# Metadata Reading

The Metadata API gives developers low-level access to the information encoded in CLI modules: type and methods definitions encoded in metadata as well as access to the CIL code and embedded resources.

Managed developers access this information using either the System.Reflection API or a library like Cecil.

To start using the Metadata API it is necessary to open an assembly or a CIL image (a .dll or .exe file) using one of the CIL image opening API calls.

## Metadata Heaps

ECMA CLI images contain four heaps that store different kinds of information, these are:

- GUID heap (called #GUID) contains 128-bit GUIDs, you can get pointer to a GUID value by calling mono_metadata_guid_heap.
- User string heap (called #US), it contains user visible strings, you can fetch a string from this heap by calling mono_metadata_user_string.
- Blob heap (called #blob), it contains binary blobs of data, you can get a blob by mono_metadata_blob_heap. You can determine the size of the blob using the mono_metadata_decode_blob_size.

The ECMA file format also has an extra section called the "#~" stream, this stream is the one that holds the metadata tables. There is a high-level API to get access to the contents of this API, described in the section Metadata Tables.

#### mono_metadata_guid_heap

#### mono_metadata_string_heap

#### mono_metadata_blob_heap

#### mono_metadata_user_string

#### mono_metadata_decode_blob_size

## Metadata Tables

Metadata is encoded in a number of tables included on every CIL image. These tables contain type definitions, member definitions and so on, these constants are defined in the ECMA 335 specification Partition II section 22. The following table shows the C constants defined in the Mono runtime and how they map to the equivalent ECMA CLI metadata table:

