In Java, methods are virtual by default (can be overridden), but in C#, a method can be overridden if it is declared explicitly as virtual. Also, in Java there is no override keyword which is mandatory in C#.

[Java]

public class C extends B
{
    public void MethodB()
    {
    }
    public void MethodA()
    {
    }
}
public class B extends A
{
    public void MethodB()
    {
    }
    public void MethodA()
    {
    }
}
public abstract class A
{
    public abstract void MethodA();
}

[C#]

public class C : B
{
    public override void MethodB()
    {
    }
    public override void MethodA()
    {
    }
}
public class B : A
{
    public virtual void MethodB()
    {
    }
    public override void MethodA()
    {
    }
}
public abstract class A
{
    public abstract void MethodA();
}