com interview questions
Top com frequently asked interview questions
Can you explain STA and MTA in your own words?
Also, what are apartment threads and do they pertain only to COM? If so, why?
Source: (StackOverflow)
It seems COM objects are general use objects which are governed by the OS. The objects follow a strict interface and allow you to query the objects to determine information. Is this what COM objects are?
Source: (StackOverflow)
EDIT: I've written the results up as a blog post.
The C# compiler treats COM types somewhat magically. For instance, this statement looks normal...
Word.Application app = new Word.Application();
... until you realise that Application is an interface. Calling a constructor on an interface? Yoiks! This actually gets translated into a call to Type.GetTypeFromCLSID() and another to Activator.CreateInstance.
Additionally, in C# 4, you can use non-ref arguments for ref parameters, and the compiler just adds a local variable to pass by reference, discarding the results:
// FileName parameter is *really* a ref parameter
app.ActiveDocument.SaveAs(FileName: "test.doc");
(Yeah, there are a bunch of arguments missing. Aren't optional parameters nice? :)
I'm trying to investigate the compiler behaviour, and I'm failing to fake the first part. I can do the second part with no problem:
using System;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;
[ComImport, GuidAttribute("00012345-0000-0000-0000-000000000011")]
public interface Dummy
{
void Foo(ref int x);
}
class Test
{
static void Main()
{
Dummy dummy = null;
dummy.Foo(10);
}
}
I'd like to be able to write:
Dummy dummy = new Dummy();
though. Obviously it'll go bang at execution time, but that's okay. I'm just experimenting.
The other attributes added by the compiler for linked COM PIAs (CompilerGenerated and TypeIdentifier) don't seem to do the trick... what's the magic sauce?
Source: (StackOverflow)
I am using a COM object (MODI) from within my .net application. The method I am calling throws a System.AccessViolationException, which is intercepted by Visual Studio. The odd thing is that I have wrapped my call in a try catch, which has handlers for AccessViolationException, COMException and everything else, but when Visual Studio (2010) intercepts the AccessViolationException, the debugger breaks on the method call (doc.OCR), and if I step through, it continues to the next line instead of entering the catch block. Additionally, if I run this outside of the visual studio my application crashes. How can I handle this exception that is thrown within the COM object?
MODI.Document doc = new MODI.Document();
try
{
doc.Create(sFileName);
try
{
doc.OCR(MODI.MiLANGUAGES.miLANG_ENGLISH, false, false);
sText = doc.Images[0].Layout.Text;
}
catch (System.AccessViolationException ex)
{
//MODI seems to get access violations for some reason, but is still able to return the OCR text.
sText = doc.Images[0].Layout.Text;
}
catch (System.Runtime.InteropServices.COMException ex)
{
//if no text exists, the engine throws an exception.
sText = "";
}
catch
{
sText = "";
}
if (sText != null)
{
sText = sText.Trim();
}
}
finally
{
doc.Close(false);
//Cleanup routine, this is how we are able to delete files used by MODI.
System.Runtime.InteropServices.Marshal.FinalReleaseComObject(doc);
doc = null;
GC.WaitForPendingFinalizers();
GC.Collect();
GC.WaitForPendingFinalizers();
}
Source: (StackOverflow)
I've seen a few COM controls which wrap the Gecko rendering engine (GeckoFX, as well as the control shipped by Mozilla - mozctlx.dll). Is there a wrapper for Webkit that can be included in a .NET Winform application?
Source: (StackOverflow)
I am creating an .idl file programmatically. How do I create UUIDs for the interfaces and Methods Programmatically.
Can I generate the UUID programmatically?
Source: (StackOverflow)
I'm trying achieve two things with DCOM (Out of process)
- Set the process wide authentication using CoInitializeSecurity and its parameter pAuthList.
- Using cloaking to change the caller's identity in special situations (COM calls)
My thoughts:
AFAIK the auth info structure contains the default authentication information (like username and password for RPC_C_AUTHN_WINNT) for all new COM calls. So instead of the process token the information in the auth structure should be used by COM.
However, all COM calls/connections are always using the process' identity instead of the applied default one.
Usually, one can use CoSetProxyBlanket to change the auth info for a proxy. This works for me. My question here is whether it must or must not work if I impersonate the token myself and call the COM function. I've read in various MSDN articles that applying EOAC_DYNAMIC_CLOAKING to CoInitializeSecurity should make it working. However, my manually "impersonated COM calls always shows the process identity on the server side.
