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Encoding.GetMaxByteCount Method

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

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

public abstract int GetMaxByteCount(
	int charCount


Type: System.Int32
The number of characters to encode.

Return Value

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


charCount is less than zero.


A fallback occurred (see Character Encoding in the .NET Framework for complete explanation)


EncoderFallback is set to EncoderExceptionFallback.

The charCount parameter actually specifies the number of Char objects that represent the Unicode characters to encode, because the .NET Framework internally uses UTF-16 to represent Unicode characters. Consequently, most Unicode characters can be represented by one Char object, but a Unicode character represented by a surrogate pair, for example, requires two Char objects.

To calculate the exact array size required by GetBytes to store the resulting bytes, the application should use GetByteCount. To calculate the maximum array size, it should use GetMaxByteCount. The GetByteCount method generally allows allocation of less memory, while the GetMaxByteCount method generally executes faster.

GetMaxByteCount retrieves a worst-case number, including the worst case for the currently selected EncoderFallback. If a fallback is chosen with a potentially large string, GetMaxByteCount retrieves large values, particularly in cases where the worst case for the encoding involves switching modes for every character. For example, this can happen for ISO-2022-JP. For more information, see the blog entry "What's with Encoding.GetMaxByteCount() and Encoding.GetMaxCharCount()?" (

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

When using GetMaxByteCount, your application should allocate the output buffer based on the maximum size of the input buffer. If the output buffer is constrained in size, the application might use the Convert method.

Note that GetMaxByteCount considers potential leftover surrogates from a previous decoder operation. Because of the decoder, passing a value of 1 to the method retrieves 2 for a single-byte encoding, such as ASCII. Your application should use the IsSingleByte property if this information is necessary.


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

The following example determines the number of bytes required to encode a character array, encodes the characters, and displays the resulting bytes.

using System;
using System.Text;

public class SamplesEncoding  {

   public static void Main()  {

      // The characters to encode:
      //    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)
      //    a high-surrogate value (U+D8FF)
      //    a low-surrogate value (U+DCFF)
      char[] myChars = new char[] { 'z', 'a', '\u0306', '\u01FD', '\u03B2', '\uD8FF', '\uDCFF' };

      // Get different encodings.
      Encoding  u7    = Encoding.UTF7;
      Encoding  u8    = Encoding.UTF8;
      Encoding  u16LE = Encoding.Unicode;
      Encoding  u16BE = Encoding.BigEndianUnicode;
      Encoding  u32   = Encoding.UTF32;

      // Encode the entire array, and print out the counts and the resulting bytes.
      PrintCountsAndBytes( myChars, u7 );
      PrintCountsAndBytes( myChars, u8 );
      PrintCountsAndBytes( myChars, u16LE );
      PrintCountsAndBytes( myChars, u16BE );
      PrintCountsAndBytes( myChars, u32 );


   public static void PrintCountsAndBytes( char[] chars, Encoding enc )  {

      // Display the name of the encoding used.
      Console.Write( "{0,-30} :", enc.ToString() );

      // Display the exact byte count.
      int iBC  = enc.GetByteCount( chars );
      Console.Write( " {0,-3}", iBC );

      // Display the maximum byte count.
      int iMBC = enc.GetMaxByteCount( chars.Length );
      Console.Write( " {0,-3} :", iMBC );

      // Encode the array of chars.
      byte[] bytes = enc.GetBytes( chars );

      // Display all the encoded bytes.
      PrintHexBytes( bytes );


   public static void PrintHexBytes( byte[] bytes )  {

      if (( bytes == null ) || ( bytes.Length == 0 ))
         Console.WriteLine( "<none>" );
      else  {
         for ( int i = 0; i < bytes.Length; i++ )
            Console.Write( "{0:X2} ", bytes[i] );



This code produces the following output.

System.Text.UTF7Encoding       : 18  23  :7A 61 2B 41 77 59 42 2F 51 4F 79 32 50 2F 63 2F 77 2D
System.Text.UTF8Encoding       : 12  24  :7A 61 CC 86 C7 BD CE B2 F1 8F B3 BF
System.Text.UnicodeEncoding    : 14  16  :7A 00 61 00 06 03 FD 01 B2 03 FF D8 FF DC
System.Text.UnicodeEncoding    : 14  16  :00 7A 00 61 03 06 01 FD 03 B2 D8 FF DC FF
System.Text.UTF32Encoding      : 24  32  :7A 00 00 00 61 00 00 00 06 03 00 00 FD 01 00 00 B2 03 00 00 FF FC 04 00


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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