Blazor JavaScript Interop

FetchData.razor

@page "/fetchdata"
@using CsvHelper
@using System.IO
@using System.Text
@inject IWeatherForecastService Service
@inject IFileService FileService

<h1>Weather forecast</h1>

<p>This component demonstrates fetching data from the server.</p>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <button onclick="@Download">Download</button>
    <table class="table">
        <thead>
            <tr>
                <th>Date</th>
                <th>Temp. (C)</th>
                <th>Temp. (F)</th>
                <th>Summary</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var forecast in forecasts)
            {
                <tr>
                    <td>@forecast.Date.ToShortDateString()</td>
                    <td>@forecast.TemperatureC</td>
                    <td>@forecast.TemperatureF</td>
                    <td>@forecast.Summary</td>
                </tr>
            }
        </tbody>
    </table>
}

@functions {
        WeatherForecast[] forecasts;

    protected override async Task OnInitAsync()
    {
        forecasts = await Service.GetAsync();
    }

    void Download()
    {
        using (var writer = new StringWriter())
        {
            using (var csv = new CsvWriter(writer))
            {
                csv.WriteRecords(forecasts);

                FileService.SaveAsAsync("data.csv", Encoding.ASCII.GetBytes(writer.ToString()));
            }
        }
    }
}

FileService.cs

public class FileService : IFileService
{
    private IJSRuntime _jsRuntime;

    public FileService(IJSRuntime jsRuntime)
    {
        _jsRuntime = jsRuntime;
    }

    public Task SaveAsAsync(string filename, byte[] data)
    {
        return _jsRuntime.InvokeAsync<object>(
            "saveAsFile",
            filename,
            Convert.ToBase64String(data));
    }
}

public interface IFileService
{
    Task SaveAsAsync(string filename, byte[] data);
}

index.html

<script type="text/javascript">
    function saveAsFile(filename, bytesBase64) {
        var link = document.createElement('a');
        link.download = filename;
        link.href = "data:application/octet-stream;base64," + bytesBase64;
        document.body.appendChild(link); // Needed for Firefox
        link.click();
        document.body.removeChild(link);
    }
</script>