Why Inno Setup can not load the proc?

Question

I need to call custom DLL function in CurStepChanged where CurStep = ssInstall.

[Files]
Source: "publish\InnoSetupHelper.dll"; Flags: dontcopy

[Code]
function StopService(ServiceName: String; TimeOut: Integer): Integer;
external 'StopService@InnoSetupHelper.dll cdecl delayload loadwithalteredsearchpath'; 

procedure CurStepChanged(CurStep: TSetupStep);
var
  ErrorCode: Integer;
begin
  case CurStep of
    ssInstall: begin
      ErrorCode := StopService('XxxService', 10000); // line 179
      if ErrorCode <> 0 then
        MsgBox(FmtMessage('Stop service failed: %1', [ErrorCode]), mbCriticalError, MB_OK);
    end;
  end;
end;

EXTERN_C_START

__declspec(dllexport) DWORD StopService(PCWSTR serviceName, DWORD waitStopTimeoutMilliseconds);

EXTERN_C_END

I tried delayload only, loadwithalteredsearchpath only, no delayload and loadwithalteredsearchpath, all not work, either:

or:

My DLL project is like:

<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <ItemGroup Label="ProjectConfigurations">
    <ProjectConfiguration Include="Debug|Win32">
      <Configuration>Debug</Configuration>
      <Platform>Win32</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Debug|x64">
      <Configuration>Debug</Configuration>
      <Platform>x64</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Release|Win32">
      <Configuration>Release</Configuration>
      <Platform>Win32</Platform>
    </ProjectConfiguration>
    <ProjectConfiguration Include="Release|x64">
      <Configuration>Release</Configuration>
      <Platform>x64</Platform>
    </ProjectConfiguration>
  </ItemGroup>
  <PropertyGroup Label="Globals">
    <VCProjectVersion>16.0</VCProjectVersion>
    <Keyword>Win32Proj</Keyword>
    <ProjectGuid>{d63e5414-c5d7-484f-8dce-6dd4996443f5}</ProjectGuid>
    <RootNamespace>InnoSetupHelper</RootNamespace>
    <WindowsTargetPlatformVersion>10.0</WindowsTargetPlatformVersion>
  </PropertyGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <UseDebugLibraries>true</UseDebugLibraries>
    <PlatformToolset>v142</PlatformToolset>
    <CharacterSet>Unicode</CharacterSet>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <UseDebugLibraries>true</UseDebugLibraries>
    <PlatformToolset>v142</PlatformToolset>
    <CharacterSet>Unicode</CharacterSet>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <UseDebugLibraries>false</UseDebugLibraries>
    <PlatformToolset>v142</PlatformToolset>
    <WholeProgramOptimization>true</WholeProgramOptimization>
    <CharacterSet>Unicode</CharacterSet>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="Configuration">
    <ConfigurationType>DynamicLibrary</ConfigurationType>
    <UseDebugLibraries>false</UseDebugLibraries>
    <PlatformToolset>v142</PlatformToolset>
    <WholeProgramOptimization>true</WholeProgramOptimization>
    <CharacterSet>Unicode</CharacterSet>
  </PropertyGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
  <ImportGroup Label="ExtensionSettings">
  </ImportGroup>
  <ImportGroup Label="Shared">
  </ImportGroup>
  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
  </ImportGroup>
  <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="PropertySheets">
    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
  </ImportGroup>
  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
  </ImportGroup>
  <ImportGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="PropertySheets">
    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
  </ImportGroup>
  <PropertyGroup Label="UserMacros" />
  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
    <ClCompile>
      <WarningLevel>Level3</WarningLevel>
      <SDLCheck>true</SDLCheck>
      <PreprocessorDefinitions>_DEBUG;INNOSETUPHELPER_EXPORTS;_WINDOWS;_USRDLL;%(PreprocessorDefinitions)</PreprocessorDefinitions>
      <ConformanceMode>true</ConformanceMode>
      <PrecompiledHeader>Use</PrecompiledHeader>
      <PrecompiledHeaderFile>pch.h</PrecompiledHeaderFile>
    </ClCompile>
    <Link>
      <SubSystem>Windows</SubSystem>
      <GenerateDebugInformation>true</GenerateDebugInformation>
      <EnableUAC>false</EnableUAC>
    </Link>
    <PostBuildEvent>
