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Encoding.GetMaxCharCount Method

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When overridden in a derived class, calculates the maximum number of characters produced by decoding the specified number of bytes.

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

Syntax

'Declaration
Public MustOverride Function GetMaxCharCount ( _
    byteCount As Integer _
) As Integer

public abstract int GetMaxCharCount(
    int byteCount
)

Parameters

  • byteCount
    Type: System.Int32
    The number of bytes to decode.

Return Value

Type: System.Int32
The maximum number of characters produced by decoding the specified number of bytes.

Exceptions

Exception Condition
ArgumentOutOfRangeException

byteCount is less than zero.
DecoderFallbackException

A fallback occurred (see Understanding Encodings for a complete explanation).

Remarks

To calculate the exact array size required by GetChars to store the resulting characters, the application should use GetCharCount. To calculate the maximum array size, it should use GetMaxCharCount. The GetCharCount method generally allocates less memory, while the GetMaxCharCount method generally executes faster.

GetMaxCharCount retrieves a worst-case number. If a fallback is chosen with a potentially large string, GetMaxCharCount retrieves large values.

In most cases, this method retrieves reasonable numbers for small strings. For large strings, you might have to choose between using very large buffers and catching errors in the rare case that a more reasonable buffer is too small. You might also want to consider a different approach using GetCharCount or Decoder.Convert.

GetMaxCharCount has no relation to GetBytes. If your application needs a similar function to use with GetBytes, it should use GetMaxByteCount.

When using GetMaxCharCount, you should allocate the output buffer based on the maximum size of the input buffer. If the output buffer is constrained in size, you might use the Encoder.Convert(array<Char[], Int32, Int32, array<Byte[], Int32, Int32, Boolean, Int32%, Int32%, Boolean%) or Decoder.Convert(array<Byte[], Int32, Int32, array<Char[], Int32, Int32, Boolean, Int32%, Int32%, Boolean%) method instead.

Note that GetMaxCharCount considers the worst case for leftover bytes from a previous encoder operation. For most code pages, passing a value of 0 to this method retrieves values greater than or equal to 1.

NoteNote:

GetMaxCharCount(N) is not necessarily the same value as N* GetMaxCharCount(1).

Notes to Implementers

All Encoding implementations must guarantee that no buffer overflow exceptions occur if buffers are sized according to the results of this method's calculations.

Examples

The following code example encodes a string into an array of bytes, and then decodes the bytes into an array of characters.

Imports System.Text

Public Class Example
   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      ' Create two instances of UTF32Encoding: one with little-endian byte order and one with big-endian byte order.
      Dim u32LE As Encoding = Encoding.GetEncoding("utf-32")
      Dim u32BE As Encoding = Encoding.GetEncoding("utf-32BE")

      ' Use a string containing the following characters:
      '    Latin Small Letter Z (U+007A)
      '    Latin Small Letter A (U+0061)
      '    Combining Breve (U+0306)
      '    Latin Small Letter AE With Acute (U+01FD)
      '    Greek Small Letter Beta (U+03B2)
      Dim myStr As String = "za" & ChrW(&H306) & ChrW(&H1FD) & ChrW(&H3B2)

      ' Encode the string using the big-endian byte order.
      ' NOTE: In VB.NET, arrays contain one extra element by default.
      '       The following line creates the array with the exact number of elements required.
      Dim barrBE(u32BE.GetByteCount(myStr) - 1) As Byte
      u32BE.GetBytes(myStr, 0, myStr.Length, barrBE, 0)

      ' Encode the string using the little-endian byte order.
      ' NOTE: In VB.NET, arrays contain one extra element by default.
      '       The following line creates the array with the exact number of elements required.
      Dim barrLE(u32LE.GetByteCount(myStr) - 1) As Byte
      u32LE.GetBytes(myStr, 0, myStr.Length, barrLE, 0)

      ' Get the char counts, and decode the byte arrays.
      outputBlock.Text &= "BE array with BE encoding : "
      PrintCountsAndChars(outputBlock, barrBE, u32BE)
      outputBlock.Text &= "LE array with LE encoding : "
      PrintCountsAndChars(outputBlock, barrLE, u32LE)
   End Sub

   Public Shared Sub PrintCountsAndChars(ByVal outputBlock As System.Windows.Controls.TextBlock, ByVal bytes() As Byte, ByVal enc As Encoding)
      ' Display the name of the encoding used.
      outputBlock.Text += String.Format("{0,-25} :", enc.ToString())

      ' Display the exact character count.
      Dim iCC As Integer = enc.GetCharCount(bytes)
      outputBlock.Text += String.Format(" {0,-3}", iCC)

      ' Display the maximum character count.
      Dim iMCC As Integer = enc.GetMaxCharCount(bytes.Length)
      outputBlock.Text += String.Format(" {0,-3} :", iMCC)

      ' Decode the bytes and display the characters.
      Dim chars As Char() = enc.GetChars(bytes)
      outputBlock.Text &= chars & vbCrLf

   End Sub 
End Class


'This code produces the following output.
'
'    BE array with BE encoding : System.Text.UTF32Encoding : 5   12  :zăǽβ
'    LE array with LE encoding : System.Text.UTF32Encoding : 5   12  :zăǽβ
'

using System;
using System.Text;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      // Create two instances of UTF32Encoding: one with little-endian byte order and one with big-endian byte order.
      Encoding u32LE = Encoding.GetEncoding("utf-32");
      Encoding u32BE = Encoding.GetEncoding("utf-32BE");

      // Use a string containing the following characters:
      //    Latin Small Letter Z (U+007A)
      //    Latin Small Letter A (U+0061)
      //    Combining Breve (U+0306)
      //    Latin Small Letter AE With Acute (U+01FD)
      //    Greek Small Letter Beta (U+03B2)
      String myStr = "za\u0306\u01FD\u03B2";

      // Encode the string using the big-endian byte order.
      byte[] barrBE = new byte[u32BE.GetByteCount(myStr)];
      u32BE.GetBytes(myStr, 0, myStr.Length, barrBE, 0);

      // Encode the string using the little-endian byte order.
      byte[] barrLE = new byte[u32LE.GetByteCount(myStr)];
      u32LE.GetBytes(myStr, 0, myStr.Length, barrLE, 0);

      // Get the char counts, and decode the byte arrays.
      outputBlock.Text += "BE array with BE encoding : ";
      PrintCountsAndChars(outputBlock, barrBE, u32BE);
      outputBlock.Text += "LE array with LE encoding : ";
      PrintCountsAndChars(outputBlock, barrLE, u32LE);
   }

   public static void PrintCountsAndChars(System.Windows.Controls.TextBlock outputBlock, byte[] bytes, Encoding enc)
   {

      // Display the name of the encoding used.
      outputBlock.Text += String.Format("{0,-25} :", enc.ToString());

      // Display the exact character count.
      int iCC = enc.GetCharCount(bytes);
      outputBlock.Text += String.Format(" {0,-3}", iCC);

      // Display the maximum character count.
      int iMCC = enc.GetMaxCharCount(bytes.Length);
      outputBlock.Text += String.Format(" {0,-3} :", iMCC);

      // Decode the bytes and display the characters.
      char[] chars = enc.GetChars(bytes);
      outputBlock.Text += chars + "\n";
   }
}


/* 
This code produces the following output.

   BE array with BE encoding : System.Text.UTF32Encoding : 5   12  :zăǽβ
   LE array with LE encoding : System.Text.UTF32Encoding : 5   12  :zăǽβ

*/

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.