"Keyword 'this' is not valid in a static property, static method, or static field initializer" when adding methods to an ExpandoObject

Question

I am try to add a dynamic method to ExpandoObject which would return the properties (added dynamically) to it, however it's always giving me error.

Is something wrong I am doing here?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Dynamic;

namespace DynamicDemo
{
class ExpandoFun
{
    public static void Main()
    {
        Console.WriteLine("Fun with Expandos...");
        dynamic student = new ExpandoObject();
        student.FirstName = "John";
        student.LastName = "Doe";
        student.Introduction=new Action(()=>
      Console.WriteLine("Hello my name is {0} {1}",this.FirstName,this.LastName);
    );

        Console.WriteLine(student.FirstName);
        student.Introduction();
    }
}
}

The Compiler is flagging the following error: Error 1

Keyword 'this' is not valid in a static property, static method, or static field initializer

D:\rnd\GettingStarted\DynamicDemo\ExpandoFun.cs 20 63 DynamicDemo

Solution 1

Well, you're using this in the lambda, which would refer to the object that is creating the Action. You cannot do that because you're in a static method.

Even if you were in an instance method, it would not work with this because it would refer to the instance of the object creating the Action, not the ExpandoObject where you're tucking it.

You need to reference the ExpandoObject (student):

student.Introduction=new Action(()=>
    Console.WriteLine("Hello my name is {0} {1}",student.FirstName,student.LastName);
);

Solution 2

There's no "this" available to you.

Capture the object when creating the lambda instead:

student.Introduction = new Action( ()=> Console.WriteLine("Hello my name is {0} {1}", student.FirstName, student.LastName) );

Then it works.

Solution 3

You can't use this keyword in the action, because here this refers to the current instance(not student), which causes the compile error because the code is in a static method. Check this:

dynamic student = new ExpandoObject();
student.FirstName = "John";
student.LastName = "Doe";
student.Introduction = new Action(() => Console.WriteLine("Hello my name is {0} {1}", student.FirstName, student.LastName));
Console.WriteLine(student.FirstName);
student.Introduction();
student.FirstName = "changed";
Console.WriteLine(student.FirstName);
student.Introduction();

Output:

John Doe
Hello my name is John Doe
changed Doe
Hello my name is changed Doe

Comments

[humblecoder]

One more question? Then how can I add a factory method to the ExpandObject which would return my a dynamic student?

[R. Martinho Fernandes]

@humblecoder: Do you mean this student.Get = new Func<dynamic>(() => student)?

[humblecoder]

Hi Fernandes!I wanted to create dynamic object (an Active Record) which would have properties (read from the database table columns) and methods (dynamic FindByXXX) where XXX is the column name, similar to NHibernate finders. What's the best approach?

[R. Martinho Fernandes]

@humblecoder: I guess that would warrant a whole other question.

[humblecoder]

Assume there is a class called student: class Student { public int ID { get; set; } public string FirstName { get; set; } public string LastName { get; set; } } I want to dynamically add the following methods:FindById(),FindByFirstName() and FindByLastName() like this: dynamic student = typeof(Student); //dynamic student = new Student(); student.FindByFirstName = new Action(() => { Console.WriteLine("I am a lookup method and search the table by FirstName"); });

[humblecoder]

and use in this way: dynamic martin = new Student(); martin.FirstName = "Martinho"; martin.LastName = "Fernandes"; martin.FindByFirstName();