Preview an Image Before It Is Uploaded in Blazor Application

In this blog post, I will show you how to upload and preview the image in Blazor Web Application.
Blazor out of the box provides a component InputFile that wraps the HTML file input element and supplies a [Stream for each file’s contents.

Let’s understand this with an example. I have a razor page Upload.razor, in which I have added two tags, as shown below.

<InputFile OnChange="LoadImage" />

<img src="@ImageUri" />

Then in the C# code, I created a method LoadImage that accepts the argument InputFileChangeEventArgs. When the user selects the image, this method triggers and provides information about the file.
For example, in the below code, you can see that I am reading the file  Stream and then converting it to a base64 string so we can show it in the img tag.

 public async Task LoadImage(InputFileChangeEventArgs inputFileChangeEventArgs)
        var image = await inputFileChangeEventArgs.File.RequestImageFileAsync("image/png", 600, 600);

        using Stream imageStream = image.OpenReadStream(1024 * 1024 * 10);
        using MemoryStream ms = new();
        //copy imageStream to Memory stream
        await imageStream.CopyToAsync(ms);

        //convert stream to base64
        ImageUri = $"data:image/png;base64,{Convert.ToBase64String(ms.ToArray())}";


Things to remember

  • RequestImageFileAsync("image/png", 600, 600)
    Convert the current image file to a new one of the specified file type and maximum file dimensions. In our case is 600x600.
  • However, this method does not ensure the file’s conversion, either before or after the conversion process. Because the modification is requested in the browser before it is transferred to the .NET function, the resulting data should not be trusted.


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