Holes in my dependency injection knowledge: IMultiTenantContextAccessor as a Singleton

[SmagPie]

Hello everyone. I have Finbuckle up and running in my app and it works, but I just want to understand how it works.

I see that on an incoming request the middleware pulls the tenantId off the claim (at least in my case). What I don't understand is how the dependancy injection stack is working.

services.AddScoped<T>(sp => sp.GetRequiredService<IMultiTenantContextAccessor<T>>().MultiTenantContext?.TenantInfo!); services.AddScoped<ITenantInfo>(sp => sp.GetService<T>()!); services.AddSingleton<IMultiTenantContextAccessor<T>, AsyncLocalMultiTenantContextAccessor<T>>(); **services.AddSingleton<IMultiTenantContextAccessor>(sp => (IMultiTenantContextAccessor)sp.GetRequiredService<IMultiTenantContextAccessor<T>>());**
Because IMultiTenantContextAccessor is a singleton, would there be crosstalk between simultaneous sessions trying to set and resolve the ITenantInfo

How is the middleware getting a scoped version of the IMultiTenantContextAccessor service to set the MultiTenantContext on ?

public async Task Invoke(HttpContext context) { var accessor = context.RequestServices.GetRequiredService<IMultiTenantContextAccessor>(); if (accessor.MultiTenantContext == null) { var resolver = context.RequestServices.GetRequiredService<ITenantResolver>(); var multiTenantContext = await resolver.ResolveAsync(context); accessor.MultiTenantContext = multiTenantContext; } await next(context); }

I am sure it is sitting right in front of my face, just not seeing it. Thanks for any enlightenment.

[AndrewTriesToCode]

Hi, you ask a very good question.

The answer, currently, is that the accessor implementation uses an AsyncLocal instance.

AsyncLocals have some special behavior whenever an async/await happens so that each new continuation gets its own copy of the instance from it's parent. So in the middleware pipeline pretty much immediately triggers an async/await so there is no conflict for tenants.

Some downsides exist though. Performance is impacted. Also any code in the async/await stack "higher" than the multitenant middleware will not see the tenants on the return trip through the pipeline.

I've also toyed with an implementation that instead uses a scoped DI variable or the HttpContext property bag.

I welcome any recommendations. For non ASP.NET Core scenarios the current implementation isn't great.

[SmagPie]

Thank you so much for the explanation! I thought I was going crazy, I will look more into this AsyncLocal, its not something I have used before. It is all working great so far, just wanted to fill in the knowledge gap.