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Blazor WebAssembly File Upload

In this article, we are going to explore file uploading in the Blazor WebAssembly application. From .Net5 onwards Blazor Assembly comes with an inbuilt file rendering blazor component like 'InputFile' Component.

Overview On InputFile Component:

  • InputFile blazor component renders an input field of type 'file' which by default supports single file upload. To support multiple file selection we need to add an attribute 'multiple' to the Html tag of the InputFile blazor component.
  • InputFile blazor component provides an event called 'OnChange'. This event gets invoked on every file selection. This 'OnChange' event has to be configured with a callback method in our blazor code block.
  • The callback method receives an argument of type 'InputFileChangeEventsArgs'. This 'InputFileChangeEventsArgs' captures all images and it information. This argument helps to read the stream of files.

Create A Sample Blazor WebAssembly:

Let's understand the file uploading steps by writing some sample code, so let's get started with the sample Blazor Assembly Application. Remember this InputFile component introduced from .Net5, so make sure your sample application created with .Net5. The IDE's for development can be chosen on your personal preference but most recommended are Visual Studio 2019 and Visual Studio Code.

Uploaded File Read And Render:

Let's understand reading the stream of the file and render the image for preview. Since our application is Blazor WebAssembly where all dotnet code runs inside of the browser sandbox.
Pages/Index.razor:(Html Part)
<div>
    <InputFile OnChange="@OnFileSelection"></InputFile>
    <div class="row">
        <img src="@imgUrl">
    </div>
</div>
  • (Line: 2) Rendered InputFile blazor component for file upload. The 'OnChange' event of the InputFile component configured with an 'OnFileSelection' call back method(.net method)
  • (Line: 4) The selected file is rendered to show the preview.
Pages/Index.razor:(Code Part)
@code{
    string imgUrl = string.Empty;
    private async Task OnFileSelection(InputFileChangeEventArgs e)
    {
        IBrowserFile imgFile = e.File;
        var buffers = new byte[imgFile.Size];
        await imgFile.OpenReadStream().ReadAsync(buffers);
        string imageType = imgFile.ContentType;
        imgUrl =$"data:{imageType};base64,{Convert.ToBase64String(buffers)}";
    }
}
  • (Line: 3) The 'OnFileSelection' method is the callback method registered with the 'OnChange' event of the InputFile blazor component. The 'InputFileChangeEventArgs' type passed parameter for 'OnChange' event.
  • (Line: 5) The 'InputFileChangeEventArgs' contains a property 'File' which returns the first image of the selection by the InputFile blazor Component. The 'File' property is of type 'Microsoft.AspNetCore.Components.Form.IBrowserFile'.
  • (Line: 6) Byte array was initialized to capture the byte data of the image stream in later steps. So the size of the byte array needs to be declared as the size of the uploaded file. The 'IBrowserFile' provides a property called 'Size' to determine the file size, this 'Size' value will be used for the byte array variable.
  • (Line: 7) The 'IBrowserFile' provides the method 'OpenReadStream' that gives the file as 'Stream' of data. Using the 'Stream' extension of ReadAsync to populate the byte data.
  • (Line: 8) The 'IBrowserFile' provides the 'ContentType' property to determine the upload file is jpg or png.
  • (Line: 9) The preview URL assigned with a base64 string image format.

Multiple Uploaded Files Read And Render:

Pages/Index.razor:(Html Part)
<div>
    <InputFile OnChange="@OnFileSelection" multiple></InputFile>
    <div class="row">
        @foreach (var imgUrl in imgUrls)
        {
            <img src="@imgUrl">
        }
    </div>
</div>
  • (Line: 2) The InputFile blazor component decorated with a 'multiple' attribute for multi-selection. of files to upload.
  • (Line: 4) The list images loop to show the preview.
Pages/Index.razor:(Code Part)
@code{
    List<string> imgUrls = new List<string>();
    private async Task OnFileSelection(InputFileChangeEventArgs e)
    {
        foreach (IBrowserFile imgFile in e.GetMultipleFiles(5))
        {
            var buffers = new byte[imgFile.Size];
            await imgFile.OpenReadStream().ReadAsync(buffers);
            string imageType = imgFile.ContentType;
            string imgUrl = $"data:{imageType};base64,{Convert.ToBase64String(buffers)}";
            imgUrls.Add(imgUrl);
        }
    }
}
  • (Line: 2) The images list container that will loop to show the preview.
  • (Line: 5) The 'InputFileChangeEventArgs' contains a method called 'GetMultipleFiles' which returns the collection of 'IBrowserFile'. The 'GetMultipleFiles' method has an optional parameter to fetch the number of files. By default, it returns the max number of 10 'IBrowserFile', here in our sample we are setting max to '5' files to read.

Create A .NetCore Web API To Save Files To Physical Location:

Create another sample .NetCore Web API application. Then we will create an endpoint to save the files to a physical location on the server. This endpoint will be consumed by our Blazor Assembly application.

