Generic class declaration in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic class named Box.

C Sharp (C#)

public class Box[1] { }

Drag options to blanks, or click blank then click option'

A<T>

B[T]

C(T)

D{T}

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets or parentheses instead of angle brackets.

Forgetting to include the generic type parameter.

Generic classes in C# use angle brackets <> to specify type parameters.

2fill in blank

medium

Complete the code to declare a generic class named Pair with two type parameters.

C Sharp (C#)

public class Pair[1] { }

Drag options to blanks, or click blank then click option'

A(T, U)

B<T>

C<T, U>

D[T, U]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses or square brackets instead of angle brackets.

Declaring only one type parameter when two are needed.

Multiple generic type parameters are declared inside angle brackets separated by commas.

3fill in blank

hard

Fix the error in the generic class declaration syntax.

C Sharp (C#)

public class Container[1] { }

Drag options to blanks, or click blank then click option'

A[T]

B<T>

C(T)

D{T}

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets or parentheses instead of angle brackets.

Omitting the generic type parameter brackets.

Generic classes must use angle brackets <> to declare type parameters.

4fill in blank

hard

Fill both blanks to declare a generic class named Result with one type parameter and a property of that type.

C Sharp (C#)

public class Result[1] {
    public [2] Value { get; set; }
}

Drag options to blanks, or click blank then click option'

A<T>

BT

Cint

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete type like int or string instead of the generic type parameter.

Forgetting to declare the generic type parameter in the class.

The class declares a generic type parameter and uses T as the property type.

5fill in blank

hard

Fill all three blanks to declare a generic class named Pair with two type parameters and two properties of those types.

C Sharp (C#)

public class Pair[1] {
    public [2] First { get; set; }
    public [3] Second { get; set; }
}

Drag options to blanks, or click blank then click option'

A<T, U>

BT

CU

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using concrete types instead of generic type parameters for properties.

Forgetting to declare both generic type parameters.

The class declares two generic type parameters and uses them as property types.

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