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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for March 4th, 2021

reg query and batch file for loop tricks that refreshes the cmd environment from the registry settings: choco/RefreshEnv.cmd at master · chocolatey/choco · GitHub

Posted by jpluimers on 2021/03/04

I bumped into a very interesting [WayBack] choco/RefreshEnv.cmd at master · chocolatey/choco · GitHub.

It allows you to refresh your cmd environment from new settings that were only applied to the registry using the SET command.

Note there is a PowerShell counterpart too: [WayBack] choco/Update-SessionEnvironment.ps1 at master · chocolatey/choco · GitHub

There are many cool tricks in it, most of which you can see in the [WayBack] new commit history, and a few you can find back in the [WayBack] old commit history of the previous repository (I have no idea why those histories have never been merged).

Intermediate batch files

The basic structure is to first create some intermediate batch files, then delete them afterwards:

  • "%TEMP%\_envget.tmp"
    • is used in :GetRegEnv to get all environment variables for the MACHINE or USER level, then loop through them and call :SetFromReg during each iteration (except for the Path environment variable which is skipped).
  • "%TEMP%\_envset.tmp"
    • is used in :SetFromReg to emit one line of SET code to "%TEMP%\_env.cmd".
  • "%TEMP%\_env.cmd"
    • Contains the SET commands for the new environment variable values.

All the above methods use quoting to ensure that environment variables having names or values containing spaces are handled correctly.

Echo without newline

I like the echo | set /p trick to echo a string without a newline allows it to start as this:

C:\>RefreshEnv
Refreshing environment variables from registry for cmd.exe. Please wait...

then finish like this by appending another string to it:

C:\>RefreshEnv
Refreshing environment variables from registry for cmd.exe. Please wait...Finished..

It is explained in the old history at [WayBack] (GH-153)(GH-134) Update PATH on cmd.exe · chocolatey/chocolatey@a09e158 · GitHub.

There is an even more interesting example of this trick in [WayBack] windows – What does /p mean in set /p? – Stack Overflow:

<nul set /p=This will not generate a new line

Spaces, what spaces

One hard thing in scripting is taking into account that path names can contain spaces. This means you need to carefully quote path names, but not overdo the quotes, otherwise the quoting works against you.

Two commits from the commit history show there were two weak spots that had to be changed in [WayBack] (GH-1227) Fix: RefreshEnv doesn’t set path w/spaces · chocolatey/choco@fdfcd06 · GitHub.

The environment has a MACHINE and USER part

Environment variables can come from two places in the registry:

  • HKLM\System\CurrentControlSet\Control\Session Manager\Environment
  • HKCU\Environment

Normally, the second overrides the first.

This means they are grabbed from the registry MACHINE and USER order, then applied to the cmd environment.

Special case PATH

The PATH environment variable is special for two reasons:

  1. In the registry it is called Path, but in the environment it is usually called PATH (this is true for both the MACHINE and USER parts of the registry). New values are applied with the Path environment variable name, so after executing RefreshEnv once, they are called Path in the cmd.exe environment too.
  2. PATH is a combination from two PATH entries in the registry in the MACHINE and USER level, so it needs to be combined as you can see in [WayBack] choco/RefreshEnv.cmd at master · chocolatey/choco · GitHub.:
    :: Caution: do not insert space-chars before >> redirection sign
    echo/set "Path=%%Path_HKLM%%;%%Path_HKCU%%" >> "%TEMP%\_env.cmd"

I am not sure why there is a space before the >>, given there is a comment above it there should not be one.

The SET command however, puts the MACHINE PATH in front of the USER PATH.

Special case USERNAME, and collateral PROCESSOR_ARCHITECTURE

The USERNAME environment variable special too. In the registry, it is only in the MACHINE part, but with a value SYSTEM.

In cmd.exe, it is actually filled with the current username, so it should not be overwritten with the one in the MACHINE part.

