如何使用表达式树类型

Question

The MSDN documentation on Lamba Expressions provides an example of how to create an expression tree type but it does not show how to use it:

using System.Linq.Expressions;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Expression<del> myET = x => x * x;
        }
    }
}

Can you complete this console application code so that it actually demonstrates the concept?

Answer 1

In general Expression Trees contain two parts. A set of parameters and a body. There is only one parameter shown in your example, which is x and the body uses that parameter by multiplying it by itself.

Essentially the behavior setup is something like this:

public int myET(int x)
{
    return x * x;
}

However, in order to access that behavior the value of the Expression must be accessed. This value is a delegate, and is accessed through compiling the expression using .Compile() . Its type will be a Func with the type parameters of your del delegate which were returning int and accepting int .

delegate int del(int i);
static void Main(string[] args)
{
    Expression<del> myET = x => x * x;
    del myFunc = myET.Compile();
}

Once compiled, the function may be called just like the method shown above with the behavior, where a value for the parameter is sent in, and the result of the code in the body is returned.

delegate int del(int i);
static void Main(string[] args)
{
    Expression<del> myET = x => x * x;
    del myFunc = myET.Compile();
    int fiveSquared = myFunc(5);
    Console.WriteLine(fiveSquared);//25
}