ECMA CLI Table Name | C Constant Name | Table Schema (Array Size + Columns Constants)
| Assembly | MONO_TABLE_ASSEMBLY | Array size: MONO_ASSEMBLY_SIZE MONO_ASSEMBLY_HASH_ALG MONO_ASSEMBLY_MAJOR_VERSION MONO_ASSEMBLY_MINOR_VERSION MONO_ASSEMBLY_BUILD_NUMBER MONO_ASSEMBLY_REV_NUMBER MONO_ASSEMBLY_FLAGS MONO_ASSEMBLY_PUBLIC_KEY MONO_ASSEMBLY_NAME MONO_ASSEMBLY_CULTURE | AssemblyOS | MONO_TABLE_ASSEMBLYOS | Array size: MONO_ASSEMBLYOS_SIZE MONO_ASSEMBLYOS_PLATFORM MONO_ASSEMBLYOS_MAJOR_VERSION MONO_ASSEMBLYOS_MINOR_VERSION | AssemblyProcessor | MONO_TABLE_ASSEMBLYPROCESSOR | Array size: MONO_ASSEMBLYPROCESSOR_SIZE MONO_ASSEMBLY_PROCESSOR | AssemblyRef | MONO_TABLE_ASSEMBLYREF | Array size: MONO_ASSEMBLYREF_SIZE MONO_ASSEMBLYREF_MAJOR_VERSION MONO_ASSEMBLYREF_MINOR_VERSION MONO_ASSEMBLYREF_BUILD_NUMBER MONO_ASSEMBLYREF_REV_NUMBER MONO_ASSEMBLYREF_FLAGS MONO_ASSEMBLYREF_PUBLIC_KEY MONO_ASSEMBLYREF_NAME MONO_ASSEMBLYREF_CULTURE MONO_ASSEMBLYREF_HASH_VALUE | AssemblyRefProcessor | MONO_TABLE_ASSEMBLYREFPROCESSOR | Array size: MONO_ASSEMBLYREFPROC_SIZE MONO_ASSEMBLYREFPROC_PROCESSOR MONO_ASSEMBLYREFPROC_ASSEMBLYREF | AssemblyRefOS | MONO_TABLE_ASSEMBLYREFOS | Array size: MONO_ASSEMBLYREFOS_SIZE MONO_ASSEMBLYREFOS_PLATFORM MONO_ASSEMBLYREFOS_MAJOR_VERSION MONO_ASSEMBLYREFOS_MINOR_VERSION MONO_ASSEMBLYREFOS_ASSEMBLYREF | ClassLayout | MONO_TABLE_CLASSLAYOUT | Array size: MONO_CLASSLAYOUT_SIZE MONO_CLASS_LAYOUT_PACKING_SIZE MONO_CLASS_LAYOUT_CLASS_SIZE MONO_CLASS_LAYOUT_PARENT | Constant | MONO_TABLE_CONSTANT | Array size: MONO_CONSTANT_SIZE MONO_CONSTANT_TYPE MONO_CONSTANT_PADDING MONO_CONSTANT_PARENT MONO_CONSTANT_VALUE | CustomAttribute | MONO_TABLE_CUSTOMATTRIBUTE | Array size: MONO_CUSTOM_ATTR_SIZE MONO_CUSTOM_ATTR_PARENT MONO_CUSTOM_ATTR_TYPE MONO_CUSTOM_ATTR_VALUE | DeclSecurity | MONO_TABLE_DECLSECURITY | Array size: MONO_DECL_SECURITY_SIZE MONO_DECL_SECURITY_ACTION MONO_DECL_SECURITY_PARENT MONO_DECL_SECURITY_PERMISSIONSET | EventMap | MONO_TABLE_EVENTMAP | Array size: MONO_EVENT_MAP_SIZE MONO_EVENT_MAP_PARENT MONO_EVENT_MAP_EVENTLIST | EventPtr | MONO_TABLE_EVENT_POINTER | Array size: MONO_EVENT_POINTER_SIZE MONO_EVENT_POINTER_EVENT | Event | MONO_TABLE_EVENT | Array size: MONO_EVENT_SIZE MONO_EVENT_FLAGS MONO_EVENT_NAME MONO_EVENT_TYPE | ExportedType | MONO_TABLE_EXPORTEDTYPE | Array size: MONO_EXPORTEDTYPE_SIZE MONO_EXP_TYPE_FLAGS MONO_EXP_TYPE_TYPEDEF MONO_EXP_TYPE_NAME MONO_EXP_TYPE_NAMESPACE MONO_EXP_TYPE_IMPLEMENTATION | Field | MONO_TABLE_FIELD | Array size: MONO_FIELD_SIZE MONO_FIELD_FLAGS MONO_FIELD_NAME MONO_FIELD_SIGNATURE | FieldLayoutt | MONO_TABLE_FIELDLAYOUT | Array size: MONO_FIELDLAYOUT_SIZE MONO_FIELD_LAYOUT_OFFSET MONO_FIELD_LAYOUT_FIELD | FieldMarshal | MONO_TABLE_FIELDMARSHAL | Array size: MONO_FIELD_MARSHAL_SIZE MONO_FIELD_MARSHAL_PARENT MONO_FIELD_MARSHAL_NATIVE_TYPE | FieldPtr | MONO_TABLE_FIELD_POINTER | Array size: MONO_FIELD_POINTER_SIZE MONO_FIELD_POINTER_FIELD | FieldRVA | MONO_TABLE_FIELDRVA | Array size: MONO_FIELDRVA_SIZE MONO_FIELD_RVA_RVA MONO_FIELD_RVA_FIELD | File | MONO_TABLE_FILE | Array size: MONO_FILE_SIZE MONO_FILE_FLAGS MONO_FILE_NAME MONO_FILE_HASH_VALUE | GenericParam | MONO_TABLE_GENERICPARAM | Array size: MONO_GENERICPARAM_SIZE MONO_GENERICPARAM_NUMBER MONO_GENERICPARAM_FLAGS MONO_GENERICPARAM_OWNER MONO_GENERICPARAM_NAME | GenericParamConstraint | MONO_TABLE_GENERICPARAMCONSTRAINT | Array size: MONO_GENERICPARAMCONSTRAINT_SIZE MONO_GENPARCONSTRAINT_GENERICPAR MONO_GENPARCONSTRAINT_CONSTRAINT
