Question 1

C#.NET — Interfaces: choose the correct statement.

Accepted Answer

Introduction / Context: This question evaluates your grasp of what interfaces are and how they relate to other types in C#.NET. It distinguishes interface inheritance from class inheritance and clarifies what members interfaces may declare. Given Data / Assumptions: We consider standard C# interface rules used in most exam contexts. Concept / Approach: An interface can inherit from one or more interfaces, enabling contract composition. Interfaces themselves do not derive from System.Object in the class-inheritance sense (their BaseType in reflection is null). Historically, interfaces contained signatures for methods, properties, indexers, and events. (Modern C# allows additional patterns, but exams typically focus on classic rules.) Step-by-Step Solution: Option B — Correct: interface IChild : IParent { … } is valid. Option A — Incorrect: interfaces are not class types and do not “derive” from Object as classes do. Option C — Incorrect: there is no “Object interface.” Option D — Incorrect: interfaces can also declare properties, indexers, and events (signatures). Option E — Incorrect in typical exam scope: traditional interfaces do not contain static data/methods. Verification / Alternative check: Compile interface I1 { int P { get; } } and interface I2 : I1 { void M(); }. It compiles, proving interface inheritance. Why Other Options Are Wrong: They either confuse interface inheritance with class inheritance or incorrectly restrict/expand allowed members. Common Pitfalls: Thinking that all types “inherit” from Object exactly the same way; applying class-only rules to interfaces. Final Answer: Interfaces can be inherited (one interface can inherit from another).

Question 2

C#.NET — Object size with only interface implementations: what is the instance size? A class implements two interfaces (each with three methods) and has no instance fields. What is the size of an object created from this class?

Accepted Answer

Introduction / Context: In managed languages such as C#, object layout is abstracted away by the runtime. Exams often check whether you incorrectly assume that “more methods or interfaces mean a larger object.” Given Data / Assumptions: The class implements interfaces only; it has no instance fields. We are asked about the size of an instance. Concept / Approach: In exam-oriented C# questions, instance size is typically considered as coming from instance fields. Methods (including interface implementations) are shared code and do not add per-object data. Therefore, with no fields, the instance is treated as having “0 byte” payload in such questions—even though actual CLR objects have a header and alignment that are not part of this simplified model. Step-by-Step Solution: Interfaces add no per-instance data in this simplified view. No instance fields → no per-instance storage → reported as 0 byte in typical MCQ keys. Remember that the real CLR adds object headers; that detail is intentionally ignored in these exam questions. Verification / Alternative check: Using reflection or unsafe code reveals real headers; however, MCQs usually abstract that away to emphasize that methods/interfaces don’t increase instance size. Why Other Options Are Wrong: They assume a fixed byte size unrelated to instance fields or mix in header sizes, which the exam does not request. Common Pitfalls: Confusing code size with object instance size; believing more methods or interfaces make each object larger. Final Answer: 0 byte

Question 3

C#.NET — Interfaces: choose the correct statement about members and implementation.

Accepted Answer

Introduction / Context: This checks your understanding of interface contracts in C#.NET: what they contain, their accessibility, and how a class satisfies them. Given Data / Assumptions: “Inherit” for interfaces means a class implements an interface’s signatures, not concrete implementations. Classic interface rules (no bodies) are assumed for exam purposes. Concept / Approach: Interface members are implicitly public and must be implemented with public members in the class. Interfaces can declare methods, properties, indexers, and events. Static is not required (and would be incorrect for instance contracts). Step-by-Step Solution: A — False: interfaces provide no implementations (again, per classic rules). B — False: interfaces may declare indexers (e.g., int this[int i] { get; }). C — True: interface members are public by definition. D — False: the implementing member must be public but not static. Verification / Alternative check: Attempt to implement an interface with a non-public member; the compiler requires public visibility. Why Other Options Are Wrong: They contradict core interface rules regarding member kinds and accessibility. Common Pitfalls: Equating interface “inheritance” with class inheritance; forgetting that interface contracts imply public accessibility. Final Answer: Interface members are automatically public.

Question 4

C#.NET — Which kinds of types can implement an interface? Consider these candidates: (1) Data (n/a), (2) Class, (3) Enum, (4) Structure, (5) Namespace. Select all that can implement an interface.

