How to check the registry value in INNOSETUP

If you want to check if the key exists and read the current value, you must use Pascal Scripting[^] in the [Code][^] section. However, this requires that you become familiar with the Pascal scripting used by Inno Setup.

The function to read a string value from the registry is RegQueryStringValue[^]. You can then for example use the Pos string function to check if the value contains a specific substring or use CompareText / CompareStr to compare complete strings.

Untested example:

var
  Value, LowerValue: String;
  Search: Integer;
begin
  if RegQueryStringValue(HKEY_LOCAL_MACHINE, 'SOFTWARE\xxx\yyy', 'KEYNAME', Value) then
  begin
    LowerValue := Lowercase(Value);
    Search := Pos('c:\x\y\z.dll', LowerValue);
    if (Search = 0) then
    begin
       Value := Value + ';C:\x\y\z.dll';
       RegWriteStringValue(HKEY_LOCAL_MACHINE, 'SOFTWARE\xxx\yyy', 'KEYNAME', Value);
    end;
  else
    RegWriteStringValue(HKEY_LOCAL_MACHINE, 'SOFTWARE\xxx\yyy', 'KEYNAME', 'C:\x\y\z.dll');
  end;
end;