Can C# extension methods access private variables?

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

Solution 1:

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.)

Solution 2:

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?