Accessing the Windows Registry

Copyright © 2000 Ernesto De Spirito

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).
 

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:

uses Registry, Windows, Dialogs;

procedure TForm1.Button1Click(Sender: TObject);
begin
  with TRegistry.Create do
    try
      RootKey := HKEY_LOCAL_MACHINE;
      if OpenKey('\Hardware\Description\System'
          + '\CentralProcessor\0', False) then begin
        ShowMessage(ReadString('Identifier'));
        CloseKey;
      end;
    finally
      Free;
    end;
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, Windows, SysUtils;

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 goto cantread;
          SetLength(s, DataSize);
          Len := Reg.ReadBinaryData(Value, PChar(s)^, DataSize);
          if Len <> DataSize then goto cantread;
          Result := s;
        end else
cantread:
          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, Windows, SysUtils;

procedure SetRegistryData(RootKey: HKEY; Key, Value: string;
  RegDataType: TRegDataType; Data: variant);
var
  Reg: TRegistry;
  s: string;
begin
  Reg := TRegistry.Create(KEY_WRITE);
  try
    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));
  finally
    Reg.Free;
  end;
end;

Sample Call

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