C# Generics MCQs

Question 1: Which of the following can be defined generic?

Answer: A,B,C

Enum cannot be generic. However, you can declare generic classes, interfaces, delegates and structs.

Learn C# generics.

Question 2:  When will be T will be replaced with the actual type in the following program?

class ValueProcessor<T>
{
    // Implementation 
}

Answer: B

Type placeholder in generics is replaced at the compile-time. C# assigns the type based on the type passed by the caller when instantiating a class. Learn C# generics.

Learn C# generics.

Question 3: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {
        DataProcessor<int> vp = new DataProcessor<int>();
        vp.Process("TEST");
        vp.Process<double>(3.5D);
    } 
}

class DataProcessor<T>
{
    public void Process<T>(T value) {
        Console.Write(value.GetType().Name + “ ”);
    }
}

Answer: D

Here, DataProcessor class and Process() method both are generic denoted by T symbol. Although a different symbol for the different generic members of the same class is recommended, the same symbol is allowed. So, DataProcessor<int> vp sets the type for the class, and the type for the Process() method is decided based on the type of the value passed during the call if not specified explicitly using angle brackets. Learn C# generics.

Learn C# generics.

Question 4: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {
        DataProcessor<int> vp = new DataProcessor<int>();
        vp.Process("TEST");
    }
       
}

class DataProcessor<T>
{
    public void Process(T value) {
        Console.Write(value.GetType().Name);
    }
}

Answer: A

Here, the class level generic symbol is applied for the method parameter. So, the value must be of integer type because an object is declared with int type (DataProcessor<int> vp); otherwise C# compiler would give an error because it replaces the type in generic at the compile-time. Learn C# generics.

Learn C# generics.

Question 5: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {            
        Processor<int> p1 = new DataProcessor<int>();
        p1.Process(100);

        DataProcessor<string> p2 = new DataProcessor<string>();
        p2.Process("TEST");
    }

}

class Processor<T>
{
    public void Process(T value)
    {
        Console.Write(value.GetType().Name + “ “);
    }
}

class DataProcessor<U> : Processor<U>
{

}

Answer: C

It will display the type of the parameter value, and the value must be of the type specified at the time of instantiating objects p1 and p2. Learn C# generics.

Learn C# generics.

Question 6: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {
        Processor<int> p1 = new DataProcessor<int>();
        p1.Process(100);

        Processor<string> p2 = new DataProcessor<string>();
        p2.Process("TEST");
    }
}

class Processor<T>
{
    public void Process(T value)
    {
        Console.Write(value.GetType().Name + “ ”);
    }
}

class DataProcessor<U> : Processor<int>
{

}

Answer: A

Here, we specified the type explicitly when defining the derived class DataProcessor<U> : Processor<int>, so a variable of the Processor type cannot have other types than int.

Question 7: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {

        DataProcessor<string> p2 = new DataProcessor<string>();
        p2.Process(100);
    }
}

class Processor<T>
{
    public void Process(T value)
    {
        Console.Write(value.GetType().Name);
    }
}

class DataProcessor<U> : Processor<int>
{

}

Answer: C

It is perfectly correct to specify any type for the variable of DataProcessor type. However, the Process() method cannot accept value other than integer because class DataProcessor<U> : Processor<int>.

Question 8: What will be the output of the following program?

static void Main()
{
    var numbers = new List<string> { 1, "two", "three"};

    // Iterate through the list.  
    foreach (var num in numbers)
    {
        Console.Write(num + “ “);
    }            
}

Answer: C

Here, numbers is a generic List of string type, so adding an integer to it will give a compile-time error. Learn C# List<T>.

Question 9: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {
        Processor<int> p = new Processor<int>();
        p.BaseValue = 1;
        int result = p.Add(10);
            
        Console.Write(result);
    }
}

class Processor<T>
{        
    public int Add(int value) {
        return this.BaseValue + value;
    }
    public T BaseValue { get; set; }
}

Answer: A

Here, BaseValue is a generic type, so the + operator cannot be applied to increment the value because no special operation is invoked on the generic types. It will give an error: "Operator '+' cannot be applied to operands of type 'T' and 'int'“.

We cannot use methods or operators specific to a particular data type. Only common methods which are available to all the objects in C# can be invoked, such as GetType(), ToString(), etc.

Question 10: What will be the output of the following program?

class Program
{
    static void Main(string[] args)
    {
        Processor<int> p = new Processor<int>();
        p.Process(100);
    }
}

class Processor<T> where T:class
{
    public void Process(T value)
    {
        Console.Write(value);
    }
}

Answer: A

Here, a generic constraint is applied to the generic class Processor<T>. A constraint where T:class specifies that the type must be of reference type. Since int is not a reference type, it will give a compile-time error. If you specify a string as a type parameter, then it will work because the string is a reference type. Similarly, you can specify any built-in or custom class as a type parameter to the Processor class. Learn generic constraints. Learn generic constraints.