Generic class declaration in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a generic class in C#?

A generic class is a class that can work with any data type. It uses a placeholder (type parameter) that is replaced with a real type when the class is used.

beginner

How do you declare a generic class named <code>Box</code> with one type parameter <code>T</code>?

You write: <pre>public class Box&lt;T&gt; { }</pre> This means <code>Box</code> can hold any type <code>T</code> you choose later.

beginner

Why use generic classes instead of normal classes?

Generic classes let you write flexible and reusable code. You avoid repeating the same code for different data types and get type safety without casting.

beginner

What does <code>T</code> represent in <code>class Box&lt;T&gt;</code>?

T is a type parameter. It is a placeholder for a real data type that you specify when creating an object of the class.

beginner

How do you create an instance of a generic class Box<T> for type int?

You write:

Box<int> intBox = new Box<int>();

This creates a Box that works with integers.

What symbol is used to declare a generic type parameter in a class?

A <>

B ()

C []

D {}

Generic type parameters are declared inside angle brackets <> after the class name.

Which of these is a correct generic class declaration?

A public class List{T} { }

B public class List(T) { }

C public class List[T] { }

D public class List<T> { }

The correct syntax uses angle brackets <> for generic type parameters.

What does the T in class Box<T> stand for?

A A specific type like int

B A placeholder for any type

C A method name

D A variable name

T is a placeholder that can be replaced by any type when the class is used.

Why are generic classes useful?

A They allow code reuse for different data types

B They make code slower

C They only work with strings

D They remove type safety

Generic classes let you write one class that works with many types, improving reuse and safety.

How do you create an object of a generic class Box<T> for strings?

A Box myBox = new Box();

B Box<T> myBox = new Box<T>();

C Box<string> myBox = new Box<string>();

D Box<string> myBox = Box<string>();

You specify the type in angle brackets when creating the object and use the new keyword.

Explain what a generic class is and why it is useful in C#.

Think about how you can write one class to work with many types.

Describe how to declare and create an instance of a generic class with an example.

Use the example of a class named Box with type T.

Practice

1.

What is the main purpose of declaring a generic class in C# using <T>?

easy

A. To allow the class to work with any data type safely

B. To restrict the class to only one specific data type

C. To make the class run faster by avoiding type checks

D. To create multiple classes with different names automatically

Solution

1. Step 1: Understand generic class purpose

   Generic classes use type parameters to work with any data type, improving code reuse and safety.

2. Step 2: Compare options with generic concept

   Only To allow the class to work with any data type safely correctly states that generics allow safe use with any data type.

3. Final Answer:

   To allow the class to work with any data type safely -> Option A

4. Quick Check:

   Generic class purpose = safe, reusable code [OK]

Hint: Generics = flexible types for safe reuse [OK]

Common Mistakes:

• Thinking generics restrict to one type
• Believing generics improve speed by skipping checks
• Confusing generics with automatic class creation

2.

Which of the following is the correct syntax to declare a generic class named Box with one type parameter?

?

easy

A. public class Box<T>() { }

B. public class Box<T> { }

C. public class Box(T) { }

D. public class Box[] { }

Solution

1. Step 1: Recall generic class syntax

   Generic classes use angle brackets with type parameters: class ClassName<T>.

2. Step 2: Check each option

   public class Box<T> { } matches correct syntax. Options A, B, and D have syntax errors or wrong symbols.

3. Final Answer:

   public class Box<T> { } -> Option B

4. Quick Check:

   Generic syntax = class Name<T> [OK]

Hint: Generic classes use angle brackets <> [OK]

Common Mistakes:

• Using parentheses instead of angle brackets
• Adding parentheses after class name
• Using square brackets instead of angle brackets

3.

What will be the output of the following C# code?

public class Container<T> {
    public T Value;
    public Container(T value) { Value = value; }
    public override string ToString() => $"Value: {Value}";
}

var intBox = new Container<int>(10);
var strBox = new Container<string>("Hello");
Console.WriteLine(intBox);
Console.WriteLine(strBox);

medium

A. Value: 10
Value: Hello

B. Value: intBox
Value: strBox

C. 10
Hello

D. Compilation error due to missing type constraints

Solution

1. Step 1: Understand class behavior

   The class stores a value of type T and overrides ToString() to print "Value: {Value}".

2. Step 2: Trace output of Console.WriteLine

   intBox.ToString() returns "Value: 10" and strBox.ToString() returns "Value: Hello".

3. Final Answer:

   Value: 10 Value: Hello -> Option A

4. Quick Check:

   ToString override prints value with prefix [OK]

Hint: ToString() shows formatted value, not variable name [OK]

Common Mistakes:

• Expecting variable names instead of values
• Ignoring ToString() override
• Thinking type constraints are required here

4.

Identify the error in this generic class declaration and fix it:

public class Pair<T, U>
{
    public T First;
    public U Second;

    public Pair(T first, U second)
    {
        First = first;
        Second = second;
    }
}

var p = new Pair<string>(5, "test");

medium

A. Generic class cannot have two type parameters

B. Constructor parameters should be swapped

C. Missing type arguments when creating instance: use new Pair<int, string>(5, "test")

D. Fields First and Second must be private

Solution

1. Step 1: Check instance creation syntax

   Generic classes require type arguments when instantiated unless using C# 7.1+ with target-typed new.

2. Step 2: Identify missing type arguments

   The code uses new Pair<string>(5, "test") without specifying <int, string>, causing error.

3. Final Answer:

   Missing type arguments when creating instance: use new Pair<int, string>(5, "test") -> Option C

4. Quick Check:

   Generic instantiation needs type args [OK]

Hint: Always specify type parameters when creating generic instances [OK]

Common Mistakes:

• Omitting type arguments on new
• Thinking generic classes can't have multiple parameters
• Changing constructor parameter order unnecessarily

5.

You want to create a generic class Cache<TKey, TValue> that stores key-value pairs and only allows reference types for TValue. Which declaration is correct?

hard

A. public class Cache<TKey, TValue> where TKey : struct { }

B. public class Cache<TKey, TValue> where TValue : struct { }

C. public class Cache<TKey, TValue> where TKey : class { }

D. public class Cache<TKey, TValue> where TValue : class { }

Solution

1. Step 1: Understand type constraints

   To restrict a generic type to reference types, use where Type : class.

2. Step 2: Apply constraint to TValue

   The question requires TValue to be a reference type, so constraint applies to TValue.

3. Final Answer:

   public class Cache<TKey, TValue> where TValue : class { } -> Option D

4. Quick Check:

   Reference type constraint = where TValue : class [OK]

Hint: Use 'where TValue : class' to restrict to reference types [OK]

Common Mistakes:

• Applying constraint to wrong type parameter
• Using 'struct' instead of 'class' for reference types
• Omitting constraints when needed