Lambda expression common syntax

问题:

Can I follow any simple synax or rules for building "lambda expression" in C#? I read some articles and understood what a lambda expression is, but if I have the general syntax or rules that would be helpful.

回答1:

There are multiple ways of expressing lambdas, depending on the exact scenario - some examples:

    // simplest form; no types, no brackets
    Func<int, int> f1 = x => 2 * x;
    // optional exlicit argument brackets
    Func<int, int> f2 = (x) => 2 * x;
    // optional type specification when used with brackets
    Func<int, int> f3 = (int x) => 2 * x;
    // multiple arguments require brackets (types optional)
    Func<int, int, int> f4 = (x, y) => x * y;
    // multiple argument with explicit types
    Func<int, int, int> f5 = (int x, int y) => x * y;

The signature of the lambda must match the signature of the delegate used (whether it is explicit, like above, or implied by the context in things like .Select(cust => cust.Name)

You can use lambdas without arguments by using an empty expression list:

    // no arguments
    Func<int> f0 = () => 12;

Ideally, the expression on the right hand side is exactly that; a single expression. The compiler can convert this to either a delegate or an Expression tree:

    // expression tree
    Expression<Func<int, int, int>> f6 = (x, y) => x * y;

However; you can also use statement blocks, but this is then only usable as a delegate:

    // braces for a statement body
    Func<int, int, int> f7 = (x, y) => {
        int z = x * y;
        Console.WriteLine(z);
        return z;
    };

Note that even though the .NET 4.0 Expression trees support statement bodies, the C# 4.0 compiler doesn't do this for you, so you are still limited to simple Expression trees unless you do it "the hard way"; see my article on InfoQ for more information.

回答2:

A lambda expression is, fundamentally, a shorthand notation for a function pointer. More commonly, a lambda expression is the propagation of input data into an expression that computes a result. In most cases, you will use lambda expressions in their more common form:

int[] numbers = new[] { 1, 3, 11, 21, 9, 23, 7, 4, 18, 7, 7, 3, 21 };

var twentyoneCount = numbers.Where(n => n == 21).Count();
var sumOfgreaterThanSeven = numbers.Sum(n => n > 7 ? n : 0);

In their less common form, lambda expression can replace more cumbersome delegate forms:

myButton.Click += new EventHandler(myButton_Click);
// ...
void myButton_Click(object sender, EventArgs e)
{
    // TODO: Implement button click handler here
}

Or the less common, and less verbose:

myButton.Click += delegate(object sender, EventArgs e)
{
    // TODO: Implement button click handler here
};

The following lambda expression achieves the same result as the above two:

myButton.Click += (s,e) =>
{
    // TODO: Implement button click handler here
};

The power of this latter form really comes from its capability to create closures. A closure is where you implement a function within a function, and "close" around parameters and variables from the parent function scope:

private void DoSomething(IList<string> input, SomeObject source)
{
    source.OnSomeEvent += (s,e) => return input.Sum();
}