Accepted Answer

Introduction / Context: The question targets where interface implementation is allowed in C#. Understanding which type categories can implement interfaces is foundational for good design. Given Data / Assumptions: Candidates: Class, Enum, Structure, etc. Concept / Approach: In C#, classes and structs (value types) can implement interfaces. Enums and namespaces cannot. “Data” is not a C# type category. Therefore, only (2) Class and (4) Structure are valid implementers among the listed options. Step-by-Step Solution: Check Class → Yes: class C : IExample { … } Check Enum → No: enums cannot implement interfaces in C# exams’ scope. Check Structure → Yes: struct S : IExample { … } Check Namespace → No: namespaces cannot implement anything; they only organize code. “Data” → Not a C# type category. Verification / Alternative check: Try compiling enum E : IExample { A } in C#; it fails. A struct/class implementing an interface compiles. Why Other Options Are Wrong: They include invalid categories (1, 3, 5) or omit necessary valid ones. Common Pitfalls: Confusing enum abilities with those of classes/structs; treating “namespace” as a type. Final Answer: 2, 4

Question 5

C#.NET — Interfaces: pick the single correct statement (clarifying implementation vs inheritance).

Accepted Answer

Introduction / Context: This question distinguishes between how interfaces are consumed by classes and how interfaces relate to each other. It also clarifies explicit interface implementation. Given Data / Assumptions: Classic C# exam scope (interfaces declare contracts; classes/structs implement them). Concept / Approach: Explicit interface implementation lets a class provide an implementation that is only accessible through the interface reference (e.g., void IFoo.M() { … }). While multiple classes can implement the same interface, this question requires a single correct choice; we therefore use the statement that is both precise and uniquely framed. Step-by-Step Solution: A — Incorrect wording: interfaces inherit from interfaces; they are not “implemented in” one another. B — Although true in general, the question asks for a single best fact; option C is more specific and instructive for exams. C — Correct: explicit implementation syntax is allowed and common for resolving name clashes or hiding members from the class’s public surface. D — False in classic rules: interface members are declarations without bodies. E — False: any concrete class or struct can implement an interface. Verification / Alternative check: Write class C : IFoo { void IFoo.M() { } } and call ((IFoo)new C()).M(); — compiles and runs. Why Other Options Are Wrong: They misuse terms (implement vs inherit), or incorrectly restrict who can implement, or claim bodies exist in interface methods. Common Pitfalls: Mixing up interface-to-interface inheritance with implementation by classes; forgetting explicit implementation exists. Final Answer: A class that implements an interface can explicitly implement members of that interface.

Question 6

C#.NET — Interface and class implementation details: analyze this code. interface IMyInterface { void fun1(); int fun2(); } class MyClass : IMyInterface { void fun1() { } int IMyInterface.fun2() { return 0; } } Which statement best describes the sit…

Accepted Answer

Introduction / Context: This problem tests understanding of interface implementation visibility and explicit interface members in C#. A class must supply public implementations for interface members unless it uses explicit interface implementation syntax for each member. Given Data / Assumptions: IMyInterface declares void fun1() and int fun2(). MyClass implements IMyInterface. MyClass defines fun1 without an access modifier (thus private by default in a class). MyClass explicitly implements fun2 as int IMyInterface.fun2(). Concept / Approach: Interface members are implicitly public contracts. To satisfy them, a class must implement them as public members or via explicit interface implementation. A method with no access modifier in a class is private; a private fun1 does not satisfy IMyInterface.fun1(). Therefore, the compiler treats MyClass as not providing a valid implementation for fun1. With an unimplemented contract, MyClass cannot be instantiated as a non-abstract class. Step-by-Step Solution: Check fun1: declared “void fun1()” with default private visibility → does not implement IMyInterface.fun1(). Check fun2: explicitly implemented “int IMyInterface.fun2()” → valid but accessible only via IMyInterface reference. Result: MyClass fails to implement fun1 publicly (or explicitly), so the type would need to be abstract, or fun1 must be declared public to compile as concrete. Verification / Alternative check: Add “public void fun1() { }” — the class compiles as a concrete type. Or change fun1 to explicit interface implementation “void IMyInterface.fun1() { }”. Why Other Options Are Wrong: A/B: Interfaces can declare methods (and other member signatures). C: Method dispatch tables are a runtime concern and not removed here. E: fun1 could be public instead; explicit syntax is not mandatory. Common Pitfalls: Forgetting default accessibility (private) on class members; misunderstanding that explicit interface members are only accessible via the interface reference. Final Answer: MyClass is an abstract class (as written) because fun1 is not publicly implemented.

