Is it possible to override a non-virtual method?

You can't override non-virtual method of any class in C# (without hacking CLR), but you can override any method of interface the class implements. Consider we have non-sealed

class GraphicsDevice: IGraphicsDevice {
    public void DoWork() {
        Console.WriteLine("GraphicsDevice.DoWork()");
    }
}

// with its interface
interface IGraphicsDevice {
    void DoWork();
}

// You can't just override DoWork in a child class,
// but if you replace usage of GraphicsDevice to IGraphicsDevice,
// then you can override this method (and, actually, the whole interface).

class MyDevice: GraphicsDevice, IGraphicsDevice {
    public new void DoWork() {
        Console.WriteLine("MyDevice.DoWork()");
        base.DoWork();
    }
}

And here's demo

class Program {
    static void Main(string[] args) {

        IGraphicsDevice real = new GraphicsDevice();
        var myObj = new MyDevice();

        // demo that interface override works
        GraphicsDevice myCastedToBase = myObj;
        IGraphicsDevice my = myCastedToBase;

        // obvious
        Console.WriteLine("Using real GraphicsDevice:");
        real.DoWork();

        // override
        Console.WriteLine("Using overriden GraphicsDevice:");
        my.DoWork();

    }
}

The answer to this question is not entirely No. It depends on how you structure your inheritance and access the instances of your classes. If your design meets the following, you would be able to override non-virtual method from base class with the new modifier:

• The method is declared in an interface that your classes inherit from
• You are accessing the class instances using the interface

Take example of the following:

interface ITest { void Test(); }
class A : ITest { public void Test(){Console.WriteLine("A");} }
class B : A { public new void Test(){Console.WriteLine("B");} }
ITest x = new B(); 
x.Test(); // output "A"

calling x.Test() will output "A" to the console. However if you re-declare the interface in the definition of class B, x.Test() will output B instead.

interface ITest { void Test(); }
class A : ITest { public void Test(){Console.WriteLine("A");} }
class B : A, ITest { public new void Test(){Console.WriteLine("B");} }
ITest x = new B();
x.Test(); // output "B"