Update DLL Server with Inno Setup

Question

How do I structure my Inno Setup script to automatically register a dll if it is the first time a user has installed my application but unregister a previous version if there is one and then register the new one (assuming the interface is different)?

I currently use the the regserver and ignoreversion flags in my Files section as seen below:

[Setup]
...

[Languages]
...

[Files]
Source: "C:\example.dll"; DestDir: "{app}"; Flags: ignoreversion regserver

In my google search I have found UnregisterServer but I not know how to add this to my script. I would gladly start tinkering around to see how this works, but I do not want to do anything that will mess-up my registry.

There is a similar post here but it does not address how this is actually accomplished.

EDIT

After hacking around in Pascal I was able to add the following to the [Code] section and it worked. Does anyone know how to use the {app} constant to dynamically define the fileName in the code below?

[Code]
const
  fileName = 'C:\Program Files\TFolderName\tigercontroller.dll';
var
  serverExists: Boolean;

function InitializeSetup(): Boolean;
begin     
  serverExists := UnregisterServer(False, fileName, False);

  if serverExists then begin
    Result:= True;
    MsgBox('This will update with the most recent version', mbInformation, mb_Ok);
  end else
    Result := True;
end;

Answer 1

What about using BeforeInstall and AfterInstall parameters for file?

Usage is:

[Files]
Source: "MYDLL.DLL"; DestDir: "{app}"; BeforeInstall: MyBeforeInstall; AfterInstall: MyAfterInstall;

BeforeInstall and AfterInstall functions must not have a return value!

procedure MyBeforeInstall();
begin
  // Your code here: If file (old) file exists call UnregisterServer() on old file
  // Use function FileExists(const Name: String): Boolean; or similar for it
  // Also you can delete the file entirely with function DeleteFile(const FileName: string): Boolean;

  // Hint: You can use 'CurrentFileName' variable to get currently processed file 
end;

procedure MyAfterInstall();
begin
  // Your (new) file was processed and now you can do additional tweaks on it
  // 'CurrentFileName' variable is still available 
  // Setup registers all files with the 'regserver' or 'regtypelib' flags as the last step of installation so in this function the file is still not registered!
end;