[c#] Public class is inaccessible due to its protection level

I have the following classes:

namespace Bla.Bla 
{
    public abstract class ClassA 
    {
        public virtual void Setup(string thing) 
        {
        }

        public abstract bool IsThingValid();

        public abstract void ReadThings();

        public virtual void MatchThings() { }

        public virtual void SaveThings() { }

        public void Run(string thing) 
        {
            Setup(thing);

            if (!IsThingValid()) 
            {

            }

            ReadThings();
            MatchThings();
            SaveThings();
        }
    }
}

namespace Bla.Bla 
{
    public class ClassB : ClassA 
    {
        ClassB() { } 

        public override void IsThingValid() 
        {
            throw new NotImplementedException();
        }

        public override void ReadThings() 
        {
            throw new NotImplementedException();
        }
    }
}

Now I try to do the following:

public class ClassC 
{
    public void Main() 
    {
        var thing = new ClassB();
        ClassB.Run("thing");
    }
}

Which returns the following error: ClassB is inaccessible due to its protection level.

But they are all public.

You could go into the designer of the web form and change the "webcontrols" to be "public" instead of "protected" but I'm not sure how safe that is. I prefer to make hidden inputs and have some jQuery set the values into those hidden inputs, then create public properties in the web form's class (code behind), and access the values that way.

Also if you want to do something like ClassB.Run("thing");, make sure the Method Run(); is static or you could call it like this: thing.Run("thing");.