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StructLayoutAttribute.Pack Field

Updated: February 2010

Controls the alignment of data fields of a class or structure in memory.

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

'Declaration
Public Pack As Integer
'Usage
Dim instance As StructLayoutAttribute 
Dim value As Integer 

value = instance.Pack 

instance.Pack = value

This field indicates memory boundaries that should be used when the LayoutKind.Sequential value is specified.

The value of Pack must be 0, 1, 2, 4, 8, 16, 32, 64, or 128:

  • A value of 0 indicates that the packing alignment is set to the default for the current platform.

  • A value of 1 indicates that data alignment occurs on byte boundaries. There are no gaps between fields with a packing value of 1.

  • Packing values of 2 and higher will cause each field to be aligned on a byte offset relative to the beginning of the structure. Therefore, data fields will start on offsets that are multiples of the requested packing value.

The following example contains a structure with three one-byte fields:

struct MyStruct
{
byte B0;
byte B1;
byte B2;
}

Byte B0 always starts at offset 0 (byte 0) regardless of the packing value.

A packing value of 0 will produce the following:

  • B0 will begin at offset 0 (byte 0).

  • B1 will begin at offset 1 (byte 1).

  • B2 will begin at offset 2 (byte 2).

Note that the default platform alignment will always pack identical types contiguously.

A packing value of 1 will produce the following:

  • B0 will still begin at offset 0 (byte 0).

  • B1 will still begin at offset 1 (byte 1).

  • B2 will still begin at offset 2 (byte 2).

However, a packing value of 2 will produce the following:

  • B0 will still begin at offset 0 (byte 0).

  • B1 will begin at offset 2 (byte 2).

  • B2 will begin at offset 4 (byte 4).

Similarily, a packing value of n will produce the following:

  • B0 will still begin at offset 0 (byte 0).

  • B1 will begin at offset n (byte n).

  • B2 will begin at offset n*2 (byte 2n).

The Pack field is frequently used when structures are exported during disk and network write operations. The field is also frequently used during platform invoke and interop operations.

Occasionally, the field is used to reduce memory requirements by producing a tighter packing size. However, this usage requires careful consideration of actual hardware constraints, and may actually degrade performance.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

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

Date

History

Reason

February 2010

Edited remarks for clarity, and removed incorrect statement pointed out in Wiki comment.

Customer feedback.

July 2009

Improved remarks.

Information enhancement.

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