What is the C# equivalent to Promise.all?

That you are looking for is Task.WhenAll. You should create as many tasks as the multiple locations from which you want to fetch your data and then feed them in this method.

To expand on @Christos's accepted answer:

Task.WhenAll appears to be about as close as you will get for a drop-in replacement for Promise.all. I actually found it to be closer than I initially thought. Here's an example using a JavaScript Promise.all implementation that you may want to replicate in C#:

const [ resolvedPromiseOne, resolvedPromiseTwo ] = await Promise.all([ taskOne, taskTwo ]);

In C# you can do something very similar with Task.WhenAll (assuming they return the same types).

var taskList = new[]
{
  SomeTask(),
  AnotherTask()
};

var completedTasks = await Task.WhenAll(taskList);

// then access them like you would any array
var someTask = completedTasks[0];
var anotherTask = completedTasks[1];

// or just iterate over the array
foreach (var task in completedTasks)
{
  doSomething(task);
}

This assumes they're both in async methods / functions.