The client looks like this (Delphi)
var
authList : SOLE_AUTHENTICATION_LIST;
authidentity : SEC_WINNT_AUTH_IDENTITY_W;
authInfo : array[0..1] of SOLE_AUTHENTICATION_INFO;
pcAuthSvc : DWORD;
asAuthSvc : array[0..0] of SOLE_AUTHENTICATION_SERVICE;
Token : TJwSecurityToken;
begin
ZeroMemory( @authidentity, sizeof(authidentity) );
authidentity.User := 'Testbenutzer';
authidentity.UserLength := Length('Testbenutzer');
authidentity.Domain := '';
authidentity.DomainLength := 0;
authidentity.Password := 'test';
authidentity.PasswordLength := 4;
authidentity.Flags := SEC_WINNT_AUTH_IDENTITY_UNICODE;
ZeroMemory( @authInfo, sizeof( authInfo ) );
// NTLM Settings
authInfo[0].dwAuthnSvc := RPC_C_AUTHN_WINNT;
authInfo[0].dwAuthzSvc := RPC_C_AUTHZ_NONE;
authInfo[0].pAuthInfo := @authidentity;
authList.cAuthInfo := 1;
authList.aAuthInfo := @authInfo;
OleCheck(CoInitializeSecurity(
NULL, // Security descriptor
-1, // Count of entries in asAuthSvc
NULL, // asAuthSvc array
NULL, // Reserved for future use
RPC_C_AUTHN_LEVEL_CONNECT, // Authentication level
RPC_C_IMP_LEVEL_IMPERSONATE, // Impersonation level
@authList, // Authentication Information
DWORd(EOAC_DYNAMIC_CLOAKING), // Additional capabilities
NULL // Reserved
));
//create COM object
int := CoSecurityTestObj.Create;
int.TestCall;
The server also has set the flag EOAC_DYNAMIC_CLOAKING. It uses CoImpersonateClient to get the thread token and the username. It also uses CoQueryClientBlanket to get the authInfo (as SEC_WINNT_AUTH_IDENTITY_W structure). However both calls always return the process identity of the client.
Also impersonating manually doesn't work (2.):
Token := TJwSecurityToken.CreateLogonUser(authidentity.User, '', authidentity.Password, LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT);
Token.ImpersonateLoggedOnUser;
int := CoSecurityTestObj.Create;
int.TestCall;
Questions again:
Am I wrong or why is the default auth info structure (WinNT with username and password) not used as default authentication in each COM connection/call ?
Am I wrong or why doesn't manual impersonation work? Be aware that I tested number 2. separately so number 1. cannot interfere.
This is basic work for the JEDI Windows Security Code Library which I extend to support COM security. So your help will go GPL/MPL.
References:
Cloaking:
- http://msdn.microsoft.com/en-us/library/ms683778%28VS.85%29.aspx
- http://msdn.microsoft.com/en-us/library/cc246058%28PROT.10%29.aspx
- http://alt.pluralsight.com/wiki/default.aspx/Keith.GuideBook/WhatIsCoInitializeSecurity.html
CoInitializeSecurity and pAuthInfo
- http://www.codeguru.cn/vc&mfc/apracticalguideusingvisualcandatl/93.htm
Getting security blanket (server side)
- http://www.codeguru.cn/vc&mfc/apracticalguideusingvisualcandatl/92.htm
Source: (StackOverflow)
I am trying to create an ActiveX controller.
I using VS2010 (.Net4)
I need to create a COM object (in C#) and have no Idea how to get started (what type of project to use ect.)
Thanks
Source: (StackOverflow)
I know how to register dlls but I've never really been sure why I'm doing it or under what conditions a dll must be registered. Could somebody explain or point me to some documentation?
Source: (StackOverflow)
I have some applications (some native, some .NET) which use manifest files so that they can be deployed in complete isolation, without requiring any global COM registration. For example, the dependency on the dbgrid32.ocx com server is declared as follows in the myapp.exe.manifest file which sits in the same folder as myapp.exe:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
<assemblyIdentity type="win32" name="myapp.exe" version="1.2.3.4" />
<dependency>
<dependentAssembly>
<assemblyIdentity type="win32" name="dbgrid32.ocx" version="5.1.81.4" />
</dependentAssembly>
</dependency>
</assembly>
The dbgrid32.ocx is deployed to the same folder, along with it's own dbgrid32.ocx.manifest file:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
<assemblyIdentity type="win32" name="dbgrid32.ocx" version="5.1.81.4" />
<file name="dbgrid32.ocx">
<typelib
tlbid="{00028C01-0000-0000-0000-000000000046}"
version="1.0"
helpdir=""/>
<comClass progid="MSDBGrid.DBGrid"
clsid="{00028C00-0000-0000-0000-000000000046}"
description="DBGrid Control" />
</file>
</assembly>
This all works fine but maintaining these manifest files manually is a bit of a pain. Is there a way to generate these files automatically? Ideally I would just like to declare the application's dependency on a list of COM servers (both native and .NET) and then let the rest be generated automatically. Is it possible?
