C# functions with multiple fat arrows

Question

I am having a hard time wrapping my head around this function with multiple fat arrows.

services.AddTransient<ServiceA>();
services.AddTransient<ServiceB>();
services.AddTransient<ServiceC>();

services.AddTransient<ServiceResolver>(serviceProvider => key =>
{
    switch (key)
    {
        case "A":
            return serviceProvider.GetService<ServiceA>();
        case "B":
            return serviceProvider.GetService<ServiceB>();
        case "C":
            return serviceProvider.GetService<ServiceC>();
        default:
            throw new KeyNotFoundException(); // or maybe return null, up to you
    }
});

In C# i have never come across this function style serviceProvider => key =>. Can anyone please explain to me what it means

Answer 1

This is an lambda expression returning another lambda expression.

Not sure what is ServiceResolver but I would say that it is a delegate looking something like this:

// IService - shared contract for ServiceA, ServiceB, ...
public delegate IService ServiceResolver(string key); 

Then the invocation services.AddTransient<ServiceResolver>(serviceProvider => ...) is a call to AddTransient method accepting a Func<T, TResult> delegate, which is constructed via lambda expression and returning another delegate constructed also via lambda expression (key => {...}).