Assembly must be COM visible
Each exposed class, interface and enumeration must have a GuidAttribute with a unique GUID value
Optionally use of ProgIdAttribute
Class must have a parameterless constructor
It seems, that the first impleted interface will be taken for COM?
No generic base classes or first generic interfaces (without inherited none generic interface) allowed
Only public members are accessible under COM (no protected etc.)
Inherited interface members might be implemented again (with new) at inheriting interface
Additional interfaces without reimplementation with new are accessible only after specific cast (implement cast methods for that)
Parameters of kind “ByRef” (ref, out) must be implemented of COM type Variant (Object)
Parameters must have an equivalent in COM (for VARIANT structure); known types without equivalent in VBScript are
- Byte and
- Structures
Parameters for object instances, stored in a variable before method calls, should have COM type Variant (Object)
Nested classes are allowed and named with a “+” (plus) as concatenation
It seems, no inheritance of classes possible? or from public classes only?
Classes, members that can’t/shouldn’t be exposed to COM must be marked with a ComVisibleAttribute of value false
For Each enumerable support for COM needs direct IEnumerator GetEnumerator() method implementation, could be done with IEnumerable interface with precidence over IEnumerable<T> (type library exporter give them the DispId of -4 / 0xFFFFFFFC for a _NewEnum implementation)
Handling arrays out and in with values needs to be of element type Object, so Linq Cast<Object>().ToArray() would help here and for in correct cast then the incoming elements
COM libraries without registration and without strong names are usable in/with manifests (use tool MT for that)

Errors

Manifest Side by side errors occurs when
- Manifest file has false structure or values
- Duplicate GUID’s or ProgId’s used
CreateObject(...) (VBA, VBScript) error will occur when
- Exception occured in constructor

Documentation

Design of COM usable classes in .NET

Errors