Source: (StackOverflow)
I'm currently writing about dynamic typing, and I'm giving an example of Excel interop. I've hardly done any Office interop before, and it shows. The MSDN Office Interop tutorial for C# 4 uses the _Worksheet interface, but there's also a Worksheet interface. I've no idea what the difference is.
In my absurdly simple demo app (shown below) either works fine - but if best practice dictates one or the other, I'd rather use it appropriately.
using System;
using System.Linq;
using Excel = Microsoft.Office.Interop.Excel;
class DynamicExcel
{
static void Main()
{
var app = new Excel.Application { Visible = true };
app.Workbooks.Add();
// Can use Excel._Worksheet instead here. Which is better?
Excel.Worksheet workSheet = app.ActiveSheet;
Excel.Range start = workSheet.Cells[1, 1];
Excel.Range end = workSheet.Cells[1, 20];
workSheet.get_Range(start, end).Value2 = Enumerable.Range(1, 20)
.ToArray();
}
}
I'm trying to avoid doing a full deep-dive into COM or Office interoperability, just highlighting the new features of C# 4 - but I don't want to do anything really, really dumb.
(There may be something really, really dumb in the code above as well, in which case please let me know. Using separate start/end cells instead of just "A1:T1" is deliberate - it's easier to see that it's genuinely a range of 20 cells. Anything else is probably accidental.)
So, should I use _Worksheet or Worksheet, and why?
Source: (StackOverflow)
I had created a COM+ domain partition then mapped it to a Windows 2008 server machine and imported a COM+ application into it.
I tried using the following C# code to activate an object from that specific partition on the server remotely:
//partition guid
Guid guidMyPartition = new Guid("41E90F3E-56C1-4633-81C3-6E8BAC8BDD70");
//parition moniker
string uri= "partition:{" + guidMyPartition + "}/new:MyObject";
Type t = Type.GetTypeFromProgID("MyObject", "MyServer");
MyObject obj = (MyObject)Activator.GetObject(t, uri);
But I get this exception:
Cannot create channel sink to connect to URL 'partition:{41e90f3e-56c1-4633-81c3-6e8bac8bdd70}/new:MyObject'. An appropriate channel has probably not been registered.
Does anybody know how such an activation can be accomplished?
Source: (StackOverflow)
I'm having an issue with a COM based client-server setup. The COM server is written in C# (.NET 4.0) and runs as a (registered) local server.
Depending on which application connects to the server, other clients will receive a Server execution failed (Exception from HRESULT: 0x80080005 (CO_E_SERVER_EXEC_FAILURE)
The underlying issue is explained here (in the section COM is integrity aware). The way I understand it, it is being caused by the fact that an elevated application creates the server with a higher integrity level. When another non-elevated application then connects, it is not allowed to connect to the same instance. The same happens when a non-elevated application creates the process, followed an elevated application connecting.
I've tried to implement the solution described on the page: modifying the registry to set a security descriptor that should allow all clients to connect. There is a code sample in C++, but this does effectively the same thing in .NET:
// Security Descriptor with NO_EXECUTE_UP
var sd = new RawSecurityDescriptor("O:BAG:BAD:(A;;0xb;;;WD)S:(ML;;NX;;;LW)");
byte[] securityDescriptor = new Byte[sd.BinaryLength];
sd.GetBinaryForm(securityDescriptor, 0);
RegistryKey key = Registry.ClassesRoot.OpenSubKey("AppID\\{APP-ID-GUID}", true);
if (key == null)
{
key = Registry.ClassesRoot.CreateSubKey("AppID\\{APP-ID-GUID}");
}
using (key)
{
key.SetValue("LaunchPermission", securityDescriptor, RegistryValueKind.Binary);
}
However, this does not have the desired effect. When the second client tries to create an instance of the object in question, Windows tries to launch a separate instance of my COM Server, but the server prevents two instances from running as the same user. Given the permissions I've set, I would not expect a second instance to launch in the first place.
Since one of the client applications is running in Medium IL and the other in High IL, I also experimented with variants on the mandatory label, like:
O:BAG:BAD:(A;;0xb;;;WD)S:(ML;;NX;;;ME)
O:BAG:BAD:(A;;0xb;;;WD)S:(ML;;NX;;;LW)(ML;;NX;;;ME)(ML;;NX;;;HI)
I've also tried setting the ROTFlags registry key to 0x1 (ROTFLAGS_ALLOWANYCLIENT) as suggested on the page, still no change in behavior.
