TaskAll

Article: https://habr.com/en/post/349352/

Using the library you can replace a code as follows:

Task<int> get1 = Get1();
Task<int> get2 = Get2();
Task<string> get3Str = Get3Str();
Task<int> get4 = Get4();

await Task.WhenAll(get1, get2, get3Str, get4);

var result = get1.Result + get2.Result + int.Parse(get3Str.Result) + get4.Result;

with

var result = await
   from val1  in Get1().AsParallel()
   from val2  in Get2().AsParallel()
   from val3S in Get3Str().AsParallel()
   from val4  in Get4().AsParallel()
   select val1 + val2 + int.Parse(val3S) + val4;

In case when an intermediate result is required you can use the library in the following way:

var result = await
   from one in Get1().AsParallel()

   let oneA = one

   from two in Get2().AsParallel()
   from freeStr in Get3Str().AsParallel()

   let free = int.Parse(freeStr)//Intermediate expr 

   from eight in  Add5(free).AsSequential()//Here all the previous results can be used

   from oneB in Get1().AsParallel()
   from twoA in Get2().AsParallel()
   
   from six in Add5(oneB).AsSequential()//Here all the previous results can be used 
   
   select one + oneA + two + int.Parse(freeStr) + free + eight + oneB + twoA + six;