNumber, balance, wday)
    printfn $"{output}"

    let wday = DaysOfWeek.Tuesday
    let output = 
        String.Format(AcctNumberFormat(),
                      "On {2}, the balance of account {0:I} was {1:C2}.",
                      acctNumber, balance, wday)
    printfn $"{output}"
    0

// The example displays the following output:
//       On Monday, the balance of account 10425-456-7890 was $16.34.
//       On Tuesday, the balance of account 104254567890 was $16.34.

Imports System.Globalization

Public Enum DaysOfWeek As Long
   Monday = 1
   Tuesday = 2
End Enum

Module TestFormatting
   Public Sub Main()
      Dim acctNumber As Long, balance As Double 
      Dim wday As DaysOfWeek 
      Dim output As String

      acctNumber = 104254567890
      balance = 16.34
      wday = DaysOfWeek.Monday

      output = String.Format(New AcctNumberFormat(), "On {2}, the balance of account {0:H} was {1:C2}.", acctNumber, balance, wday)
      Console.WriteLine(output)

      wday = DaysOfWeek.Tuesday
      output = String.Format(New AcctNumberFormat(), "On {2}, the balance of account {0:I} was {1:C2}.", acctNumber, balance, wday)
      Console.WriteLine(output)
   End Sub
End Module
' The example displays the following output:
'    On Monday, the balance of account 10425-456-7890 was $16.34.
'    On Tuesday, the balance of account 104254567890 was $16.34.

備註

GetFormat 是一種回調方法，格式化與解析方法會呼叫它，用以在解析操作中取得輸入字串格式或格式化操作中輸出字串格式的資訊。 在參數中 formatType ，格式化或解析方法傳遞執行其操作所需的物件類型。 如果實 IFormatProvider 作能提供這個格式化或解析物件，則會回傳該物件。 若不行，則返回 null。

例如，在呼叫 Int32.ToString(IFormatProvider) 該方法時，method 參數是一個 IFormatProvider 物件，提供關於當前整數實例字串表示可能格式化的資訊。 執行時執行該方法時，會呼叫 IFormatProvider 物件的方法 GetFormat ，並傳入 Type 一個代表該型別的 NumberFormatInfo 物件。 如果 IFormatProvider 物件能提供物件， NumberFormatInfo 它就會回傳該物件。 若無法提供該類型的物件，則回傳 null。

你可以在類別中實作 IFormatProvider 介面和 GetFormat 方法，提供自訂格式化或解析服務。 接著，實IFormatProvider作作為參數傳遞給任何帶有型態IFormatProvider為 、 String.Format(IFormatProvider, String, Object[])Int32.ToString(String, IFormatProvider)、 或 DateTime.Parse(String, IFormatProvider)的參數的解析或格式方法的超載。