I've established that the LaunchPermission registry value is being used in some way. I cannot discover where it's being read using Process Monitor, but when I use the dcomcnfg.exe tool to set the same key, I can force the server to fail loading by denying launch permissions.
I would like to point out that my server process does not need elevation. How do I make both elevated and non-elevated processes capable of connecting to a single server instance?
Source: (StackOverflow)
Update: XE2 Update 2 fixes the bug described below.
The program below, cutdown from the real program, fails with an exception in XE2. This is a regression from 2010. I don't have XE to test on but I'd expect that the program works fine on XE (thanks to Primož for confirming that the code runs fine on XE).
program COMbug;
{$APPTYPE CONSOLE}
uses
SysUtils, Variants, Windows, Excel2000;
var
Excel: TExcelApplication;
Book: ExcelWorkbook;
Sheet: ExcelWorksheet;
UsedRange: ExcelRange;
Row, Col: Integer;
v: Variant;
begin
Excel := TExcelApplication.Create(nil);
try
Excel.Visible[LOCALE_USER_DEFAULT] := True;
Book := Excel.Workbooks.Add(EmptyParam, LOCALE_USER_DEFAULT) as ExcelWorkbook;
Sheet := Book.Worksheets.Add(EmptyParam, EmptyParam, 1, EmptyParam, LOCALE_USER_DEFAULT) as ExcelWorksheet;
Sheet.Cells.Item[1,1].Value := 1.0;
Sheet.Cells.Item[2,2].Value := 1.0;
UsedRange := Sheet.UsedRange[LOCALE_USER_DEFAULT] as ExcelRange;
for Row := 1 to UsedRange.Rows.Count do begin
for Col := 1 to UsedRange.Columns.Count do begin
v := UsedRange.Item[Row, Col].Value;
end;
end;
finally
Excel.Free;
end;
end.
In XE2 32 bit the error is:
Project COMbug.exe raised exception class $C000001D with message 'system exception (code 0xc000001d) at 0x00dd6f3e'.
The error occurs on the second execution of UsedRange.Columns.
In XE2 64 bit the error is:
Project COMbug.exe raised exception class $C0000005 with message 'c0000005 ACCESS_VIOLATION'
Again, I think that the error occurs on the second execution of UsedRange.Columns, but the 64 bit debugger steps through the code in a slightly weird way so I'm not 100% sure of that.
I have submitted a QC report for the issue.
I looks very much to me as though something in the Delphi COM/automation/interface stack is comprehensively broken. This is a complete show-stopper for my XE2 adoption.
Does anyone have any experience of this problem? Does anyone have any tips and advice as to how I might attempt to work around the problem? Debugging what's really going on here is outside my area of expertise.
Source: (StackOverflow)
I can fire up Excel with the following script. But in ghci (7.4.1) I have a segmentation fault when I run it.
I don't know where to search from now. I don't have this error if I remove the line
workSheets <- workBook # propertyGet_0 "Worksheets"
Here is the code. May be I forgot something. I read the source code of com.hs here, but it doesn't give me any clue.
import System.Win32.Com
import System.Win32.Com.Automation
--
-- createObjectExcel
-- coming from Automation.hs and com.hs
--
iidIDispatch_unsafe = mkIID "{00020400-0000-0000-C000-000000000046}"
createObjExl :: IO (IDispatch ())
createObjExl = do
clsidExcel <- clsidFromProgID "Excel.Application"
pExl <- coCreateInstance clsidExcel Nothing LocalProcess iidIDispatch_unsafe
return pExl
fichierTest2 = "E:/Programmation/haskell/Com/qos1.xls"
main = coRun $ do
pExl <- createObjExl
workBooks <- pExl # propertyGet_0 "Workbooks"
workBook <- workBooks # propertyGet_1 "Open" fichierTest2
workSheets <- workBook # propertyGet_0 "Worksheets"
workBooks # method_1_0 "Close" (0::Int)
pExl # method_0_0 "Quit"
mapM release [workSheets,workBook, workBooks, pExl]
Edit with Gonzalez's advice I tried debugging, but no information arose. I tried the code by hand in ghci, and it seemed that the guilty party was the release function.
When I entered these in ghci, I got the segmentation fault:
*Main> coInitialize
*Main> pExl <- createObjExl
*Main> release pExl
0
Now if I hit "pExl" I have a reference. Shouldn't that be set to Null?
*Main> pExl
<interface pointer = 0x020844cc>
*Main> coUnInitialize
*Main> :q
leaving Ghci
Segmentation Fault/access violation ...
Source: (StackOverflow)