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.

⚠️ Breaking changes in version 2.0.0.20200 : Changes Root Namespace from Blazor.FileReader to Tewr.Blazor.FileReader. Update your using statements and / or type references.

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.

Setup IoC for IFileReaderServiceas in (Program.cs):

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

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

services.AddFileReaderService();

For server-side hosting, bufferSize + metadata (up to ~30%, depending on buffersize) should not exceed the SignalR MaximumReceiveMessageSize setting, or you will encounter a client-side exception if the file is larger than bufferSize. Make sure MaximumReceiveMessageSize exceeds your bufferSize with 30% to be on the safe side. It is also recommended to set a fixed upper file size in the input tag or validate file.Size in code before starting the uploading. The default settings is 32KB. Thus, if you leave this setting untouched, you should not use a buffer size exceeding 22KB.

You can set the MaximumReceiveMessageSize like this in Startup.cs (creds @ADefWebserver for mentioning this). Microsoft Docs

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

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.

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).

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 2.0.0.20242 Fixes a bug when working with file larger than 2Gb in InteropStream.Seek (#153)

Version 2.0.0.20200 ⚠️ Breaking changes: Changes Root Namespace from Blazor.FileReader to Tewr.Blazor.FileReader to avoid conflicts.

• CancellationToken can now be used in most relevant methods to cancel ongoing upload.
• Native support for displaying progress. See demo project for usage.