Exception calling remote SOAP call from thread

Question

This is an extension / next step of this question I asked a few minutes ago.

I've a Delphi application with a main form and a thread. Every X seconds the thread makes a web services request for a remote object. It then posts back to the main form which handles updating the UI with the new information.

I was previously using a TTimer object in my thread, and when the TTimer callback function ran, it ran in the context of the main thread (but the remote web services request did work). This rather defeated the purpose of the separate thread, and so I now have a simple loop and sleep routine in my thread's Execute function. The problem is, an exception is thrown when returning from GetIMySOAPService().

procedure TPollingThread.Execute;
var
SystemStatus : TCWRSystemStatus;
begin
while not Terminated  do
begin
  sleep(5000);
  try
    SystemStatus := GetIMySOAPService().GetSystemStatus;
    PostMessage( ParentHandle, Integer(apiSystemStatus), Integer(SystemStatus), 0 );
    SystemStatus.DataContext := nil;
    LParam(SystemStatus) := 0;
  except
  end;
end;
end;

Can anyone advise as to why this exception is being thrown when calling this function from the thread? I'm sure I'm overlooking something fundamental and simple.

Thanks, Duncan

Answer 1

For my future-self...I needed to CoInitialize and CoUnInitialize in my Execute method:

procedure TPollingThread.Execute;
begin

  CoInitialize(nil);

  while not Terminated  do
  begin
  // ...
  end;


   CoUnInitialize;
end;

Answer 2

In your Execute() method, you must call CoInitialize and CoUnitialize to setup and tear down the COM library.

Your main thread automatically does this in the Application.Initialize() procedure, however, other threads require the call to CoInitialize before calling COM functions.

Ensure you call CoInitialize in the Execute() method and not in the constructor because the constructor is executed in the parent thread (usually the main thread). That's not where you need it. It must be called from the thread that you plan on making COM calls from.

I recommend using this format:

try
  // OleCheck will raise an error if the call fails
  OleCheck(CoInitializeEx(NIL, COINIT_MULTITHREADED or COINIT_SPEED_OVER_MEMORY));
  try
    // Do work here
  finally
    CoUninitialize;
  end;
except
  // We failed, do something
end;

This allows you to trap the error if it fails to initialize and ensures that you call CoUninitialize.