Search
Module:
Directory

   Desktop Functions:

   Smart Device Functions:


Show Recent Changes
Subscribe (RSS)
Misc. Pages
Comments
FAQ
Helpful Tools
Playground
Suggested Reading
Website TODO List
Download Visual Studio Add-In

VarBstrFromDisp (oleaut32)
 
.
Summary
Handy function Invoke the default value(s) on a ComObject reference within C#/VB.NET. The full code to retrieve DispId(0) (Default Value) from a Dispatch interface is ugly. This pinvoke is elegant and quick. Also supported on 64 bit of course.

C# Signature:

[DllImport("oleaut32.dll", SetLastError=false, ExactSpelling=true)]
static extern int VarBstrFromDisp(IntPtr Disp, int lcid, int dwFlags, [MarshalAs(UnmanagedType.BStr)] out string pbstrOut);

VB Signature:

Declare Function VarBstrFromDisp Lib "oleaut32.dll" (Byval Disp as IntPtr, Byval lcid As Integer, Byref pbstrOut as String) As Integer

User-Defined Types:

None.

Alternative Managed API:

There is no alternative API in .NET except for Dispatch Invoke the default value (DispId == 0)

However, you could use the dynamic keyword if you do know the name of the default Dispatch property, such as 'Value' or 'Text'. But if you don't know the default property, the IDispatch Invoke() method comes in, which is ugly. Whynot have OleAut32.dll do it for you?

However, you could use the dynamic keyword. But this expects you to know the name of the default property. (Which can be Value but Text or any name as the original Dispatch interface developer (eg in VB6) had intended to).

Notes:

The sample cannot be used on DCOM (remote COM). It is not tested.

Tips & Tricks:

Please add some!

Sample Code:

object pComObject = rs.Fields[0]; // ADODB COM stuff just to illustrate how it works. Normally, use a typelibrary or the dynamic keyword

object pComObject = rs.Fields[0]; // ADODB COM stuff

if (Marshal.IsComObject(pComObject)) // shows as System.__ComObject

{

   string pbstrOut;
   int hr = VarBstrFromDisp(Marshal.GetIDispatchForObjectInContext(pComObject), Thread.CurrentThread.CurrentCulture.LCID, 0, out pbstrOut);
   if (hr != 0)
   {
       Marshal.ThrowExceptionForHR(hr);
   }
}


Documentation

Please edit this page!

Do you have...

  • helpful tips or sample code to share for using this API in managed code?
  • corrections to the existing content?
  • variations of the signature you want to share?
  • additional languages you want to include?

Select "Edit This Page" on the right hand toolbar and edit it! Or add new pages containing supporting types needed for this API (structures, delegates, and more).

 
Access PInvoke.net directly from VS:
Terms of Use
Edit This Page
Find References
Show Printable Version
Revisions