Как создать дополнительные методы с лямбда-выражениями

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;
    }

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;
            };
        }

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
    ); 

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
);

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.