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Can C# extension methods access private variables?

Posted by: admin November 30, 2017 Leave a comment

Questions:

Is it possible to access an object’s private variables using an extension method?

Answers:

No. You can do the same in an extension method as in a “normal” static method in some utility class.

So this extension method

public static void SomeMethod(this string s)
{
    // do something with 's'
}

is equivalent to some static helper method like this (at least regarding what you can access):

public static void SomeStringMethod(string s)
{
    // do something with 's'
}

(Of course you could use some reflection in either method to access private members. But I guess that’s not the point of this question.)

Questions:
Answers:

No it cannot.

However, you will be interested to know that the other answers are incorrect in saying that normal static methods cannot access private fields. A static method can access private non-static member fields in its own class. The following code is perfectly valid and shows a static method accessing a private field:

public class Foo
{
    private bool _field;

    public static bool GetField(Foo foo)
    {
        return foo._field;
    }
}

Now… back to your question. You might think that an extension method should be able to do the same thing, given the (non-existent) “equivalence” to static methods that other answers claim exists. However, you cannot declare extension methods inside a nested class. So if you try to do the following:

public class Foo
{
    private bool _field;

    public static class Extensions
    {
        public static bool GetField(this Foo foo)
        {
            return foo._field;
        }
    }
}

You will get a compile error saying

Extension method must be defined in a top level static class; Extensions is a nested class

Note that, interestingly enough, removing the this keyword causes the code to compile fine. The reasons for this are discussed here:

  1. Why are extension methods only allowed in non-nested, non-generic static class?
  2. Why not allow Extension method definition in nested class?
Questions:
Answers:

No:

public class Foo
{
    private string bar;
}

public static class FooExtensions
{
    public static void Test(this Foo foo)
    {
        // Compile error here: Foo.bar is inaccessible due to its protection level  
        var bar = foo.bar;
    }
}

Questions:
Answers:

No, unless you give some kind of access to them through public properties or a proxy pattern.

Questions:
Answers:

If you own the class that you are extending, you an always declare the class partial, then extend the class & have access to all the private members in a different file… But you wouldn’t really be using extension methods.

Questions:
Answers:

An extension method is essentially a static method so all you have access to are the public members of the instance on which the extension method is invoked on

Questions:
Answers:

Not recommended, but you could possibly access any private variable of any type using another extension method like so:

public static T GetFieldValue<T>(this object obj, string name) {
    var field = obj.GetType().GetField(name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
    return (T)field?.GetValue(obj);
}

And then access a private field of an arbitrary type:

Foo foo = new Foo();
string privateBar = foo.GetFieldValue<string>("_bar");