Now create a payload object for our endpoint as below.
Models/SaveFile.cs:
using System.Collections.Generic;

namespace FileUploadSample.Api.Models
{
    public class SaveFile
    {
        public List<FileData> Files { get; set; }
    }

    public class FileData
    {
        public byte[] Data { get; set; }
        public string FileType { get; set; }
        public long Size { get; set; }
    }
}
  • (Line: 12) 'Data' property of type byte[] which holds the file as byte data.
  • (Line: 13) 'FileType' property holds the file info like 'png', 'jpeg', etc.
  • (Line: 14) 'Size' property holds the size of the file uploading.
Now write an endpoint that has an implementation of saving the file to the physical folder location.
Controllers/FileUploadController.cs:
using FileUploadSample.Api.Models;
using Microsoft.AspNetCore.Mvc;
using System;
using System.Threading.Tasks;

namespace FileUploadSample.Api.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class FileUploadController : ControllerBase
    {
        [HttpPost]
        [Route("save-file-to-physicallocation")]
        public async Task<IActionResult> SaveToPhysicalLocation([FromBody] SaveFile saveFile)
        {
            foreach (var file in saveFile.Files)
            {
                string fileExtenstion = file.FileType.ToLower().Contains("png") ? "png" : "jpg";
                string fileName = $@"D:\MyTest\{Guid.NewGuid()}.{fileExtenstion}";
                using(var fileStream = System.IO.File.Create(fileName))
                {
                    await fileStream.WriteAsync(file.Data);
                }
            }
            return Ok();
        }
    }
}
  • (Line: 16) looping the collection of file information.
  • (Line: 19) the full path of the file name. It is always recommended to save the files in non-application folders on the server.
  • (Line: 20) The 'System.IO.File.Create("image_path")'  generates the file stream at the specified path.
  • (Line: 22) Saving the files array of byte data to the specified physical path.
To consume this endpoint by our Blazor Application enable cors.
Startup.cs:
namespace FileUploadSample.Api
{
// existing code hidden for display purpose
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers();
            services.AddCors(options =>
            {
                options.AddPolicy(name: "MyAllowSpecificOrigins",
                                  builder =>
                                  {
                                      builder.AllowAnyOrigin();
                                      builder.AllowAnyHeader();
                                      builder.AllowAnyMethod();
                                  });
            });
        }
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {

            app.UseRouting();

            app.UseCors("MyAllowSpecificOrigins");
        }
    }
}
  • (Line: 11) registered cors policy.
  • (Line: 25) configure the cors middleware bypassing the registered cors policy name.

Integrate HttpClient Call In Our Blazor WebAssembly Application:

Add our web API domain to HttpClient configuration in Program.cs file.
Program.cs:
builder.Services.AddScoped(sp => new HttpClient { BaseAddress = new Uri("https://localhost:5002/") });
  • Replace the Url with our web API domain in HttpClient configurations
Add a model class in our Blazor WebAssembly application for API payload this class will be the exact match of our API model we have created in prior steps
Models/SaveFile.cs:
using System.Collections.Generic;
namespace BlazorWasmFileUpload.Models
{
    public class SaveFile
    {
        public List<FileData> Files { get; set; }
    }

    public class FileData
    {
        public byte[] Data { get; set; }
        public string FileType { get; set; }
        public long Size { get; set; }
    }
}
Pages/index.razor:(Html Part)
@page "/"
@using BlazorWasmFileUpload.Models;
@inject HttpClient Http

<div>
    <InputFile OnChange="@OnFileSelection" multiple></InputFile>
    <div class="row">
        @foreach (var imgUrl in imgUrls)
        {
            <img src="@imgUrl">
        }
    </div>
    <div>
        <button type="button" class="btn btn-primary" @onclick="SaveToServer" >Upload To Server</button>
    </div>
</div>
  • (Line: 3) Injected the HttpClient
  • (Line: 14) Added a new button trigger web API to post the file data. The 'SaveToServer' is a call back method of the button click
Pages/Index.razor:(Code Part)
@code{
    List<string> imgUrls = new List<string>();
    List<FileData> fileData = new List<FileData>();
    private async Task OnFileSelection(InputFileChangeEventArgs e)
    {
        foreach (IBrowserFile imgFile in e.GetMultipleFiles(5))
        {
            var buffers = new byte[imgFile.Size];
            await imgFile.OpenReadStream().ReadAsync(buffers);
            string imageType = imgFile.ContentType;
            string imgUrl = $"data:{imageType};base64,{Convert.ToBase64String(buffers)}";
            imgUrls.Add(imgUrl);
            fileData.Add(new FileData
            {
                Data = buffers,
                FileType = imageType,
                Size = imgFile.Size
            });
        }
    }

    private async Task SaveToServer()
    {
        if (fileData.Count > 0)
        {
            var payload = new SaveFile { Files = fileData };
            await Http.PostAsJsonAsync("/api/FileUpload/save-file-to-physicallocation", payload);
        }
    }
}
  • (Line: 3) Initialized collection of 'FileData' object to hold the selected files.
  • (Line: 13) Selected files info dumped into the collection of the 'FileData' object.
  • (Line: 22) The 'SaveToServer' is the callback method of the button we created above.
  • (Line: 26) Preparing the final payload to post.
  • (Line: 27) Invoked the HttpClient post method.



Wrapping Up:

Hopefully, I think this article delivered some useful information on the Blazor WebAssembly File Upload. I love to have your feedback, suggestions, and better techniques in the comment section below.

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