      <Command>XCOPY $(OutDir)$(TargetFileName) $(SolutionDir)publish\ /Y</Command>
    </PostBuildEvent>
  </ItemDefinitionGroup>
  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
    <ClCompile>
      <WarningLevel>Level3</WarningLevel>
      <SDLCheck>true</SDLCheck>
      <PreprocessorDefinitions>_DEBUG;INNOSETUPHELPER_EXPORTS;_WINDOWS;_USRDLL;%(PreprocessorDefinitions)</PreprocessorDefinitions>
      <ConformanceMode>true</ConformanceMode>
      <PrecompiledHeader>Use</PrecompiledHeader>
      <PrecompiledHeaderFile>pch.h</PrecompiledHeaderFile>
    </ClCompile>
    <Link>
      <SubSystem>Windows</SubSystem>
      <GenerateDebugInformation>true</GenerateDebugInformation>
      <EnableUAC>false</EnableUAC>
    </Link>
    <PostBuildEvent>
      <Command>XCOPY $(OutDir)$(TargetFileName) $(SolutionDir)publish\ /Y</Command>
    </PostBuildEvent>
  </ItemDefinitionGroup>
  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
    <ClCompile>
      <WarningLevel>Level3</WarningLevel>
      <FunctionLevelLinking>true</FunctionLevelLinking>
      <IntrinsicFunctions>true</IntrinsicFunctions>
      <SDLCheck>true</SDLCheck>
      <PreprocessorDefinitions>NDEBUG;INNOSETUPHELPER_EXPORTS;_WINDOWS;_USRDLL;%(PreprocessorDefinitions)</PreprocessorDefinitions>
      <ConformanceMode>true</ConformanceMode>
      <PrecompiledHeader>Use</PrecompiledHeader>
      <PrecompiledHeaderFile>pch.h</PrecompiledHeaderFile>
      <RuntimeLibrary>MultiThreaded</RuntimeLibrary>
    </ClCompile>
    <Link>
      <SubSystem>Windows</SubSystem>
      <EnableCOMDATFolding>true</EnableCOMDATFolding>
      <OptimizeReferences>true</OptimizeReferences>
      <GenerateDebugInformation>true</GenerateDebugInformation>
      <EnableUAC>false</EnableUAC>
    </Link>
    <PostBuildEvent>
      <Command>XCOPY $(OutDir)$(TargetFileName) $(SolutionDir)publish\ /Y</Command>
    </PostBuildEvent>
  </ItemDefinitionGroup>
  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
    <ClCompile>
      <WarningLevel>Level3</WarningLevel>
      <FunctionLevelLinking>true</FunctionLevelLinking>
      <IntrinsicFunctions>true</IntrinsicFunctions>
      <SDLCheck>true</SDLCheck>
      <PreprocessorDefinitions>NDEBUG;INNOSETUPHELPER_EXPORTS;_WINDOWS;_USRDLL;%(PreprocessorDefinitions)</PreprocessorDefinitions>
      <ConformanceMode>true</ConformanceMode>
      <PrecompiledHeader>Use</PrecompiledHeader>
      <PrecompiledHeaderFile>pch.h</PrecompiledHeaderFile>
      <RuntimeLibrary>MultiThreaded</RuntimeLibrary>
    </ClCompile>
    <Link>
      <SubSystem>Windows</SubSystem>
      <EnableCOMDATFolding>true</EnableCOMDATFolding>
      <OptimizeReferences>true</OptimizeReferences>
      <GenerateDebugInformation>true</GenerateDebugInformation>
      <EnableUAC>false</EnableUAC>
    </Link>
    <PostBuildEvent>
      <Command>XCOPY $(OutDir)$(TargetFileName) $(SolutionDir)publish\ /Y</Command>
    </PostBuildEvent>
  </ItemDefinitionGroup>
  <ItemGroup>
    <ClInclude Include="framework.h" />
    <ClInclude Include="InnoSetupHelper.h" />
    <ClInclude Include="pch.h" />
  </ItemGroup>
  <ItemGroup>
    <ClCompile Include="dllmain.cpp" />
    <ClCompile Include="pch.cpp">
      <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">Create</PrecompiledHeader>
      <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">Create</PrecompiledHeader>
      <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Release|x64'">Create</PrecompiledHeader>
      <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">Create</PrecompiledHeader>
    </ClCompile>
    <ClCompile Include="Service.cpp" />
  </ItemGroup>
  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
  <ImportGroup Label="ExtensionTargets" />
</Project>

Answer 1

1. Your DLL must be 32-bit. Your project has 64-bit target.
2. You should not link against Runtime DLL (or when you do, you have to distribute the Runtime DLL along). I suggest you link the Runtime statically. In project Properties, go to C/C++ > Code Generation > Runtime Library and select Multi-threaded (not DLL).
3. You are missing files: prefix in your external declaration.

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See also Problem with importing DLL into Inno Setup