Why keep two variables for one content in C# UWP?

Question

In C#, I notice that many coders do something like the following:

class X
{
    private int test_;

    public int test
    {
        get { return test_; }
        set { if (test_ != value) test_ = value; }
    }
}

My question is why keep a private, and a public variable for the same content?

Why did we not do this instead?

class X
{
    public int test
    {
        get; set;
    }
}

I mean, we are changing the private variable within anyway. What is the point of not using a single public variable instead?

Answer 1

The code:

class X
{
    public int test
    {
        get; set;
    }
}

...is a direct equivalent for this:

class X
{
    private int test_;

    public int test
    {
        get { return test_; }
        set { test_ = value; }
    }
}

The first example is an auto-implemented property. The C# compiler automatically produces the second example when you compile.

Now, the code you presented first though had this line:

set { if (test_ != value) test_ = value; }

You'll notice that it's doing something different to the auto-property equivalent code. And that's where the difference lies in these two approaches.

When you use a backing field for your properties you can introduce specific rules that you need your code to follow.

For example, you might want to set the audio volume on a music app, so your code might be like this:

public class Music
{
    private int _volume = 7;

    public int Volume
    {
        get { return _volume; }
        set
        {
            if (value >= 0 && value <= 10)
            {
                _volume = value;
            }
        }
    }
}

It is common to have a private field variable when your property contains logic like this.

Answer 2

These are not two variables. One is a field and other one is a property. A property is optional getter and setter methods in disguise.

Your proposal of the solution is in fact part of the language, called auto-implemented properties: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/auto-implemented-properties