Why does the backing field of my property not change its value?

Question

namespace tutor4
{
    class Class1
    {
        int _num = 2;
        public int num
        {
            get
            {
                return _num;
            }         


        

Answer 1

Your setter for num property is wrong.

It should not be

set
{
     _num = num;
}

because in this case it does nothing (sets _num back to its value since getter for num returns _num so this line is equivalent to _num = _num)

It should be

set
{
     _num = value;
}

MSDN explanation about value keyword:

The contextual keyword value is used in the set accessor in ordinary property declarations. It is similar to an input parameter on a method. The word value references the value that client code is attempting to assign to the property

Also note: your num property is just simple wrapper of _num field of class. If you don't need some complex logic in getter and setter for this property - you can change it to auto-implemented property like this:

class Class1
{
    public int num { get; set;}

    public Class1
    {
        num = 2;
    }
}

Until C# version 6 you should assign default value to auto-implemented property in class constructor.

In C# version 6 (not yet released, should be available this summer) you will be able to assign default value to auto-implemented property in declaration:

public int num { get; set;} = 2;