Generic function for loading a function from a DLL library

Question

I am trying to write a generic function for loading a function from a DLL library. I am in no way an expert on DLLs, that is why I ask.

We have discussed it first in my Stack Overflow question to discover the most obvious errors, and there were some crucial ones.

---

Current version of my code follows:

function LoadFunctionFromLibrary(const LibraryName, FunctionName: string; out FunctionPointer: Pointer): Boolean;
var
 LibraryHandle: THandle;
begin
 Result := False;
 FunctionPointer := nil;
 LibraryHandle := Winapi.Windows.LoadLibrary(PChar(LibraryName));
 if LibraryHandle = 0 then Exit;
 FunctionPointer := Winapi.Windows.GetProcAddress(LibraryHandle, PChar(FunctionName));
 if Assigned(FunctionPointer) then
 Result := True;
end;

---

This version of the function has been successfully tested on my EnableInput function:

function EnableInput(const Enable: Boolean): Boolean;
var
 BlockInput: function(Block: BOOL): BOOL; stdcall;
begin
 Result := LoadFunctionFromLibrary('User32.dll', 'BlockInput', @BlockInput) and
 BlockInput(not Enable);
end;

Answer 1

Just like Dangph, I wonder why you need this. There are other better approaches suggested in the previous comments.

Even if this is useful, Your solution is not great: 1. You repeatedly load the library and the function. 2. There is no way for you to free the loaded libraries.

Here is a better way to do it:

type
 TFunctionLoader = class
 private
 FLibraries: TStrings; // This stores the library handles and names
 FFunctions: TStrings; // This stores the function pointers and names
 public
 constructor Create;
 destructor Destroy; override;
 function LoadFunction(const LibraryName, FunctionName: string; 
 out FunctionPointer: Pointer): Boolean;
 end;
{ TFunctionLoader }
constructor TFunctionLoader.Create;
begin
 FLibraries := TStringList.Create;
 FFunctions := TStringList.Create;
end;
destructor TFunctionLoader.Destroy;
var
 i: Integer;
begin
 for i := 0 to FLibraries.Count - 1 do // free all opened libraries
 Windows.FreeLibrary(THandle(FLibraries.Objects[i]));
 FLibraries.Free; // free other objects
 FFunctions.Free;
end;
function TFunctionLoader.LoadFunction(const LibraryName, FunctionName: string;
 out FunctionPointer: Pointer): Boolean;
var
 i: Integer;
 LibraryHandle: THandle;
begin
 i := FFunctions.IndexOf(FunctionName); // Is the function already loaded?
 if i >= 0 then
 begin // Yes, just return the stored pointer
 FunctionPointer := Pointer(FFunctions.Objects[i]);
 Exit(True);
 end;
 i := FLibraries.IndexOf(LibraryName); // No, test if the library is already loaded
 if i < 0 then
 begin // No, load it and store in FLibraries
 LibraryHandle := Windows.LoadLibrary(PChar(LibraryName));
 if LibraryHandle = 0 then Exit(False); // Failed, quit
 i := FLibraries.AddObject(LibraryName, TObject(LibraryHandle));
 end;
 // Load the function from the library
 FunctionPointer := Windows.GetProcAddress(THandle(FLibraries.Objects[i]),
 PChar(FunctionName));
 Result := Assigned(FunctionPointer); // succeeded?
 if Result then // Add the function to FFunctions
 FFunctions.AddObject(FunctionName, TObject(FunctionPointer));
end;

With above you can define var FunctionLoader: TFunctionLoader; somewhere and add this to the end of the unit

initialization
 FunctionLoader := TFunctionLoader.Create;
finalization
 FunctionLoader.Free;
end.

Use it as

Result := FunctionLoader.LoadFunction('User32.dll', 'BlockInput', @BlockInput)
 and BlockInput(not Enable);

All functions and libraries will be loaded only once and will be automatically freed.