C#7: How to use tuples in generic methods (LINQ select example)-CodePudding

I have some heavily repeating code, which has always the same structure, just using different columns in a database for accessing it and doing similar stuff

A typical query looks like:

var portfolioIds = context.PortSelMotorSeries
                    .Select(x => new { x.Id, x.InstallationAltitudeMax })
                    .ToList();

Now I want to use a generic function for dependency inversion and to pass the selector function as a delegate to the query:

private void ForEachIterate<T1>(Func<MotorSeriesDb, T1> selectorFunc): where T1 : (int Id, double Value)
{
  ...
  var portfolioIds = context.PortSelMotorSeries
                        .Select(selectorFunct)
                        .ToList();
  ...
}

So that I can call the query with my own selector:

ForEachIterate(x => new { Id = x.Id, Value = x.InstallationAltitudeMax });
ForEachIterate(x => new { Id = x.Id, Value = x.TemperatureMax });

Specifying the constraint with "where T1 : (int Id, double Value)" leads to a compiler error CS0701.

Leaving it away leads to other compiler errors.

Is there any way to use tuples in generic functions?

CodePudding user response：

For one thing you're confusing tuples (the (int Id, double Value) thing) with anonymous classes (the new { Id = x.Id, Value = x.TemperatureMax }). They aren't even related, so your code would never work as is.

For another, if all you want is to force the user to output a tuple of some specific type, you can do something like this:

private void ForEachIterate(Func<MotorSeriesDb, (int, double)> selectorFunc)
{
  ...
  var portfolioIds = context.PortSelMotorSeries
                        .Select(selectorFunct)
                        .ToList();
  ...
}

// call like:
ForEachIterate(x => (x.Id, x.InstallationAltitudeMax));

Note that there's nothing generic about your function at all. Which leads me to my third point: you're missing the entire point of Linq. You talk about inversion of control, but you're the one who's inverting it in the wrong direction to begin with.

You already have a construct that allows arbitrary selection: context.PortSelMotorSeries. Simply use Linq to select what you want out of it in the call site and you're done.

CodePudding user response：

If you try this it could works:

private static void ForEachIterate<T1>(Func<MotorSeriesDb, T1> selectorFunc) where T1 : Tuple<int, double>
{
    var portfolioIds = context.PortSelMotorSeries
                .Select(selectorFunct)
                .ToList();
}