BackgroundService that doesn't call base.StartAsync leads to NullReferenceException - .NET 6 breaking change

[davidni]

Description

.NET 6.0 introduced an undocumented breaking change. IHostedService startup is now special-cased for BackgroundService, where the Host takes a direct dependency on BackgroundService.ExecuteTask, which can be null in some cases that previously worked.

runtime/src/libraries/Microsoft.Extensions.Hosting/src/Internal/Host.cs

Lines 80 to 84 in 6195d98

	private async Task TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)
	{
	try
	{
	await backgroundService.ExecuteTask.ConfigureAwait(false);

When BackgroundService.ExecuteTask is null (e.g. because the override for BackgroundService.StartAsync decided not to call the base impl), this throws a NullReferenceException and causes the Host to abort (instead of previous behavior where IHostedService.StartAsync was called and that was it.

Example BackgroundService impl that breaks

class MyBackgroundService : BackgroundService
{
    public override Task StartAsync(CancellationToken cancellation)
    {
        return Task.CompletedTask;
    }

    protected override Task ExecuteAsync(CancellationToken stoppingToken)
    {
        return Task.CompletedTask;
    }
}

Configuration

• Which version of .NET is the code running on? .NET 6.0.100-rc.1.21463.6
• What OS and version, and what distro if applicable? Windows 10, but issue is not platform-specific
• What is the architecture (x64, x86, ARM, ARM64)? x64, but issue is not platform-specific
• Do you know whether it is specific to that configuration? Not platform-specific

Regression?

Yes, this is a breaking change in .NET 6. Previous versions did not special-case BackgroundService, and instead treated all IHostedService's uniformly. The new behavior IMHO violates the Principle of Least Astonishment.

Other information

Breaking change was introduced in #42981

Expectations for this issue

Either fix the regression, or document it as a breaking change in https://docs.microsoft.com/en-us/dotnet/core/compatibility/6.0

[ghost]

Tagging subscribers to this area: @eerhardt, @maryamariyan
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Issue Details

---

Description

.NET 6.0 introduced a =n undocumented breaking change. IHostedService startup is now special-cased for BackgroundService, where the Host takes a direct dependency on BackgroundService.ExecuteTask, which can be null in some cases that previously worked.

runtime/src/libraries/Microsoft.Extensions.Hosting/src/Internal/Host.cs

Lines 80 to 84 in 6195d98

	private async Task TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)
	{
	try
	{
	await backgroundService.ExecuteTask.ConfigureAwait(false);

When BackgroundService.ExecuteTask is null (e.g. because the override for BackgroundService.StartAsync decided not to call the base impl), this throws a NullReferenceException and causes the Host to abort (instead of previous behavior where IHostedService.StartAsync was called and that was it.

Example BackgroundService impl that breaks

class MyBackgroundService : BackgroundService
{
    public override Task StartAsync(CancellationToken cancellation)
    {
        return Task.CompletedTask;
    }

    protected override Task ExecuteAsync(CancellationToken stoppingToken)
    {
        return Task.CompletedTask;
    }
}

Configuration

• Which version of .NET is the code running on? .NET 6.0.100-rc.1.21463.6
• What OS and version, and what distro if applicable? Windows 10, but issue is not platform-specific
• What is the architecture (x64, x86, ARM, ARM64)? x64, but issue is not platform-specific
• Do you know whether it is specific to that configuration? Not platform-specific

Regression?

Yes, this is a breaking change in .NET 6. Previous versions did not special-case BackgroundService, and instead treated all IHostedService's unifrmly. The new behavior IMHO violates the Principle of Least Astonishment.

Other information

Breaking change was introduced in #42981

Expectations for this issue

Either fix the regression, or document it as a breaking change in https://docs.microsoft.com/en-us/dotnet/core/compatibility/6.0

Author:	davidni
Assignees:	-
Labels:	untriaged, area-Extensions-Hosting
Milestone:	-

[eerhardt]

I think we can fix the NRE in this scenario for 6.0.

[eerhardt]

Re-opening to backport to 6.0.

[eerhardt]

Fixed in 6.0 with #60177.