Question 7

C#.NET — Which statements about interfaces are correct?  1) An interface can contain properties, methods, and events. 2) The keyword 'must implement' forces implementation of an interface. 3) Interfaces can be overloaded. 4) Interfaces can be implem…

Accepted Answer

Introduction / Context: Interfaces in C#.NET define contracts that types agree to fulfill. This question checks core facts: what members an interface may declare, who can implement it, and how adding implementations affects existing code. Given Data / Assumptions: C# interface syntax and capabilities are considered in the classic sense (properties, methods, events, indexers). We assume no Visual Basic keywords (like MustInherit/Implements) in C#. “Enhanced implementations without breaking code” refers to adding new implementing types while keeping the interface unchanged. Concept / Approach: Interfaces can declare members such as methods, properties, events, and indexers. Both classes and structs can implement interfaces. Adding new implementations (new classes/structs that implement the same interface) does not break existing consumers because existing binaries compile against the unchanged interface contract. In contrast, changing the interface (e.g., adding new required members) can break implementers. Step-by-Step Solution: 1) True — interfaces can include properties, methods, events, and indexers.2) False — “must implement” is not a C# keyword (it is VB-related).3) False — you do not “overload” interfaces; you may overload members declared in them, but the interface itself is not an overloadable entity.4) True — classes and structs can implement interfaces.5) True — you can add new implementing types with no breaking change to callers. Verification / Alternative check: Define an interface with properties/methods/events; implement it in a class and a struct; add another class that implements it later. Existing code continues to work. Why Other Options Are Wrong: Option A includes a false statement (2). Option C includes statement 3 (false) and omits 1 and 5. Option D only keeps the false statement 3. Option E is invalid because there is a correct set: 1, 4, 5. Common Pitfalls: Confusing VB keywords with C#. Also, assuming you can change interface members freely without breaking existing implementers. Final Answer: 1, 4, 5

Question 8

C#.NET — Which members can be declared inside an interface?  Consider: 1) Properties 2) Methods 3) Enumerations 4) Events 5) Structures

Accepted Answer

Introduction / Context: C# interfaces specify contracts. Knowing which members are allowed within an interface definition is essential for proper API design. Given Data / Assumptions: Standard C# rules apply for interface declarations. We compare typical members: properties, methods, events vs. type declarations like enums/structs. Concept / Approach: Interfaces can declare members that describe behavior: methods, properties, events, and indexers. They do not declare fields. Type declarations (enums, structs, classes) are not declared inside an interface in classic C# usage for this curriculum. Step-by-Step Solution: Evaluate 1) Properties → allowed in interfaces.Evaluate 2) Methods → allowed in interfaces.Evaluate 3) Enumerations → not declared inside interfaces in this context.Evaluate 4) Events → allowed in interfaces.Evaluate 5) Structures → not declared inside interfaces in this context. Verification / Alternative check: Write a small interface: interface IEx { int P { get; set; } event EventHandler E; void Do(); } — compiles. Attempt to place a nested enum or struct in the interface and note it is outside the scope of this syllabus and not used for typical design. Why Other Options Are Wrong: A/C/D include members not declared in interfaces here (enums/structs). E is incorrect because 1, 2, and 4 are valid. Common Pitfalls: Confusing interface members (behavioral contracts) with type definitions, which belong at the namespace or type level, not inside an interface for this exam. Final Answer: 1, 2, 4

Question 9

C#.NET — Is the following interface definition valid?  interface IPerson {  string FirstName { get; set; }  string LastName { get; set; }  void Print();  void Stock();  int Fun(); }

Accepted Answer

Introduction / Context: Interfaces in C# define member signatures without implementation. This problem verifies whether properties and methods can be declared in an interface and whether bodies are required. Given Data / Assumptions: IPerson declares two properties and three methods. No method or property accessor includes a body. Concept / Approach: In classic C#, interface members have signatures only. Implementing types must provide bodies. Properties in an interface are declared with get; and/or set; accessors — again, without bodies. Methods (void or returning values) are allowed and, similarly, have no bodies in the interface. Step-by-Step Solution: Check properties FirstName/LastName → valid with get; set; signatures only.Check methods Print/Stock/Fun → valid method signatures; no bodies necessary.Therefore the interface compiles and is usable as a contract for implementing types. Verification / Alternative check: Create a class Employee : IPerson and implement each member. The compiler will require bodies in the class. Why Other Options Are Wrong: A/E are false because interfaces can declare properties and methods. C/D are false because bodies are not provided in interfaces (for this syllabus; default interface implementations came later and are not assumed here). Common Pitfalls: Trying to put code into interface accessors or methods; implementation belongs in classes/structs that implement the interface. Final Answer: This is a perfectly workable interface.

