Sort a list of Expressions in Optano C#

Question

Edit: To reword my opening explanation to hopefully make it clearer: "How can I write an expression that represents the mean of the lowest N expressions in an unordered list?" The list includes negative values.

TLDR: how do I make a list of Expressions that is sorted by the values to which each Expression would evaluate?

Full question: I'm using Optano with the Gurobi solver in C# (using VS2019) to minimise the sum of the lowest 5% of a set of weighted values (the weights being the variables added to the model).

To the find the lowest 5% I need to sort the values calculated by the model. One way or another, I need to be able to compare two values, but I can't work out how. If I have List the Sort() method doesn't work for obvious reasons.

Instead, because it's more efficient and is much simpler code than a sorting algorithm (which I have also tried to no avail, using selection sorting), I have a little foreach that finds the correct position in a list into which to insert the new value so that I end up with a sorted list.

Expression[] expressionPairForMin = new Expression[2];
expressionPairForMin[0] = weightedValueTemp; //The new Expression I want to insert into the sorted list.
foreach (Expression weightedValue in allWeightedValues)
{
    expressionPairForMin[1] = weightedValue;
    //if (weightedValueTemp < weightedValue) { break; } //What I want to do, which doesn't work of course.
    if (Expression.Equals(Expression.Minimum(expressionPairForMin), weightedValueTemp)) { break; } //One of several ideas I've tried.
    insertPosition++;
}
portfolioWeightedReturns.Insert(insertPosition, weightedValueTemp);

The if I have there is trying to say: if the minimum of the two Expressions is the same as the first Expression, then the first Expression must be the smaller of the two.

Later, I take the sum (using Expression.Sum) of the first n values (which is what's returned as an Expression for the Objective).

My code works if I use Expression.Evaluate on the objective with hardcoded weights as input variables and use the double values more directly, but not when I run the solver. It just doesn't sort the list in that case, as the if never returns true and so weightedValueTemp is always inserted at the end of the list.