Preload Script is Not Loaded

[AeonSake]

I'm trying to expose certain utility functions to the renderer process. Electron advises using a preload script and expose the relevant functions via the contextBridge. Unfortunately, it seems the preload script is not called when specifying the file path via the BrowserWindowOptions.WebPreferences.Preload option during window creation. Furthermore, it seems the preload file is not checked at all because even proving an invalid file path won't lead to an exception (which I somewhat expected in that scenario).

• NuGet Version: 0.4.0
• Electron Version: 39.2.6
• Electron Builder Version: 26.0.12
• Framework: net10.0
• Target: win-x64 (specified via publish profile)
• IDE: Visual Studio 2026 18.1.1

Steps to Reproduce:

1. Create a new Blazor Web App, targeting net10.0
2. Add ElectronNET.Core and ElectronNET.Core.AspNet version 0.4.0 as NuGet packages
3. Add builder.UseElectron(args, ElectronAppReady); to the builder call and the following method for creating the window:

private static async Task ElectronAppReady()
{
    var options = new BrowserWindowOptions
    {
        Show = false,
        IsRunningBlazor = true,
        WebPreferences = new WebPreferences
        {
             Preload = Path.Combine(AppContext.BaseDirectory, "preload.js")
        }
    };
    var browserWindow = await Electron.WindowManager.CreateWindowAsync(options);
    browserWindow.OnReadyToShow += () => browserWindow.Show();
}

4. Create a file called preload.js in the root directory of the project with the following content (example from the Electron docs):

const { contextBridge, webUtils } = require('electron');

contextBridge.exposeInMainWorld('electronApi', {
    getPathForFile: (file) => {
        return webUtils.getPathForFile(file);
    }
});

5. Add the following to the project configuration (.csproj file) to ensure the preload script is copied to the output directory:

<ItemGroup>
    <Content Include="preload.js" CopyToOutputDirectory="PreserveNewest" />
</ItemGroup>

6. Run the application and try to access window.electronApi.getPathForFile in JavaScript

[AeonSake]

Looking into the bridge API, it seems the preload path is overwritten when using Blazor:

Electron.NET/src/ElectronNET.Host/api/browserWindows.ts

Lines 222 to 229 in 456135a

	if (options.isRunningBlazor) {
	options.webPreferences["preload"] = path.join(
	__dirname,
	"..",
	"scripts",
	"blazor-preload.js"
	);
	}

First of all, I think this should be documented somewhere, ideally in the Preload property itself and probably in the IsRunningBlazor property. But more importantly: how can we still have custom preload scripts when running Blazor?

[AeonSake]

Thinking about this further, I would much prefer to have a user-specified preload script always load and only fall back to the blazor-preload.js if no value is set. The special script only contains the two lines:

global.process = undefined;
global.module = undefined;

I think people are capable to add those to their custom preload script when the documentation has a warning for it (when using Blazor and a preload script). I'm really not a fan of silently ignoring user-specified options. I would prefer the framework to provide sensible defaults but give the user the option to shoot themselves in the foot if they wish (or know what they are doing).

[AeonSake]

Just for completeness or for anyone with the same problem: the current workaround is setting BrowserWindowOptions.IsRunningBlazor to false (or removing it) and adding the two lines from above to the custom preload script. Naturally, this only affects applications using Blazor.

[FlorianRappl]

Well, honestly, the ideal world is simple:

1. Blazor does not do weird defaults and checks that make no sense to go into a mode that anyway does not have any reason to exist; so the preload script would not be necessary
2. If what you wrote is true then IsRunningBlazor would not be needed anyway - however, many people (not only in this / .NET community, but also in plain Electron world) would not struggle to identify why two lines like that would be needed

So long story short: Just don't opt-into IsRunningBlazor if you want to have full control.

From this issue I see a documentation enhancement - nothing more.

[AeonSake]

Yea, I get that too. It's just a question of which option property should have priority; either IsRunningBlazor or Preload. From my standpoint, I would have assumed to give it to Preload as this is a native option property of electron, rather than the admittedly hack for Blazor. I don't mind it either way though.