How to get an enumeration value when using IMetada

Question

Short version

How to you get the numeric value associated with an enum from a *.winmd file when using IMetadataImport?

A good example is ApplicationHighContrastAdjustment enumeration:

//Windows.UI.Xaml.ApplicationContrastMode (@020000006)
public enum ApplicationHighContrastAdjustment : uint
{
    None = 0u,
    Auto = 4294967295u
}

Most enumerations are 0, 1, 2, .... But this one has other values specified on the enum members:

• 0
• 4294967295

How do i read get those UInt32 values

Note: The question doesn't have to apply to just WinRT. The same interfaces are used in the C# world to inspect .NET managed assemblies. WinRT happens to use the same assembly file format.

Long version

I'm using IMetadataImport to read the contents of an *.winmd (the modern version of TLBs for WinRT applications). But the question applies equally to reading metadata about a .NET managed assembly.

The abridged version of how to get up and running reading winmd metadata file:

// Create your metadata dispenser:
IMetadataDispsener dispener;
MetaDataGetDispenser(CLSID_CorMetaDataDispenser, IMetaDataDispenser, out dispenser);

//Open the winmd file we want to dump
String filename = "C:\Windows\System32\WinMetadata\Windows.UI.Xaml.winmd";

IMetaDataImport reader; //IMetadataImport2 supports generics
dispenser.OpenScope(filename, ofRead, IMetaDataImport, out reader); //"Import" is used to read metadata. "Emit" is used to write metadata.

Getting information about the enum (Auto, None)

We now have a reader. Rather than enumerating the types in the assembly, i can jump right to the interesting one for this question: 0x02000006:

//Get metadata for enum Windows.UI.Xaml.ApplicationHighContrastAdjustment
mdToken tokenID = 0x02000006; //Windows.UI.Xaml.ApplicationHighContrastAdjustment

//btw, this is all hypothetical code that is vaguely C#/Java-like.

Pointer enum = null;
mdToken memberID;
int nCount;
while (reader.EnumMembers(ref enum, tokenID, out memberID, 1, out nCount) == S_OK)
{
   //out MemberID receives the TokenID of each member of the enumeration
}
reader.CloseEnum(enum);

The call to EnumMembers returns us the three members of the enumeration:

• Windows.UI.Xaml.ApplicationContrastMode (@02000006)
  • value__ (@04000439, private)
  • None (@0400043A, public)
  • Auto (@0400043B, public)

Getting info on each enum value

We actually find out their names (and that fact that one is private) through a call to GetMemberProps:

IMetaDataImporter.GetMemberProps(0x0400043A, ...); //"None"
IMetaDataImporter.GetMemberProps(0x0400043B, ...); //"Auto"  

Note: GetMemberProps is a helper function. From Microsoft:

This is a simple helper method: if md is a MethodDef, then we call GetMethodProps; if md is a FieldDef, then we call GetFieldProps. See these other methods for details.

The GetMemberProps method returns a whole host of information about each enum value - but not their actual enum value:

| Metadata          | @0400043A         | @0400043B       |
|-------------------|-------------------|-----------------|
| Name              | "None"            | "Auto"          |
| Attributes        | 0x00008056        | 0x00008056      |
| Signature         | 06 11 A3 95       | 06 11 A3 95     |
| CodeRVA           | 0x00000000        | 0x00000000      |
| CPlusTypeFlag     | ELEMENT_TYPE_U4   | ELEMENT_TYPE_U4 |
| DefaultValue      | (none)            | (none)          |

I cannot find anything in the member properties that indicates the enumeration's assigned value. And looking at other IMetadataImporter methods:

• IMetdataImporter
  • GetMemberProps (GetMemberProps is a helper that calls GetMethodProps or GetFieldProps depending on the type)
    • GetMethodProps
    • GetFieldProps
  • GetPropertyProps
  • GetEventProps
  • GetParamProps
  • GetInterfaceImplProps
  • GetCustomAttributeProps
  • GetTypeDefProps
  • GetTypeRefProps
  • GetScopeProps
  • GetPermissionSetProps
  • GetModuleRefProps
  • GetNestedClassProps
  • GetMemberRefProps

Bonus Reading

• MSDN Blogs: Metadata Unmanaged API (a preliminary PDF version of an old Word document that, as far as i can tell, is the only Microsoft documentation for the Metadata API) (archive)

Answer 1

Given the tokenID of an enumeration member i want the Value of:

@0400043B = Windows.UI.Xaml.ApplicationHighContrastMode.Auto

You need to iterate through the Constant table (0x0B), and find where the Parent column (columnIndex=1) is the element you want.

The Constant table looks like:

Rid  Type (iBYTE)         Parent (iCodedToken)  Value (iBLOB)
===  ===================  ====================  ===============
1    ELEMENT_TYPE_I4 (8)  @04000002             00 00 00 00
2    ELEMENT_TYPE_I4 (8)  @04000003             01 00 00 00
3    ELEMENT_TYPE_I4 (8)  @04000005             00 00 00 00
...
883  ELEMENT_TYPE_I4 (8)  @0400040A             02 00 00 00
884  ELEMENT_TYPE_U4 (9)  @0400043A             00 00 00 00
885  ELEMENT_TYPE_U4 (9)  @0400043B             FF FF FF FF
886  ELEMENT_TYPE_I4 (8)  @0400043D             00 00 00 00
...

Starting with the IMetadataImporter, you need to QueryInterface for it's IMetadataTables interface:

//Get the tables interface
IMetadataTables tables = reader as IMetadataImporter;

//get the number of rows in the Constant (11) table
UInt32 tabConstant = 11; //the "Constant" table

UInt32 rowSize;
UInt32 rowCount;
UInt32 columnCount;
UInt32 keyColumn;
String tableName;
tables.GetTableInfo(tabConstant,
       out rowSize,
       out rowCount,
       out columnCount,
       out keyColumn,
       out tableName);

Now with the scunt work out of the way, you have to actually manually iterate the table:

//Loop over ever row in the Constants table
//and look for Parent (columnIndex=1) is the parent we want
//all code released into the public domain; no attribution required
UInt32 desiredToken = 0x0400043B;

UInt32 colParent = 1; // Parent (iCodedToken)
UInt32 colValue  = 2; // Value  (iBLOB)

for (int i = 0 to rowCount-1)
{
   //Get the Parent codedToken of this row
   UInt32 value;
   tables.GetColumn(tabConstant, colParent, i, outvalue);

   // Is it the parent we're interested in (i.e. @0400043A)
   if (value != desiredToken)
      continue;

   // We found it! Get the value from the "Value" (iBLOB) column 2
   tables.GetColumn(tabConstant, colValue, i, out value);

   //Convert blob UInt32 to a pointer to data
   UInt32 dataLen;
   Pointer data;
   tables.GetBlob(value, out dataLen, out dataLen, out data);

   //Convert the dataLen bytes pointed to by data to a UInt32
   UInt32 enumValue = PUInt32(data)^;

   return enumValue;
}