如何使用lambda表达式创建扩展方法

Question

Currently i am creating an extension method that accepts parameters. Using the below example, how could one convert this using lambda expressions?

public static decimal ChangePercentage(this IEnumerable<Trade> trades, DateTime startDate, DateTime endDate)
{
    var query = from trade in trades
                where trade.TradeTime >= startDate
                where trade.TradeTime <= endDate
                orderby trade.TradeTime descending
                select trade;
    return (query.First().Value - query.Last().Value) / query.First().Value * 100;
}

What are the pro/cons using lambda vs normal method parameters?

Thanks

Answer 1

One way you could change the sample to use lambda expressions is to use a filter.

public static decimal ChangePercentage(this IEnumerable<Trade> trades, 
                                       Func<Trade,bool> pred)
        {
            var query = from trade in trades
                        where pred(trade);
                        orderby trade.TradeTime descending
                        select trade;
            return (query.First().Value - query.Last().Value) / query.First().Value * 100;
        }

    someTrades.ChangePercentage(x => x.TradeDate >= startDate && x.TradeTime <= endDate);

The biggest pro this gives you is flexbility. Instead of having a method which does date based filtering for calculation. You have a method with a flexible filter method for calculating percentages.

Answer 2

Did you want to replace the startDate and endDate parameters with a single lambda expression?

public static decimal ChangePercentage(this IEnumerable<Trade> trades, DateTime startDate, DateTime endDate)
{
    return trades.ChangePercentage(trade => trade.TradeTime >= startDate 
        && trade.TradeTime <= endDate);
}

public static decimal ChangePercentage(this IEnumerable<Trade> trades, Func<Trade, bool> filter)
    {
        var query = from trade in trades
                    where filter(trade)
                    orderby trade.TradeTime descending
                    select trade;
        return (query.First().Value - query.Last().Value) / query.First().Value * 100;
    }

Answer 3

Your method is implicitly using lambda expressions already.

When you say

trade.TradeTime >= startDate

What you're really saying is "given a Trade called " trade ", return a bool by evaluating the following: trade.TradeTime >= startDate ."

That is the definition of this lambda expression:

Func<Trade, bool> expr = (trade => trade.TradeTime >= startDate);

And in fact, minus the declaration of expr , this is how you would express it if you were using the function composition syntax for LINQ instead of the query syntax.

Answer 4

If you don't want parameters, you can move the filtering outside.

public static decimal ChangePercentage(this IEnumerable<Trade> trades)
{
  var query = trades.OrderByDescending(t => t.TradeTime);

  if (query.Any())
    return (query.First().Value - query.Last().Value) / query.First().Value * 100;
  else
    return 0;
}

Then, it can be called like this:

DateTime startDate, DateTime endDate

decimal answer = ChangePercentage
(
  from trade in trades
  where trade.TradeTime >= startDate
  where trade.TradeTime <= endDate
  select trade
);

Answer 5

Continuing on Tim's answer , you could also provide a lambda to perform the calculation:

    public static decimal ChangePercentage(
        this IEnumerable<Trade> trades, 
        Func<Trade, bool> filter, 
        Func<Trade, Trade, decimal> calc)
    {
        var query = from trade in trades
                    where filter(trade)
                    orderby trade.TradeTime descending
                    select trade;
        return calc(query.First(), query.Last());
    }

Usage:

    trades.ChangePercentage(
        trade => (trade.TradeTime >= startDate && trade.TradeTime <= endDate), 
        (t1, t2) => (t1.Value - t2.Value) / t1.Value * 100
    ); 

Answer 6

It's important to understand that Lambda expressions serve a different purpose than extension methods. Lambda expressions are used primarily as a compact syntax for defining a delegate implementation or function implementaion. An added benefit of lambda expressions is that you can define event handlers and functions within the body of another function, useful if you have a simple function that is used only within a specific method. Just define the function using the Func<> or Action<> type with lamda syntax.

I would recommend picking up a copy of Jon Skeet's C# In Depth. It covers these topics in detail.

Here's this function as a lambda expression

private void Form1_Load(object sender, EventArgs e)
        {
            //signature of our function
            Func<IEnumerable<Trade>, DateTime, DateTime, decimal> changePercentage = null;

            //function implemented using lambda expression syntax
            changePercentage += (trades, startDate, endDate) => 
            {
                var query = from trade in trades
                            where trade.TradeTime >= startDate
                            where trade.TradeTime <= endDate
                            orderby trade.TradeTime
                            descending
                            select trade;
                return (query.First().Value - query.Last().Value) / query.First().Value * 100;
            };
        }