AspNetCore.Docs/aspnetcore/blazor/file-downloads.md

title	author	description	monikerRange	ms.author	ms.custom	no-loc	ms.date	uid
ASP.NET Core Blazor file downloads	TanayParikh	Learn how to download files using Blazor.	>= aspnetcore-6.0	taparik	mvc	Home Privacy Kestrel appsettings.json ASP.NET Core Identity cookie Cookie Blazor Blazor Server Blazor WebAssembly Identity Let's Encrypt Razor SignalR	11/09/2021	blazor/file-downloads

ASP.NET Core Blazor file downloads

[!WARNING] Always follow security best practices when allowing users to download files. For more information, see the Security considerations section.

The following example demonstrates how to download a file. Native byte[] streaming interop is used to ensure efficient transfer to the client.

In a Razor component (.razor), add @using and @inject directives for the following:

• xref:System.IO?displayProperty=fullName
• xref:Microsoft.JSInterop.IJSRuntime?displayProperty=fullName

@using System.IO
@inject IJSRuntime JS

Add a button to trigger the file download:

<button @onclick="DownloadFileFromStream">
    Download File From Stream
</button>

Create a method that retrieves a xref:System.IO.Stream for the file that's downloaded to clients (GetFileStream in the following example). You may choose to retrieve a file from storage or dynamically generate a file.

For this demonstration, the app creates a 50 KB file of random data from a new byte array (new byte[]). The bytes are wrapped with a xref:System.IO.MemoryStream to serve as the example's dynamically-generated binary file:

@code {
    private Stream GetFileStream()
    {
        var randomBinaryData = new byte[50 * 1024];
        var fileStream = new MemoryStream(randomBinaryData);

        return fileStream;
    }
}

The following DownloadFileFromStream method performs the following steps:

• Retrieves the xref:System.IO.Stream from GetFileStream.
• Specify a file name when file is saved on the user's machine. The following example names the file log.bin.
• Wraps the xref:System.IO.Stream in a xref:Microsoft.JSInterop.DotNetStreamReference, which allows streaming the file data to the client.
• Invokes downloadFileFromStream, which is a JavaScript function that accepts the data on the client. The downloadFileFromStream function is shown later in this article.

@code {
    private async Task DownloadFileFromStream()
    {
        var fileStream = GetFileStream();
        var fileName = "log.bin";

        using var streamRef = new DotNetStreamReference(stream: fileStream);

        await JS.InvokeVoidAsync("downloadFileFromStream", fileName, streamRef);
    }
}

The JavaScript downloadFileFromStream function accepts the file name and data stream and triggers the client-side download. The function performs the following steps:

• Read the provided stream into an ArrayBuffer.
• Create a Blob to wrap the ArrayBuffer.
• Create an object URL to serve as the address for the file to be downloaded.

The fileName and object url are passed to triggerFileDownload, which performs the following steps:

• Create an HTMLAnchorElement (<a> element).
• Assign the url and fileName for the download.
• Trigger the download via click.
• Remove the anchor (<a>) element.

At this point, the file download is triggered. It should be safe to revoke the temporary object URL by calling revokeObjectURL on the URL. This is an important step to ensure memory isn't leaked on the client.

async function downloadFileFromStream(fileName, contentStreamReference) {
  const arrayBuffer = await contentStreamReference.arrayBuffer();
  const blob = new Blob([arrayBuffer]);

  const url = URL.createObjectURL(blob);

  triggerFileDownload(fileName, url);

  URL.revokeObjectURL(url);
}

function triggerFileDownload(fileName, url) {
  const anchorElement = document.createElement('a');
  anchorElement.href = url;

  if (fileName) {
    anchorElement.download = fileName;
  }

  anchorElement.click();
  anchorElement.remove();
}

File streams

In Blazor WebAssembly apps, file data is streamed directly from .NET code into the browser. In Blazor Server apps, file data is streamed over the SignalR connection from .NET code into the browser.

Security considerations

Use caution when providing users with the ability to download files from a server. Attackers may execute denial of service (DOS) attacks or attempt to compromise networks and servers in other ways.

Security steps that reduce the likelihood of a successful attack are:

• Download files from a dedicated file download area on the server, preferably from a non-system drive. Using a dedicated location makes it easier to impose security restrictions on downloadable files. Disable execute permissions on the file download area.
• Verify that client-side checks are also performed on the server. Client-side checks are easy to circumvent.

Additional resources

• xref:blazor/file-uploads
• xref:mvc/models/file-uploads#security-considerations
• xref:blazor/forms-validation