Pack Field

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

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

public int Pack

This field indicates memory boundaries for aligning fields. It affects both LayoutKind..::..Sequential and LayoutKind..::..Explicit. By default, the value is 0, indicating the default packing size for the current platform.

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.

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