Type Memory Layout

Note

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Sometimes it is useful to know details about the memory layout of a type at runtime. Knowing the memory layout of a type can help in various processes, including:

• Getting the size of a type at runtime.
• Calculating field pointer offsets within a type.

AsmResolver provides an API to statically infer information about the memory layout of any given blittable type. It supports structures marked as SequentialLayout and ExplicitLayout, as well as field alignments and custom field offsets.

To get access to the API, you must include the following namespace:

using AsmResolver.DotNet.Memory;

Obtaining the type layout

To get the memory layout of any ITypeDescriptor, use the following extension method:

ITypeDescriptor type = ...
TypeMemoryLayout typeLayout = type.GetImpliedMemoryLayout(is32Bit: false);

Warning

Only value types that are marked with the SequentialLayout or ExplicitLayout structure layout are fully supported. If AutoLayout was provided, a sequential layout is assumed. This might not be the case for all implementations of the CLR.

Warning

If the type contains a cyclic dependency (e.g. a field with field type equal to its enclosing class), this method will throw an instance of the CyclicStructureException class.

The size of a type

After the memory layout is inferred, you can query the Size property to obtain the total size in bytes of the type.

uint size = typeLayout.Size;

Getting field offsets

The TypeMemoryLayout provides an indexer property that takes an instance of FieldDefinition, and returns an instance of FieldMemoryLayout.

FieldDefinition field = ...
FieldMemoryLayout fieldLayout = typeLayout[field];

This class contains the offset of the queried field within the type:

uint offset = fieldLayout.Offset;

It also provides another instance of TypeMemoryLayout to get the layout of the contents of the field:

TypeMemoryLayout contentsLayout = fieldLayout.ContentsLayout;

This can be used to recursively find fields and their offsets.

Note

A TypeMemoryLayout describing the layout of type T might be different from a TypeMemoryLayout that was associated to a field, even if this field has field type T as well. This is due to the fact that the CLR might layout nested fields differently when a structure defines a field with a compound field type.

Getting fields by offset

It is also possible to turn an offset (relative to the start of the type) to the field definition that is stored at that offset. This is done by using the TryGetFieldAtOffset method.

uint offset = ...
if (typeLayout.TryGetFieldAtOffset(offset, out var fieldLayout))
{
    // There is a field defined at this offset.
}

Sometimes, offsets within a structure refer to a field within a nested field. For example, consider the following sample code:

[StructLayout(LayoutKind.Sequential, Size = 17)]
public struct Struct1
{
    public int Dummy1;
}

[StructLayout(LayoutKind.Sequential, Size = 23, Pack = 2)]
public struct Struct2
{
    public Struct1 Nest1;
}

[StructLayout(LayoutKind.Sequential, Size = 87, Pack = 64)]
public struct Struct3
{
    public Struct1 Nest1;

    public Struct2 Nest2;
}

To get a collection of fields to access to reach a certain offset within the type, use the TryGetFieldPath method. This method will return true if the offset refers to the beginning of a field, and false otherwise.

var struct3Definition = (TypeDefinition) Module.LookupMember(
    typeof(Struct3).MetadataToken);
var struct3Layout = struct3Definition.GetImpliedMemoryLayout(false);

uint offset = 20;
bool isStartOfField = layout.TryGetFieldPath(offset, out var path);

// This results in:
//  - isStartOfField: true.
//  - path: {Struct3::Nest2, Struct2::Nest1, Struct1::Dummy1}.