Accessing the Windows Registry - Delphi

What is the Registry?
---------------------

It is where Windows stores many of its configuration options and also allows applications to access this data as well as save their own data. If you want to take a look at the registry, just execute the REGEDIT.EXE application located in the Windows directory. Be careful not to change anything or you could end up ruining your installation! Now, the data in the registry is stored in a tree structure. There are many roots (many trees):

  HKEY_CLASSES_ROOT
  HKEY_CURRENT_USER
  HKEY_LOCAL_MACHINE
  HKEY_USERS
  HKEY_PERFORMANCE_DATA
  HKEY_CURRENT_CONFIG
  HKEY_DYN_DATA

Each root can have values and keys. The values are data stored under item names (right panel of RegEdit). Keys can have values and other keys, forming a tree structure (left panel of RegEdit).

For example, the tree HKEY_CURRENT_USER has many keys, like AppEvents, Control Panel, Identities, Network, Software, etc. Each key may have sub-keys. For example, Control Panel has some sub-keys, like Accessibility, Appearance, Colors, Cursors, Desktop, International, etc. All keys have at least one value (the first value in the right panel of RegEdit), which is the default value (the name of the value is the empty string), not necessarily set. A key may have more values. For example, let's see the key Control Panel\Colors under HKEY_CURRENT_USER. Appart from the default value, it has values like ActiveBorder, ActiveTitle, AppWorkspace, Background, etc. In turn, each "value" has a "data" (the actual value, sort to speak). For example, the data of the value ActiveTitle would be "0 0 128" (may be different if you are not using the standard Windows colors).


TRegistry
---------

Before getting into GetRegistryData and SetRegistryData, let's see how we would accomplish the same tasks the hard way, using TRegistry.

The TRegistry class is declared in the Registry unit, so you will have to add this unit to the Uses clause of the unit or program where you want to use it. To access a value in the registry, first you should create an object of this class, assign the root to its RootKey property (the values are defined in the Windows unit) and then try to open a key with the OpenKey function method, which will return True if successful. Then you can read (with the ReadXxxx functions) or write (with the WriteXxxx procedures) the values of the open key and, after that, you should close the key with CloseKey. When you are done with the registry, you should free the registry object you created. Let's see an example of how to obtain the name of the processor in our computer:

  procedure TForm1.Button1Click(Sender: TObject);
  var
    Reg: TRegistry;
  begin
    Reg := TRegistry.Create;
    Reg.RootKey := HKEY_LOCAL_MACHINE;
    if Reg.OpenKey('\Hardware\Description\System'
        + '\CentralProcessor\0', False) then begin
      ShowMessage(Reg.ReadString('Identifier'));
      Reg.CloseKey;
    end; // if
    Reg.Free;
  end;

You can see another example in the article Determining the associated application. Of course, there are many more things you can do with the registry, like creating and deleting keys and values...

The TRegistryIniFile class makes it simpler for applications to write and read their configuration information to and from the registry, while TRegistry operates at a lower level.


GetRegistryData
---------------

To simplify reading a data value from the registry you can use the following function that can read any data type from the registry and returns it as a variant (string or integer). The function performs exception handling.

  uses Registry;

  function GetRegistryData(RootKey: HKEY; Key,
                           Value: string): variant;
  var
    Reg: TRegistry;
    RegDataType: TRegDataType;
    DataSize, Len: integer;
    s: string;
  label cantread;
  begin
    Reg := nil;
    try
      Reg := TRegistry.Create(KEY_QUERY_VALUE);
      Reg.RootKey := RootKey;
      if Reg.OpenKeyReadOnly(Key) then begin
        try
          RegDataType := Reg.GetDataType(Value);
          if (RegDataType = rdString) or
             (RegDataType = rdExpandString) then
            Result := Reg.ReadString(Value)
          else if RegDataType = rdInteger then
            Result := Reg.ReadInteger(Value)
          else if RegDataType = rdBinary then begin
            DataSize := Reg.GetDataSize(Value);
            if DataSize = -1 then
              raise Exception.Create(SysErrorMessage(ERROR_CANTREAD));
            SetLength(s, DataSize);
            Len := Reg.ReadBinaryData(Value, PChar(s)^, DataSize);
            if Len <> DataSize then
              raise Exception.Create(SysErrorMessage(ERROR_CANTREAD));
            Result := s;
          end else
            raise Exception.Create(SysErrorMessage(ERROR_CANTREAD));
        except
          s := ''; // Deallocates memory if allocated
          Reg.CloseKey;
          raise;
        end;
        Reg.CloseKey;
      end else
        raise Exception.Create(SysErrorMessage(GetLastError));
    except
      Reg.Free;
      raise;
    end;
    Reg.Free;
  end;

Sample Call:

  ShowMessage(GetRegistryData(HKEY_LOCAL_MACHINE,
    '\Hardware\Description\System\CentralProcessor\0', 'Identifier'));


SetRegistryData
---------------

To simplify writing a data value to the registry you can use the following procedure that can write any data type to the registry. The procedure performs exception handling.

  uses Registry;

  procedure SetRegistryData(RootKey: HKEY; Key, Value: string;
    RegDataType: TRegDataType; Data: variant);
  var
    Reg: TRegistry;
    s: string;
  begin
    Reg := nil;
    try
      Reg := TRegistry.Create(KEY_WRITE);
      Reg.RootKey := RootKey;
      if Reg.OpenKey(Key, True) then begin
        try
          if RegDataType = rdUnknown then
            RegDataType := Reg.GetDataType(Value);
          if RegDataType = rdString then
            Reg.WriteString(Value, Data)
          else if RegDataType = rdExpandString then
            Reg.WriteExpandString(Value, Data)
          else if RegDataType = rdInteger then
            Reg.WriteInteger(Value, Data)
          else if RegDataType = rdBinary then begin
            s := Data;
            Reg.WriteBinaryData(Value, PChar(s)^, Length(s));
          end else
            raise Exception.Create(SysErrorMessage(ERROR_CANTWRITE));
        except
          Reg.CloseKey;
          raise;
        end;
        Reg.CloseKey;
      end else
        raise Exception.Create(SysErrorMessage(GetLastError));
    except
      Reg.Free;
      raise;
    end;
    Reg.Free;
  end;

Sample Call:

  SetRegistryData(HKEY_LOCAL_MACHINE,
    '\Software\Microsoft\Windows\CurrentVersion',
    'RegisteredOrganization', rdString, 'Latium Software');