Expression Lambda versus Statement Lambda

问题:

Fundamentally, is there any difference between a single-line expression lambda and a statement lambda? Take the following code, for example:

private delegate void MyDelegate();

protected static void Main()
{
    MyDelegate myDelegate1 = () => Console.WriteLine("Test 1");
    MyDelegate myDelegate2 = () => { Console.WriteLine("Test 2"); };

    myDelegate1();
    myDelegate2();

    Console.ReadKey();
}

While I prefer the first because I find the brackets to be ugly, is there anything different between the two (besides the obvious part about requiring brackets for multi-line statements)?

回答1:

You need statement lambda for multistatement lambdas. In addition statement lambdas are not supported by expression providers like LINQ to SQL. Before .NET 4.0 the .NET Framework did not have support for statement expression trees. This was added in 4.0 but as far as I know no provider uses it.

Action myDelegate1 = () => Console.WriteLine("Test 1");
Expression<Action> myExpression = () => { Console.WriteLine("Test 2") }; //compile error unless you remove the { }
myDelegate1();
Action myDelegate2 = myExpression.Compile();
myDelegate2();

Otherwise they are the same.

回答2:

Reflector to the rescue! The disassembled code looks like this:

private static void Main(string[] args)
{
    MyDelegate myDelegate1 = delegate {
        Console.WriteLine("Test 1");
    };
    MyDelegate myDelegate2 = delegate {
        Console.WriteLine("Test 2");
    };
    myDelegate1();
    myDelegate2();
    Console.ReadKey();
}

So no, there is no real difference between the two. Be happy.

回答3:

The two are the same - the first is syntactic sugar to the second and both will compile to the same IL.

回答4:

No, there is no difference in this example. If the body of the lambda is only one expression, you can drop the brackets. However, once the lambda contains more than one expression, like so:

MyDelegate myDelegate2 = () => { 
  Console.WriteLine("Test 2");                      
  Console.WriteLine("Test 2"); 
};

the brackets are mandatory.

回答5:

If the delegate returns a value, return is necessary in statement lambda as follows.

        Func<int, int, bool> foo = (x, y) => { return x == y; };
        Func<int, int, bool> goo = (x, y) => x == y;

回答6:

Personnaly, i prefer the Lambda Expression. The Expression have a value where the Statement does not.

i think the following links can help you :

http://lambda-the-ultimate.org/node/1044

回答7:

Same for the OP example, but after C# 6.0, allowing you to use same expression syntax to define normal non-lambda methods within a class. For example:

public static double AreaOfTriangle(double itsbase, double itsheight)
{
    return itsbase * itsheight / 2;
}

The above code snippet can be written only if the method can be turned into a single expression. In short, it can be used the expression lambda syntax, but not the statement lambda syntax.

public static double 
              AreaOfTrianglex(double itsbase, double itsheight) => itsbase * itsheight / 2;