The CLI supports both sequential and explicit layout control, see §II.10.1.2. For explicit layout it is also necessary to specify the precise layout of an instance; see also §II.22.18 and §II.22.16.
| FieldDecl ::= |
|---|
[ '[' Int32 ']' ] FieldAttr* Type Id |
The optional int32 specified in brackets at the beginning of the declaration specifies the byte offset from the beginning of the instance of the type. (For a given type t, this beginning refers to the start of the set of members explicitly defined in type t, excluding all members defined in any types from which type t directly or indirectly inherits.) This form of explicit layout control shall not be used with global fields specified using the at notation §II.16.3.2).
Offset values shall be non-negative. It is possible to overlap fields in this way, though offsets occupied by an object reference shall not overlap with offsets occupied by a built-in value type or a part of another object reference. While one object reference can completely overlap another, this is unverifiable.
Fields can be accessed using pointer arithmetic and ldind to load the field indirectly or stind to store the field indirectly (see Partition III). See §II.22.16 and §II.22.18 for encoding of this information. For explicit layout, every field shall be assigned an offset.
The .pack directive specifies that fields should be placed within the runtime object at byte addresses which are a multiple of the specified number, or at natural alignment for that field type, whichever is smaller. For example, .pack 2 would allow 32-bit-wide fields to be started on even addresses, whereas without any .pack directive, they would be naturally aligned; that is, placed on addresses that are a multiple of 4. The integer following .pack shall be one of the following: 0, 1, 2, 4, 8, 16, 32, 64, or 128. (A value of zero indicates that the pack size used should match the default for the current platform.) The .pack directive shall not be supplied for any type with explicit layout control.
The .size directive indicates a minimum size, and is intended to allow for padding. Therefore, the amount of memory allocated is the maximum of the size calculated from the layout and the .size directive. Note that if this directive applies to a value type, then the size shall be less than 1 MByte.
[Note: Metadata that controls instance layout is not a "hint," it is an integral part of the VES that shall be supported by all conforming implementations of the CLI. end note]
[Example: The following class uses sequential layout of its fields:
.class sequential public SequentialClass
{ .field public int32 a // store at offset 0 bytes
.field public int32 b // store at offset 4 bytes
}
The following class uses explicit layout of its fields:
.class explicit public ExplicitClass
{ .field [0] public int32 a // store at offset 0 bytes
.field [6] public int32 b // store at offset 6 bytes
}
The following value type uses .pack to pack its fields together:
.class value sealed public MyClass extends [mscorlib]System.ValueType
{ .pack 2
.field public int8 a // store at offset 0 bytes
.field public int32 b // store at offset 2 bytes (not 4)
}
The following class specifies a contiguous block of 16 bytes:
.class public BlobClass
{ .size 16
}
end example]