Accessing the Windows Registry

Problem/Question/Abstract:

How can I simply save and get data from the Windows Registry? The purpose of this article is to introduce GetRegistryData and SetRegistryData as an alternative to TRegistry, making it easy to read and write values from and to the Windows Registry, allowing developers to access the registry in a practical way.

Answer:

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');

You can find another example of SetRegistryData in my article Making an application run automatically when Windows starts.

Component Download: http://www.latiumsoftware.com/download/delphi-2.zip

Copyright (c) 2001 Ernesto De Spirito
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