c# build a list of tasks before executing

Well, another approach, (which I find very direct)

        var list = new List<Task>();
        for (var i = 0; i < 10; ++i)
        {
            var i2 = i;
            var t = new Task(() =>
                {
                    Thread.Sleep(100);
                    Console.WriteLine(i2);
                });
            list.Add(t);
            t.Start();
        }

        Task.WaitAll(list.ToArray());

Instead of using Task.Factory.StartNew to create the tasks (the clue is in the name), instead just create them by using new Task(...) with your lambdas, then simply use taskName.Start() inside the condition you want to begin them.

You can create an array of Action based on a flag, and then use Parallel.Invoke() to run in parallel all the actions in the array and wait for them to finish.

You can use lambdas for the actions which will allow you to assign their return values to a local variable, if you want.

Here's a complete compilable example. Try it with getFlag() returning true and again with it returning false:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    sealed class Program
    {
        void run()
        {
            bool flag = getFlag();
            var results = new string[5];
            Action[] actions;

            if (flag)
            {
                actions = new Action[]
                {
                    () => results[0] = function("f1"),
                    () => results[1] = function("f2"),
                    () => results[2] = function("f3")
                };
            }
            else
            {
                actions = new Action[]
                {
                    () => results[3] = function("f4"),
                    () => results[4] = function("f5")
                };
            }

            Parallel.Invoke(actions); // No tasks are run until you call this.

            for (int i = 0; i < results.Length; ++i)
                Console.WriteLine("Result {0} = {1}", i, results[i]);
        }

        private bool getFlag()
        {
            return true; // Also try with this returning false.
        }

        string function(string param)
        {
            Thread.Sleep(100); // Simulate work.
            return param;
        }

        static void Main(string[] args)
        {
            new Program().run();
        }
    }
}