在自动属性中访问支持字段

Question

Is there any way to access the backing field for a property in order to do validation, change tracking etc.?

Is something like the following possible? If not is there any plans to have it in .NET 4 / C# 4?

public string Name
{
    get;
    set
    {
        if (value != <Keyword>)
        {
            RaiseEvent();
        }
        <Keyword> = value;
    }
}

The main issue I have is that using auto properties doesn't allow for the same flexibility in validation etc. that a property with a explicit backing field does. However an explicit backing field has the disadvantage in some situations of allowing the class it is contained in to access the backing field when it should be accessing and reusing the validation, change tracking etc. of the property just like any other class that may be accessing the property externally.

In the example above access to the backing field would be scoped to the property thus preventing circumvention of the property validation, change tracking etc.

Edit: I've changed < Backing Field > to < Keyword >. I would propose a new keyword similar to value. field would do nicely although I'm sure it's being used in a lot of existing code.

Answer 1

No there isn't. If you want to access the backing field, then don't use auto properties and roll your own.

I agree that it would be great to have a field that was only accessible by the property and not by the rest of the class. I would use that all the time.

Answer 2

As the MSDN states:

"In C# 3.0 and later, auto-implemented properties make property-declaration more concise when no additional logic is required in the property accessors. They also enable client code to create objects When you declare a property as shown in the following example, the compiler creates a private, anonymous backing field can only be accessed through the property's get and set accessors."

Since you have additional logic in you accessors, the use of auto-implemented properties is not appropriate in your scenario.

While the backing field does exist, it is given a mangled name to stop you referencing it easily - the idea is that you never reference the field directly . For interests sake, you can use Reflector to disassemble your code and discover the field name, but I would recommend you not use the field directly as this name may indeed be volatile, so your code could break at any time.

Answer 3

Having read your comments in Mehrdad's answer, I think I understand your problem a bit better.

It appears that you are concerned about the ability of the developer to access private state in the class they are writing, bypassing your validation logic, etc. This suggests that the state should not be contained in the class at all.

I would suggest the following strategy. Write a generic class that represents a ValidatedValue. This class holds only the backing value and only allows access/mutation via get and set methods. A delegate is passed to the ValidatedValue to represent the validation logic:

public class ValidatedValue< T >
{
    private T m_val;
    public ValidationFn m_validationFn;

    public delegate bool ValidationFn( T fn );

    public ValidatedValue( ValidationFn validationFn )
    {
        m_validationFn = validationFn;
    }

    public T get()
    {
        return m_val;
    }

    public void set(T v)
    {
        if (m_validationFn(v))
        {
            m_val = v;
        }
    }
}

You could, of course, add more delegates as required (eg, to support pre/post change notification).

Your class would now use the ValidatedValue in place of a backing store for your property.

The example below shows a class, MyClass, with an integer that is validated to be less than 100. Note that the logic to throw an exception is in MyClass, not the ValidatedValue. This allows you to do complex validation rules that depend on other state contained in MyClass. Lambda notation was used to construct the validation delegate - you could have bound to a member function instead.

public partial class MyClass
{
    private ValidatedValue<int> m_foo;

    public MyClass()
    {
        m_foo = new ValidatedValue<int>(
            v => 
            {
                if (v >= 100) RaiseError();
                return true;
            }
        );
    }

    private void RaiseError()
    {
        // Put your logic here....
        throw new NotImplementedException();
    }

    public int Foo
    {
        get { return m_foo.get(); }
        set { m_foo.set(value); }
    }
}

Hope that helps - somewhat off the original topic, but I think it's more inline with your actual concerns. What we have done is taken the validation logic away from the property and put it on the data, which is exactly where you wanted it.

Answer 4

No, but you can in a subclass:

public class Base
{
    public string Name
    {
        get;
        virtual set;
    }
}

public class Subclass : Base
{
    // FIXME Unsure as to the exact syntax.
    public string Name
    {
        override set
        {
            if (value != base.Name)
            {
                RaiseEvent();
            }

            base.Name = value;
        }
    }
}

Answer 5

If you're gonna do so, why you are using auto properties?!

A simple property has done it way back in 1.0. I don't think it makes sense to add complexity to the language for every special case. You either need the property to do plain store/retrieve model or need more than that. In the latter case, a normal property will do.

Answer 6

You can't do this I'm afraid. That's one of the reasons I started writing MoXAML Power Toys , to provide the ability to convert automatic properties into Notify properties.