ASP.NET CoreアプリケーションでMagicodes.IEを使用してExcelファイルをエクスポートする際の基本的な実装手順を以下に示します。 必要なNuGetパッケージをインストールし、名前空間を参照します。

using Magicodes.ExporterAndImporter.Excel.AspNetCore;

Excelエクスポートの実装例を以下に示します。

[ApiController]
[Route("api/[controller]")]
public class ExcelExportController : ControllerBase
{
    [HttpGet("export-by-bytes")]
    public async Task<ActionResult> ExportByBytes()
    {
        var sampleData = GenerateSampleData(100);
        var exporter = new ExcelDataGenerator();
        var byteData = await exporter.GenerateExcelBytes(sampleData);
        return new ExcelExportResult(byteData);
    }

    [HttpGet("export-by-stream")]
    public async Task<ActionResult> ExportByStream()
    {
        var sampleData = GenerateSampleData(100);
        var exporter = new ExcelDataGenerator();
        var byteData = await exporter.GenerateExcelBytes(sampleData);
        var stream = new MemoryStream(byteData);
        return new ExcelExportResult(stream, "sample_data.xlsx");
    }

    [HttpGet("export-by-list")]
    public async Task<ActionResult> ExportByList()
    {
        var dataList = GenerateSampleData(100);
        return new ExcelExportResult(dataList);
    }

    private List<SampleModel> GenerateSampleData(int count)
    {
        var data = new List<SampleModel>();
        for (int i = 0; i < count; i++)
        {
            data.Add(new SampleModel { Id = i + 1, Name = $"Sample{i}", Value = i * 100 });
        }
        return data;
    }
}

カスタムActionResultの実装は以下の通りです。

public class ExcelExportResult<T> : ExcelExportBase where T : class, new()
{
    public ExcelExportResult(IEnumerable<T> dataSource, string fileName = null)
    {
        DownloadName = fileName;
        Data = dataSource;
    }

    public string DownloadName { get; }
    public IEnumerable<T> Data { get; }

    public async override Task ExecuteResultAsync(ActionContext httpContext)
    {
        var generator = new ExcelDataGenerator();
        var byteData = await generator.GenerateBytes(Data);
        var memoryStream = new MemoryStream(byteData);
        await SaveExcelFile(httpContext, memoryStream, DownloadName);
    }
}

public class ExcelExportResult : ExcelExportBase
{
    public ExcelExportResult(Stream stream, string fileName = null)
    {
        OutputStream = stream;
        DownloadName = fileName;
    }

    public ExcelExportResult(byte[] bytes, string fileName = null)
    {
        OutputStream = new MemoryStream(bytes);
        DownloadName = fileName;
    }

    public Stream OutputStream { get; }
    public string DownloadName { get; }

    public async override Task ExecuteResultAsync(ActionContext httpContext)
    {
        await SaveExcelFile(httpContext, OutputStream, DownloadName);
    }
}

public class ExcelExportBase : ActionResult
{
    protected virtual async Task SaveExcelFile(ActionContext context, Stream stream, string fileName)
    {
        var response = context.HttpContext.Response;
        response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";

        if (string.IsNullOrEmpty(fileName))
        {
            fileName = Guid.NewGuid().ToString("N") + ".xlsx";
        }
        else if (!fileName.EndsWith(".xlsx", StringComparison.OrdinalIgnoreCase))
        {
            fileName += ".xlsx";
        }

        response.Headers["Content-Disposition"] = $"attachment; filename={Uri.EscapeDataString(fileName)}";
        await stream.CopyToAsync(response.Body);
    }
}