What does VB.NET do differently than C# that Excel exits after the console-app ends without releasin-CodePudding

I've got two .net-6 console-apps:

one in C#

using Excel = Microsoft.Office.Interop.Excel;

Console.WriteLine("Hello, World!");

var xl = new Excel.Application();
var wb = xl.Workbooks.Open(@"c:\temp\test-2.xlsx");

wb.Close(0);
xl.Quit();

System.Runtime.InteropServices.Marshal.ReleaseComObject(wb);
System.Runtime.InteropServices.Marshal.ReleaseComObject(xl);

wb = null;
xl = null;

the other one in VB.NET.

Imports System
Imports Excel = Microsoft.Office.Interop.Excel

Module Program
    Sub Main(args As String())
        Console.WriteLine("Hello World!")

        Dim xl = New Excel.Application()
        Dim wb = xl.Workbooks.Open("c:\\temp\\test-2.xlsx")

        wb.Close(0)
        xl.Quit()

        'System.Runtime.InteropServices.Marshal.ReleaseComObject(wb)
        'System.Runtime.InteropServices.Marshal.ReleaseComObject(xl)

        'wb = Nothing
        'xl = Nothing


    End Sub
End Module

The project configuration is in both consoles is the same:

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net6.0</TargetFramework>
        <RootNamespace>OfficeInteropTest</RootNamespace>        
    </PropertyGroup>

    <ItemGroup>
        <COMReference Include="Microsoft.Office.Interop.Excel">
            <WrapperTool>tlbimp</WrapperTool>
            <VersionMinor>9</VersionMinor>
            <VersionMajor>1</VersionMajor>
            <Guid>00020813-0000-0000-c000-000000000046</Guid>
            <Lcid>0</Lcid>
            <Isolated>false</Isolated>
            <EmbedInteropTypes>true</EmbedInteropTypes>
        </COMReference>
    </ItemGroup>

</Project>

When I run each of them the C# app does not close Excel after the console exits while the VB.NET app does even though I commented out the last four lines.

So, I was wondering what do they do differently and why does it look like VB.NET is better with Office interop?

CodePudding user response：

It turns out that Dai's susspicion was correct. VB.NET adds STAThread implicitly. I've looked into the ILCode and there it is:

.method public static void
    Main(
      string[] args
    ) cil managed
  {
    .entrypoint
    .custom instance void [System.Runtime]System.STAThreadAttribute::.ctor()
      = (01 00 00 00 )
    .maxstack 16
    .locals init (
      [0] class Microsoft.Office.Interop.Excel.Application xl,
      [1] class Microsoft.Office.Interop.Excel.Workbook wb
    )

    // [5 5 - 5 31]
    IL_0000: nop

    // [6 9 - 6 42]
    IL_0001: ldstr        "Hello World!"
    IL_0006: call         void [System.Console]System.Console::WriteLine(string)
    IL_000b: nop

Adding the [STATHread] attribute to a C# app fixes the issue with leaving Excel open after the app quits and makes the last four lines unecessary.