Blazor Wasm - cannot convert from 'method group' to 'EventCallback'

Question

I have a simple Blazor Server app that works ok.

I'm trying to create an equivalent app using Blazor WebAssembly with dotnet 5.

In the working app, I have a razor component that enables the user to upload files using the BlazorFileUploader nuget package

Markup:

<InputFile multiple OnChange="HandleFileSelected"/>

Code behind:

public async Task HandleFileSelected(IFileListEntry[] files)
        {
            if (!ValidateFiles(files, out _errorMsg))
            {
                return;
            }
etc...

When I try to compile the wasm app, I get the following error

CS1503 Argument 2: cannot convert from 'method group' to 'EventCallback'

Any ideas what I'm doing wrong?

Answer 1

Jonas is correct; your method needs to accept InputFileChangeEventArgs instead of IFileListEntry[]. If you hover over OnChange in Visual Studio 2019, you should see something like this:

The event handler expects a delegate EventCallback that can accept up to 1 parameter of type InputFileChangeEventArgs. Think of a delegate as a method interface: your declaration of IFileListEntry[] does not match the interface, so you are getting an error.

If you inspect InputFileChangeEventArgs, you can see how to get the files:

public async Task HandleFileSelected(InputFileChangeEventArgs args)
{
    IReadOnlyList<Microsoft.AspNetCore.Components.Forms.IBrowserFile> files =
        args.GetMultipleFiles(args.FileCount);
    // ...

Answer 2

I dont think the OnChange eventcallback supports a parameter of type IFileListEntry[]

Try to change your code behind to:

public async Task HandleFileSelected(InputFileChangeEventArgs e)
{
    var files = e.GetMultipleFiles();
}