Set a read only property defined in a interface within a concrete class

Question

I have an interface with a read only property

public interface IPerson
{
    string Name { get; }
}

and a concrete class...

public class Person : IPerson
{

    public Person()
    {
        Name = "Person";
    }

    public string Name
    {
        get
        {
            return Name;
        }
    }
}

I want Name to be read only externally to this class, but how do I set it from within the concrete class?

Error: Person.Name cannot be assigned to.

How do I set the value of this property from within Person class?

Answer 1

This doesn't have anything to do with the interface, you're just declaring the property incorrectly. In C# 6, you can create a read-only property like this:

public class Person : IPerson
{
    public Person()
    {
        Name = "Person";
    }

    public string Name { get; }
}

In earlier versions, you can use a read-only backing field which you can set:

public class Person : IPerson
{
    private readonly string _name;

    public Person()
    {
        _name = "Person";
    }

    public string Name
    {
        get { return _name; }
    }
}

Note that the interface only requires the property has a getter, the implementation doesn't have to be read-only. You could add a setter if you had reason to modify the value:

public class Person : IPerson
{
    public Person()
    {
        Name = "Person";
    }

    public string Name { get; set; }
}

The setter could be private if you only needed to be able to change the value from within the class.

Answer 2

You can use a private property to hold the value.

public class Person : IPerson
{
    private string _name;
    public Person()
    {
        _name = "Person";
    }

    public string Name
    {
        get
        {
            return _name;
        }
    }
}

Answer 3

Right now, you're trying to read the property by reading the property. Needless to say, this will result in an endless loop. Instead, you either need to use full-blown auto-properties, or a manual backing field.

public class Person : IPerson
{
    public Person()
    {
        Name = "Person";
    }

    public string Name { get; private set; }
}

Answer 4

You simply have a private setter (pre c#6):

public class Person : IPerson
{

    public Person()
    {
        Name = "Person";
    }

    public string Name { get; private set; }

}

Answer 5

Try using accessors:

private string _name;

public string Name
{
    get
    {
        return _name;
    }
}

You can then set the value of _name inside any method or the constructor.

Answer 6

I want Name to be read only externally to this class, but how do I set it from within the concrete class

First of all, let's realize that: an interface only describes a set of public requirements; it does not prevent us from implementing other public members, nor does it limit our ability to create private members.

Therefore, to make a property writeable from inside the class but read-only elsewhere, we can declare the set method with private scope:

public class Person : IPerson
{
    // Name is read-only outside the class, but can be set internally
    public string Name { get; private set; } = "DefaultName";

    public Person() { }

    public Person(string name)
    {
        // Potentially do some validation before setting the name
        if (!IsValidName(name)) 
            throw new ArgumentException("Name cannot be null, empty, or whitespace.");

        Name = name;
    }

    private bool IsValidName(string name)
    {
        return !string.IsNullOrWhitespace(name);
    }

    // Continued below...

And if we want to, we can give access to the private setter through a method. This is often done in order to make the renaming of a person very intentional (for example, it prevents accidental assignments when a comparison was intended). The client will get a design-time error from the compiler if they try person.Name = "Foo"; but they can write person.RenameAs("Foo");

    public void RenameAs(string newName)
    {
        // Potentially do some validation before setting the name
        if (IsValidName(newName)) Name = newName;
    }
}