Currently this is resolved by storing a copy of the old value of USERNAME and PROCESSOR_ARCHITECTURE in [WayBack] (GH-902) Fix: User changed to SYSTEM during env update · chocolatey/choco@cb6b92c · GitHub.

I am not sure why PROCESSOR_ARCHITECTURE is also stores.

In any case, this means that setting a USERNAME or PROCESSOR_ARCHITECTURE in the USER part of the registry, will not be reflected by RefreshEnv.

I am not sure yet when that can cause problems, so this is a reminder to myself that if ever it does, then this logic needs to be changed.

–jeroen

Posted in Batch-Files, Development, Scripting, Software Development | Leave a Comment »

Short Delphi tip: ensuring RTTI for classes is included

Posted by jpluimers on 2021/03/04

When using RTTI in Delphi, you really want the RTTI to be available.

The compiler includes RTTI for classes, as soon as it found that a class is touched by code that will be executed.

So in order to include RTTI for classes into the executable, you have to ensure you touch the class.

Basically there are two tricks for that.

  1. A small one step process which incurs a tiny bit of string overhead:
    class function TObject.ClassName: string;
    begin
      Result := UTF8ToString(_PShortStr(PPointer(PByte(Self) + vmtClassName)^)^);
    end;
    • Touch the [WayBack] ClassName class function for each class, for instance in an initialization section or registration method, like this:
      TMyClass.ClassName;
      TMyOtherClass.ClassName;
  2. A small two step process
    1. Create a method like this: procedure EnsureRttiIsAvailable(const Classes: array of TClass); begin end;
    2. Pass the classes to it like this:
      EnsureRttiIsAvailable([TMyClass, TMyOtherClass]);

I like the second solution more, as it clearly states the intent.

The first trick is calling a function without using the result. This is a Pascal construct that looks odd, but is perfectly valid to use: basically you discard the result.

–jeroen

Posted in Conference Topics, Conferences, Delphi, Development, Event, Software Development, Undocumented Delphi | Leave a Comment »

Delphi: combining intrinsic functions and inline to have no-code checks on concrete generic instantiation

Posted by jpluimers on 2021/03/04

The title might sound like a lot of gibberish, but the mechanism it describes helps solving a problem when using generics: lack of generic constraints in the compiler.

For instance, you cannot constrain on enumeration types (C# could not do this either: you could not do T GetEnumFromString<T>(string valueString, T defaultValue) where T : Enum, but [WayBack] since C# 7.3, you can do ... where T : Enum) unless you [WayBack] did some real hackery.

For Delphi, you still cannot do the constraint, but with some hackery, you can avoid code generation. Spring4d uses this in [WayBack] Spring.pas, from which I copied these fragments:

class function TType.Kind<T>: TTypeKind;
{$IFDEF DELPHIXE7_UP}
begin
  Result := System.GetTypeKind(T);
{$ELSE}
var
  typeInfo: PTypeInfo;
begin
  typeInfo := System.TypeInfo(T);
  if typeInfo = nil then
    Exit(tkUnknown);
  Result := typeInfo.Kind;
{$ENDIF}
end;

class procedure Guard.CheckTypeKind<T>(expectedTypeKind: TTypeKind;
  const argumentName: string);
begin
  if TType.Kind<T> <> expectedTypeKind then
    RaiseArgumentException(TType.Kind<T>, argumentName);
end;

class function TEnum.IsValid<T>(const value: Integer): Boolean;
var
  data: PTypeData;
begin
  Guard.CheckTypeKind<T>(tkEnumeration, 'T');
  data := GetTypeData(TypeInfo(T));
  Result := (value >= data.MinValue) and (value <= data.MaxValue);
end;

When <T> is an enumeration type, any code for that call is eliminated by the compiler.

Related

Concrete versus generic type:

–jeroen

Posted in Delphi, Development, Software Development | Leave a Comment »

 
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