Question 10

C#.NET — Consider explicit interface implementation with a partially implemented interface.  interface IMyInterface { void fun1(); void fun2(); } class MyClass : IMyInterface {  private int i;  void IMyInterface.fun1() { /* code */ }  // fun2 not im…

Accepted Answer

Introduction / Context: In C#, a non-abstract class that declares it implements an interface must provide implementations for all interface members. Explicit interface implementation is permitted, but completeness is still required. Given Data / Assumptions: IMyInterface declares fun1 and fun2. MyClass uses explicit implementation for fun1 only. MyClass is not declared abstract. Concept / Approach: When a class lists an interface after the colon, the compiler enforces that every declared member of the interface is implemented (either explicitly or implicitly). If any member is missing and the class is not abstract, compilation fails. Declaring the class abstract would defer the unimplemented members to a subclass. Step-by-Step Solution: Identify interface members: fun1, fun2.Check implementations in MyClass: only fun1 is present (explicitly).MyClass is not abstract → must implement fun2 as well → violation.Result: compiler error stating MyClass does not implement interface member fun2. Verification / Alternative check: Add either an implicit public void fun2() { ... } or an explicit void IMyInterface.fun2() { ... }. Compilation will then succeed. Why Other Options Are Wrong: A: The class is not declared abstract. B: Classes can contain instance data alongside interface implementations. C: False — fun2 is missing. D: Interfaces do not inherit from Object; classes do. Common Pitfalls: Forgetting that explicit implementation still requires completeness; it only affects accessibility and call syntax. Final Answer: The compiler will report an error since the interface IMyInterface is only partially implemented.

Question 11

C#.NET — Choose the correct implementation for the interface:  interface IMyInterface { double MyFun(Single i); }

Accepted Answer

Introduction / Context: Interfaces declare member signatures. Implementations may be provided either implicitly (public members matching the signature) or explicitly (qualified with the interface name). This item asks you to recognize valid C# syntax for explicit interface implementation. Given Data / Assumptions: Interface: double MyFun(Single i). Language: C# (not VB). Concept / Approach: Explicit interface implementation syntax is: returnType InterfaceName.MemberName(parameters) { ... } and appears within a class that lists the interface after the colon. No access modifier is used on explicit implementations. Step-by-Step Solution: Option D uses correct C#: class MyClass : IMyInterface { double IMyInterface.MyFun(Single i) { ... } }Option A uses non-existent “as” syntax.Option B uses VB-like “As” and omits modifiers/return type position.Option C uses VB-like “implements” keywords; not C#. Verification / Alternative check: Compile Option D: succeeds. Attempt A/B/C in a C# project: they fail with syntax errors. Why Other Options Are Wrong: A/B/C borrow from other languages or invent syntax not supported by C#. Common Pitfalls: Mixing VB and C# syntax when writing interface implementations; remember explicit implementations omit access modifiers and are qualified by the interface name. Final Answer: class MyClass : IMyInterface { double IMyInterface.MyFun(Single i) { / code */ } }

Question 12

C#.NET — Which statement about interfaces is correct?

Accepted Answer

Introduction / Context: Understanding how classes fulfill interface contracts is fundamental to C# design. This question targets partial implementations and what is permitted inside interfaces. Given Data / Assumptions: A concrete (non-abstract) class must implement every member of its interfaces. Classic C# interface rules are assumed for this curriculum. Concept / Approach: If a class does not implement all interface members, it cannot be concrete; it must be declared abstract so that derived types are required to finish the job. Historically, interfaces did not contain static members or data (modern C# adds advanced features, but the exam context assumes classic rules). Multiple interface inheritance is allowed for classes and interfaces. Step-by-Step Solution: Option A: Correct — partial implementation requires the class to be abstract.Option B: Incorrect — partial implementation is allowed only if the class is abstract.Option C/D: Incorrect in the classic ruleset — interfaces declare behavior, not static members/data.Option E: Incorrect — classes can implement multiple interfaces, and interfaces can inherit multiple interfaces. Verification / Alternative check: Try compiling a non-abstract class that leaves one interface member unimplemented; the compiler errors. Marking the class abstract defers the requirement to subclasses. Why Other Options Are Wrong: They contradict C# interface semantics in the scope of this exam (multiple interfaces allowed; no static data in interfaces; static methods not part of the classic model). Common Pitfalls: Forgetting to mark a partially implementing class as abstract, leading to “does not implement interface member …” compile errors. Final Answer: If a class implements an interface partially, then it should be an abstract class.

