BlazorFileReader

Blazor library exposing read-only file streams in Blazor using <input type="file" /> and FileReader. Drag and drop targets may also be used to initialize streams.

Here is a Live demo that contains the output of the wasm demo project. Currently, its a build based on v3.2.0.

Installation

Use Nuget: Install-Package Tewr.Blazor.FileReader

Make sure your environment is up to date with the appropriate SDK and VS2019 16.6. See this article for more details. Depending on your project type, use one of the two examples below. For a complete use-case, see the client or server-side demo projects.

Client-side / Wasm Project type / "CSB"

Setup IoC for IFileReaderServiceas in (Program.cs):

services.AddFileReaderService(options => options.UseWasmSharedBuffer = true);

Server-side / asp.net core Project type / "SSB"

Setup IoC for IFileReaderService as in the example (Startup.cs):

services.AddFileReaderService();

Documentation

The public API is documented here, generated from the XML comments.

To get started, the best is to look at the example razor files in the demo project.

services.AddServerSideBlazor().AddHubOptions(o =>
{
    o.MaximumReceiveMessageSize = 10 * 1024 * 1024; // 10MB
});

Gotcha's

Problems with reading strings using StreamReader in while header

When publishing or compiling in Release mode, the Optimize flag is set by default. Compiling with this flag set may result in problems if you are using StreamReader. An bug is open on this subject, being investigated by the mono team. Tracked locally here. A simple workaround is available in this issue. Basically, don't call await in the while header, call it somewhere else. This has been fixed in Blazor 5rc1.

IFileReference.CreateMemoryStreamAsync()

The IFileReference.CreateMemoryStreamAsync() method (without any argument) is basically the same as calling IFileReference.CreateMemoryStreamAsync(bufferSize: file.Size). Calling IFileReference.CreateMemoryStreamAsync() may thus be unsuitable for large files (at least for client-side Blazor as the UI will be blocked during the transfer).

Usage in a Blazor View

The code for views looks the same for both client- and server-side projects. The demo projects also contains a drag and drop example. While the demo projects are the reference, examples also exist in the wiki.

@page "/MyPage"
@using Tewr.Blazor.FileReader
@using System.IO;
@inject IFileReaderService fileReaderService;

<input type="file" @ref=inputTypeFileElement /><button @onclick=ReadFile>Read file</button>

@code
{
    private ElementReference inputTypeFileElement;

    public async Task ReadFile()
    {
        foreach (var file in await fileReaderService.CreateReference(inputTypeFileElement).EnumerateFilesAsync())
        {
            // Read into buffer and act (uses less memory)
            await using (Stream stream = await file.OpenReadAsync()) {
                // Do (async) stuff with stream...
                await stream.ReadAsync(buffer, ...);
                // The following will fail. Only async read is allowed.
                stream.Read(buffer, ...)
            }

            // Read file fully into memory and act
            using (MemoryStream memoryStream = await file.CreateMemoryStreamAsync(4096)) {
                // Sync calls are ok once file is in memory
                memoryStream.Read(buffer, ...)
            }
        }
    }
}

Version notes

Version 3.3.2.23201 Fixes a a bug in 3.3.2.23185 related to drag and drop

Version 3.3.2.23185 Adds the possibility to drop multiple files and directories on elements.

Version 3.3.1.21360 Fixes a bug related to platform detection under .net6

Version 3.3.0.21348 Adds .Net6 support.

Version 3.2.0.21211 Adds Copy/Paste support.

Version 3.1.0.21158 Adds two new methods (.NET5 only): JsObjectReference for files, and CreateObjectUrl for files (built on JsObjectReference). Adds a new overload for IFileReaderRef.RegisterDropEventsAsync that provides extensibility points for custom scripts.

Version 3.0.0.20340 Add support for .NET5 and fixes a small issue with Platform detection.

Standard upload component

📰 01.10.2020 Microsoft has released a built-in file upload component called InputFile. For the most common and simple scenarios, I'd recommend using the built-in component rather than BlazorFileReader. The BlazorFileReader library gives more control over buffer size and memory usage, which may be to your advantage when working with large files. Also, not really being a blazor component, but rather a service that binds to an element, it may at times offer more flexibility.