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 v2.0.0.

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.

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();

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