Option 1: Provide IMPLEMENTATION for all the members inheirted from the INTERFACE.

namespace INTERFACES
{
interface Interface1
{
VOID Interface1Method();
}
class BaseClass1 : Interface1
{
public void Interface1Method()
{
Console.WriteLine("Interface1 Method");
}
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
}

Option 2: If the class does not wish to provide Implementation for all the members inheirted from the interface, then the class has to be marked as abstract.

namespace Interfaces
{
interface Interface1
{
void Interface1Method();
}
abstract class BaseClass1 : Interface1
{
abstract public void Interface1Method();
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
}

Option 1: Provide Implementation for all the members inheirted from the interface.

namespace Interfaces
{
interface Interface1
{
void Interface1Method();
}
class BaseClass1 : Interface1
{
public void Interface1Method()
{
Console.WriteLine("Interface1 Method");
}
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
}

Option 2: If the class does not wish to provide Implementation for all the members inheirted from the interface, then the class has to be marked as abstract.

namespace Interfaces
{
interface Interface1
{
void Interface1Method();
}
abstract class BaseClass1 : Interface1
{
abstract public void Interface1Method();
public void BaseClass1Method()
{
Console.WriteLine("BaseClass1 Method");
}
}
}