return tuple result in async method .net core

Question

I have an async method. This method get list of 10 row from database and getting total count of query result. Because I'm using for grid pagination. So, I'm using tuple which one element is List and other element is count of total number. But I can't return result because of compilation error. How i can do this?

public async Task<Tuple<List<IdNamePair>, int> GetStudents(QueryFilter queryObject)
{
    var query = studentEntity.Select(p => new IdNamePair
    {
        ID = p.ID.ToString(),
        Name = p.StudentNameSurname
    }).ToListAsync();

    int totalCount = await query.CountAsync();

    query = query.ApplyPaging(queryObject);//like Skip(20).Take(10)

    var students = query.ToListAsync();

    return await new Tuple<List<IdNamePair>, int>(students, totalCount); //ERROR
}

Answer 1

ToListAsync returns task, remove it to return IQueryable

public async Task<Tuple<List<IdNamePair>, int>> GetStudents(QueryFilter queryObject)
{
    var query = studentEntity.Select(p => new IdNamePair
    {
        ID = p.ID.ToString(),
        Name = p.StudentNameSurname
    });

    int totalCount = await query.CountAsync();

    var students = await query.ApplyPaging(queryObject).ToListAsync();

    return new Tuple<List<IdNamePair>, int>(students, totalCount); 
}

Answer 2

You have some minor errors here.

First, the return type, the int is declared outside of the tuple. Add it inside.

Then, you have mixed the await keyword. Use it where you want to wait for async operations to finish.

Try it like this

public async Task<Tuple<List<IdNamePair>, int>> GetStudents(QueryFilter queryObject)
{
    var query = studentEntity.Select(p => new IdNamePair
    {
        ID = p.ID.ToString(),
        Name = p.StudentNameSurname
    });

    int totalCount = await query.CountAsync();

    query = query.ApplyPaging(queryObject);//like Skip(20).Take(10)

    var students = await query.ToListAsync();

    return new Tuple<List<IdNamePair>, int>(students, totalCount);
}

Extra 1: You could also use the shorter tuple declaration here

public async Task<(List<IdNamePair>, int)> GetStudents()
{
    ...
    return (students, totalCount);
}

Extra 2: In C# 7, You could also name the tuple items which brings more clarity to the consumer of the method

public async Task<(List<IdNamePair> Students, int TotalCount)> GetStudents()
{
    ...
    return (students, totalCount);
}

Answer 3

You have used await keyword on wrong statement. This keyword is used with asynchronous method calls only.

Sample structure for async and await with given return type -

public async Task<Tuple<List<string>, int>> GetTupleResultAsync()
{
        List<string> listd = new List<string>() { "A", "A", "D", "E" };

        //Test code to await
        await Task.Run(() => "");

        return new Tuple<List<string>, int>(listd, listd.Count);
}

Try below modified code -

public async Task<Tuple<List<IdNamePair>>, int> GetStudents(QueryFilter queryObject)
{
  var query = studentEntity.Select(p => new IdNamePair
  {
    ID = p.ID.ToString(),
    Name = p.StudentNameSurname
  });

  int totalCount = await query.CountAsync();

  query = query.ApplyPaging(queryObject);//like Skip(20).Take(10)

  var students = await query.ToListAsync();

  return new Tuple<List<IdNamePair>, int>(students, totalCount); //ERROR
}