Question 13

C#.NET — Which is a correct implementation of the IPerson interface?  interface IPerson { string FirstName { get; set; } }

Accepted Answer

Introduction / Context: The problem asks you to identify correct C# syntax for implementing a simple interface property. It also tests awareness of explicit vs. implicit interface implementation and the correct inheritance syntax. Given Data / Assumptions: IPerson declares string FirstName { get; set; }. We want a valid class that compiles and implements the property. Concept / Approach: To implement an interface implicitly, the class must list the interface after its name using a colon and then provide a public member with the same signature. Explicit interface implementation would use the qualified name IPerson.FirstName, but the class must still declare that it implements IPerson. Step-by-Step Solution: Option A: Correct — uses : IPerson and a public property matching the signature.Option B: Attempts explicit naming but the class does not implement IPerson; also access modifiers are invalid for explicit interface members in this form.Option C: Uses non-C# keyword “implements”; invalid syntax. Verification / Alternative check: Compile Option A — succeeds. Compile B/C — fail with syntax/interface errors. Why Other Options Are Wrong: B lacks : IPerson and misuses explicit implementation syntax; C is not C# syntax. Common Pitfalls: Confusing VB “Implements” with C#’s colon syntax, and forgetting that explicit property implementations need the interface qualification and no access modifiers. Final Answer: class Employee : IPerson { ... public string FirstName { get; set; } }

Question 14

C#.NET — Which statement about interfaces is correct?

Accepted Answer

Introduction / Context: This item checks fundamental truths about C# interfaces: multiple implementation, interface inheritance, and the kinds of members interfaces can declare. Given Data / Assumptions: C# allows a class or struct to implement multiple interfaces. Interfaces can inherit from multiple interfaces. Interfaces can declare properties, methods, events, and indexers. Concept / Approach: Eliminate false claims: single-interface-only implementation is false; exclusivity of implementation to a single class is false; prohibition on multiple base interfaces is false; and “interfaces cannot be inherited” is false. The remaining positive fact is that properties are allowed inside interfaces. Step-by-Step Solution: Reject A: Classes may implement many interfaces.Reject B: Any number of classes can implement the same interface.Reject C: New interfaces may inherit multiple base interfaces.Accept D: Properties are valid interface members.Reject E: Interfaces can inherit other interfaces. Verification / Alternative check: Write interface IX { int P { get; set; } } and interface IY : IX, IDisposable { }. Implement both in various classes; compilation succeeds. Why Other Options Are Wrong: They contradict the multiple-interface implementation/inheritance features of C#. Common Pitfalls: Confusing class single inheritance (only one base class) with interface multiple inheritance (many interfaces). Final Answer: Properties can be declared inside an interface.

Question 15

C#.NET — Which statements about interfaces are correct?  1) A class can implement multiple interfaces. 2) Structures cannot inherit a class but can implement an interface. 3) In C#.NET, the colon (:) is used to signify that a class member implements…

Accepted Answer

Introduction / Context: This question tests nuanced understanding of interface usage across classes and structs, the correct meaning of the colon syntax, and restrictions on static members with interface implementations. Given Data / Assumptions: We assess five statements; some are partly true, partly misworded. Classic C# rules apply for this syllabus. Concept / Approach: 1) True on its face: classes can implement multiple interfaces. 2) True: structs cannot inherit classes, but they can implement interfaces. 3) False as written: the colon (:) is used after the type name to list base types and interfaces that the type implements or inherits; it does not apply to a “class member.” 4) False: interfaces do not implement classes; classes implement interfaces; interfaces can inherit other interfaces. 5) False in classic C#: methods implementing interface members are instance members; marking them static does not implement the interface member for instances. Step-by-Step Solution: Identify that options provided in the answers do not match the correct subset (1 and 2 only).Check available choices: (A) includes 3 (wrong); (B) includes 4 (wrong); (C) includes 3 and 5 (both wrong); (D) None of the above → best reflects that none of the offered groupings are correct; (E) is not in the original options list for this question block. Verification / Alternative check: Declare class C : IComparable, IDisposable { ... } and struct S : IComparable { ... }. Observe that implementations are instance methods, not static. Why Other Options Are Wrong: A/B/C include demonstrably false statements. Common Pitfalls: Misreading (:) as applying to members, and assuming static methods can satisfy instance interface contracts. Final Answer: None of the above.

Interfaces Questions