ImplMap | MONO_TABLE_IMPLMAP | Array size: MONO_IMPLMAP_SIZE MONO_IMPLMAP_FLAGS MONO_IMPLMAP_MEMBER MONO_IMPLMAP_NAME MONO_IMPLMAP_SCOPE
| InterfaceImpl | MONO_TABLE_INTERFACEIMPL | Array size: MONO_INTERFACEIMPL_SIZE MONO_INTERFACEIMPL_CLASS MONO_INTERFACEIMPL_INTERFACE | ManifestResource | MONO_TABLE_MANIFESTRESOURCE | Array size: MONO_MANIFESTRESOURCE_SIZE MONO_MANIFEST_OFFSET MONO_MANIFEST_FLAGS MONO_MANIFEST_NAME MONO_MANIFEST_IMPLEMENTATION
MemberRef | MONO_TABLE_MEMBERREF | Array size: MONO_MEMBERREF_SIZE MONO_MEMBERREF_CLASS MONO_MEMBERREF_NAME MONO_MEMBERREF_SIGNATURE
| MethodImpl | MONO_TABLE_METHODIMPL | Array size: MONO_METHODIMPL_SIZE MONO_METHODIMPL_CLASS MONO_METHODIMPL_BODY MONO_METHODIMPL_DECLARATION | MethodSpec | MONO_TABLE_METHODSPEC | Array size: MONO_METHODSPEC_SIZE MONO_METHODSPEC_METHOD MONO_METHODSPEC_SIGNATURE
MethodSemantics | MONO_TABLE_METHODSEMANTICS | Array size: MONO_METHOD_SEMA_SIZE MONO_METHOD_SEMA_SEMANTICS MONO_METHOD_SEMA_METHOD MONO_METHOD_SEMA_ASSOCIATION
| Moduleref | MONO_TABLE_MODULEREF | Array size: MONO_MODULEREF_SIZE MONO_MODULEREF_NAME
Module | MONO_TABLE_MODULE | Array size: MONO_MODULE_SIZE Columns: MONO_MODULE_GENERATION MONO_MODULE_NAME MONO_MODULE_MVID MONO_MODULE_ENC MONO_MODULE_ENCBASE
| TypeRef | MONO_TABLE_TYPEREF | Array size: MONO_TYPEREF_SIZE MONO_TYPEREF_SCOPE MONO_TYPEREF_NAME MONO_TYPEREF_NAMESPACE
MethodPtr | MONO_TABLE_METHOD_POINTER | Array size: MONO_METHOD_POINTER_SIZE MONO_METHOD_POINTER_METHOD
| Method | MONO_TABLE_METHOD | Array size: MONO_METHOD_SIZE MONO_METHOD_RVA MONO_METHOD_IMPLFLAGS MONO_METHOD_FLAGS MONO_METHOD_NAME MONO_METHOD_SIGNATURE MONO_METHOD_PARAMLIST
NestedClass | MONO_TABLE_NESTEDCLASS | Array size: MONO_NESTEDCLASS_SIZE MONO_NESTED_CLASS_NESTED MONO_NESTED_CLASS_ENCLOSING
ParamPtr | MONO_TABLE_PARAM_POINTER | Array size: MONO_PARAM_POINTER_SIZE MONO_PARAM_POINTER_PARAM
| Param | MONO_TABLE_PARAM | Array size: MONO_PARAM_SIZE MONO_PARAM_FLAGS MONO_PARAM_SEQUENCE MONO_PARAM_NAME | PropertyMap | MONO_TABLE_PROPERTYMAP | Array size: MONO_PROPERTY_MAP_SIZE MONO_PROPERTY_MAP_PARENT MONO_PROPERTY_MAP_PROPERTY_LIST | PropertyPtr | MONO_TABLE_PROPERTY_POINTER | Array size: MONO_PROPERTY_POINTER_SIZE MONO_PROPERTY_POINTER_PROPERTY | Property | MONO_TABLE_PROPERTY | Array size: MONO_PROPERTY_SIZE MONO_PROPERTY_FLAGS MONO_PROPERTY_NAME MONO_PROPERTY_TYPE
StandaloneSig | MONO_TABLE_STANDALONESIG | Array size: MONO_STAND_ALONE_SIGNATURE_SIZE MONO_STAND_ALONE_SIGNATURE
TypeDef | MONO_TABLE_TYPEDEF | Array size: MONO_TYPEDEF_SIZE MONO_TYPEDEF_FLAGS MONO_TYPEDEF_NAME MONO_TYPEDEF_NAMESPACE MONO_TYPEDEF_EXTENDS MONO_TYPEDEF_FIELD_LIST MONO_TYPEDEF_METHOD_LIST
TypeSpec | MONO_TABLE_TYPESPEC | Array size: MONO_TYPESPEC_SIZE MONO_TYPESPEC_SIGNATURE

