Documentation

Index

General .NET

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• Design of COM usable classes in .NET
  • Errors
  • Samples for params
    • Default implemention (accessible for VBScript)
    • Required implementation for VB98/VB6
  • Sample for For Each enumerator with generic IEnumerable<T> implementation

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Design of COM usable classes in .NET

• 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 implemented 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
• Use of public abstract classes are possible
• 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 exports as IEnumVARIANT and 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

• Handling of optional parameters by params in (VB/VBA/VBScript as ParamArray) requires an implementation as COM type Variant (Object) array. The framework handles this as a SAFEARRAY(VARIANT). When exporting the type library (TLB) with TlbExp it becomes a vararg attribute there also. Specific implementation and change in TLB/IDL is required for VB, while VBScript can direct use it.

  • VB98/VB6 require a pointer to the SAFEARRAY(VARIANT). In that case it needs:
    • implement the parameter as ref Object[]
    • compile your library or application
    • export the type library with TlbExp
    • open the type library with the tool oleview
    • save the content as IDL file
    • add the atribute vararg after the attribute id
    • compile the IDL file with midl (requires the cl and includes)
• 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

Samples for params

Default implemention (accessible for VBScript)

MyParamArrayMethod(params Object[] values)

Results in VB

MyParamArrayMethod(ByVal ParamArray values() As Object)

Being in IDL

[id(1), vararg]
HRESULT MyParamArrayMethod([in] SAFEARRAY(VARIANT) values);

Required implementation for VB98/VB6

MyParamArrayMethod(ref Object[] values)

Should result in VB

MyParamArrayMethod(ByVal ParamArray values() As Object)

Requires in IDL

[id(1), vararg]
HRESULT MyParamArrayMethod([in] SAFEARRAY(VARIANT)* values);

Sample for For Each enumerator with generic IEnumerable<T> implementation

public class ForEachSample :
  IEnumerable<String>
{
  private readonly List<String> _list = new List<String>();

  public ForEachSample()
  {
  }

  public IEnumerator GetEnumerator() => ((IEnumerable<String>)this).GetEnumerator();
  
  IEnumerator<String> IEnumerable<String>.GetEnumerator() => _list.GetEnumerator();
}

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