新規にブックを作成する

ExcelVBA

Sub Sample()
    Dim bookObj As workbook
    
    Set bookObj = Workbooks.Add
    
    Set bookObj = Nothing
End Sub

Excel(COM)

using System;
using System.Runtime.InteropServices;
using Excel = Microsoft.Office.Interop.Excel;

namespace SampleCode; // C#10～

internal class SampleExcel
{
    static void Main(string[] args)
    {
        Excel.Application? application = null;
        Excel.Workbooks? workbooks = null;
        Excel.Workbook? workbook = null;

        try
        {
            // Excelが起動
            application = new Excel.Application();

            // 表示
            application.Visible = true; // 処理を確認したいときだけtrueにする

            // ブックを追加
            workbooks = application.Workbooks;
            workbook = workbooks.Add();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
        finally
        {
            CleanUpComObject(ref workbook, true, false);
            CleanUpComObject(ref workbooks);
            CleanUpComObject(ref application);

            // RCW強制解放（残留プロセス対策）
            GC.Collect();
            GC.WaitForPendingFinalizers();
            GC.Collect();
            GC.WaitForPendingFinalizers();
        }
    }

    static void CleanUpComObject<T>(ref T comObject, bool shouldClose = true, bool saveChanges = false)
    {
        // フラグによってWorkbookはClose / ApplicationはQuitする
        if (shouldClose && comObject != null)
        {
            // 型をチェックする
            if (comObject is Microsoft.Office.Interop.Excel.Workbook workbook)
            {
                // Workbookの場合
                workbook.Close(saveChanges);
            }
            else if (comObject is Microsoft.Office.Interop.Excel.Application application)
            {
                // Applicationの場合
                application.Quit();
            }
        }

        // Objectを解放
        if (comObject != null && Marshal.IsComObject(comObject))
        {
            Marshal.ReleaseComObject(comObject);
            comObject = default!;
        }
    }
}

補足：Microsoft.Office.Interop.Excel でオブジェクトの解放について

ClosedXML

using ClosedXML.Excel;
using System;

namespace SampleCode; // C#10～

internal class SampleClosedXML
{
    static void Main(string[] args)
    {
        try
        {
            using (var workbook = new XLWorkbook())
            {
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

EPPlus

using OfficeOpenXml;
using System;
namespace SampleCode; // C#10～

internal class SampleEPPlus
{
    static void Main(string[] args)
    {
        // Ver8.0のソースです
        // 非商用個人利用の場合　名前を設定
        ExcelPackage.License.SetNonCommercialPersonal("SampleTarou");

        try
        {
            using (var package = new ExcelPackage())
            {
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

ExcelDataReader

読み取りのみのライブラリのためなし

NPOI

□xls

using NPOI.HSSF.UserModel;
using System;


namespace SampleCode; // C#10～

internal class SampleNPOI_xls
{
    static void Main(string[] args)
    {
        try
        {
            using (var workbook = new HSSFWorkbook())
            {
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

□xlsx

using NPOI.XSSF.UserModel;
using System;

namespace SampleCode; // C#10～

internal class SampleNPOI_xlsx
{
    static void Main(string[] args)
    {
        try
        {
            // xlsx形式
            using (var workbook = new XSSFWorkbook())
            {
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

□xls/xlsx対応版

Program.cs

using System;
namespace SampleCode; // C#10～

internal class SampleNPOI
{
    static void Main(string[] args)
    {
        try
        {   // xls
            using (var workbook_xls = WorkbookFactory.Create(".xls"))
            {
            }

            // xlsx
            using (var workbook_xlsx = WorkbookFactory.Create(".xlsx"))
            {
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }
}

WorkbookFactory.cs

using NPOI.HSSF.UserModel;
using NPOI.SS.UserModel;
using NPOI.XSSF.UserModel;
using System;
using System.IO;

namespace SampleCode; // C#10

public class WorkbookFactory
{
    public static IWorkbook Create(Stream stream, string extension)
    {
        extension = extension.ToLower();

        return extension switch
        {
            ".xls" => new HSSFWorkbook(stream),
            ".xlsx" => new XSSFWorkbook(stream),
            _ => throw new NotSupportedException("対象外の拡張子です")
        };
    }

    public static IWorkbook Create(string extension)
    {
        extension = extension.ToLower();

        return extension switch
        {
            ".xls" => new HSSFWorkbook(),
            ".xlsx" => new XSSFWorkbook(),
            _ => throw new NotSupportedException("対象外の拡張子です")
        };
    }
}