Each table can contain zero or more rows, you must call the mono_metadata_table_rows to obtain the number of rows in a table, and then you can extract individual row values by using the mono_metadata_decode_row or the mono_metadata_decode_row_col. When decoding rows you must provide an guint32 array large enough to hold as many columns as the table contains.

The metadata tables are stored in the MonoImage, you obtain a pointer to the MonoTableInfo by calling the mono_image_get_table_info and then you can scan those tables, for example:

    	/*
    	 * Dumps a few fields from the AssemblyRef table
    	 */
    	void DumpAssemblyRefs (MonoImage *image)
    	{
    		/* Get a pointer to the AssemblyRef metadata table */
    		MonoTableInfo *t = mono_image_get_table_info (image, MONO_TABLE_ASSEMBLYREF);

    		/* Fetch the number of rows available in the table */
    		int rows = mono_table_info_get_rows (t);
    		int i;

    		/* For each row, print some of its values */
    		for (i = 0; i < rows; i++){
    			/* Space where we extract one row from the metadata table */
    			guint32 cols [MONO_ASSEMBLYREF_SIZE];

    			/* Extract the row into the array cols */
    			mono_metadata_decode_row (t, i, cols, MONO_ASSEMBLYREF_SIZE);

    			fprintf (output, "%d: Version=%d.%d.%d.%d\n\tName=%s\n", i + 1,
                            	cols [MONO_ASSEMBLYREF_MAJOR_VERSION],
                            	cols [MONO_ASSEMBLYREF_MINOR_VERSION],
                            	cols [MONO_ASSEMBLYREF_BUILD_NUMBER],
                            	cols [MONO_ASSEMBLYREF_REV_NUMBER],
                            	mono_metadata_string_heap (image, cols [MONO_ASSEMBLYREF_NAME]));
    		}
    	}

The above program shows the following output when ran on the C# compiler:

    	1: Version=1.0.5000.0
    		Name=mscorlib
    	2: Version=1.0.5000.0
    		Name=System
    	3: Version=1.0.5000.0
    		Name=System.Xml

### Metadata Tables API

These are the APIs for dealing with tables:

#### mono_image_get_table_info

#### mono_image_get_table_rows

#### mono_metadata_decode_row_col

#### mono_metadata_decode_row

#### mono_metadata_compute_size

#### mono_metadata_custom_attrs_from_index

#### mono_metadata_decode_signed_value

#### mono_metadata_decode_value

#### mono_metadata_encode_value

## Metadata access API

This is the low-level API for accessing the metadata images.

#### mono_pe_file_open

#### mono_metadata_events_from_typedef

#### mono_metadata_decode_table_row

#### mono_metadata_decode_table_row_col

#### mono_metadata_field_info

#### mono_metadata_free_array

#### mono_metadata_free_marshal_spec

#### mono_metadata_free_mh

#### mono_metadata_free_type

#### mono_metadata_get_constant_index

#### mono_metadata_get_marshal_info

#### mono_metadata_implmap_from_method

#### mono_metadata_interfaces_from_typedef

#### mono_metadata_locate

#### mono_metadata_locate_token

#### mono_metadata_methods_from_event

#### mono_metadata_methods_from_property

#### mono_metadata_nested_in_typedef

#### mono_metadata_nesting_typedef

#### mono_metadata_packing_from_typedef

#### mono_metadata_properties_from_typedef

#### mono_metadata_token_from_dor

#### mono_metadata_translate_token_index

#### mono_metadata_typedef_from_field

#### mono_metadata_typedef_from_method

#### mono_metadata_type_equal

#### mono_metadata_type_hash

#### mono_metadata_declsec_from_index

#### mono_metadata_free_method_signature

### Retrieving Objects from Tokens

#### mono_metadata_parse_array

#### mono_metadata_parse_custom_mod

#### mono_metadata_parse_field_type

#### mono_metadata_parse_marshal_spec

#### mono_metadata_parse_method_signature

#### mono_metadata_parse_mh

#### mono_metadata_parse_param

#### mono_metadata_parse_signature

#### mono_metadata_parse_typedef_or_ref

#### mono_metadata_parse_type

## Generics Support

#### mono_metadata_generic_class_is_valuetype

## Tokens

#### mono_ldtoken

#### mono_ldstr

#### mono_exception_from_token

## OpCodes

#### mono_opcode_name

#### mono_opcode_value