OOPS Concepts: What is the difference in passing object reference to interface and creating class object in C#? -

i have class, customernew, , interface, icustomer:

public class customernew : icustomer {     public void a()     {         messagebox.show("class method");     }      void icustomer.a()     {         messagebox.show("interface method");     }       public void b()     {         messagebox.show("class method");     } }  public interface icustomer {     void a(); } 

i confused between these 2 code of lines.

icustomer objnew = new customernew(); customernew objcustomernew = new customernew(); objnew.b(); // why wrong? objcustomernew.b(); // correct because using object of class 

the first line of code means passing object reference of customernew class in objnew, correct? if yes, why can not access method b() of class interface objnew?

can explain these 2 in detail.

actually interface type (you can't create instances of interfaces since they'are metadata).

since customernew implements icustomer, instance of customernew can upcasted icustomer. when customernew typed icustomer can access icustomer members.

this because c# strongly-typed language, thus, in order access particular member (i.e. methods, properties, events...) need object reference qualified type defines members want access (i.e. need store customernew object in reference of type customernew access method b).

update

op said:

so due upcasting can access methods inside interface, correct ? upcasting main reason behind ?

yes. easy explanation object implements icustomer shouldn't mandatorily customernew. need downcast icustomer reference customernew able access customernew members.

as both classes , interfaces types, , c# strongly-typed language, access object members providing actual type. why need use casts.

for example, code implicit upcast:

// implicit cast that's equivalent // icustomer objnew = (icustomer)new customernew() icustomer objnew = new customernew(); 

implicit upcasts possible because compiler knows customernew implements icustomer introspecting customernew metadata, while can't implictly-downcast icustomer reference because knows implements icustomer? can either customernew or other class or struct:

icustomer asinterface = new customernew();  // error: won't compile, need provide explicit downcast customernew asclass1 = asinterface;  // ok. you're telling compiler know asinterface // reference guaranteed customernew too! customernew asclass2 = (customernew)asinterface; 

if you're not sure icustomer customernew, can use as operator, won't throw exception during run-time if cast isn't possible:

// if asinterface isn't customernew, expression set null customernew asclass3 = asinterface customernew;