Generic method declaration in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a generic method call

What is the output of this C# program that uses a generic method to print the type and value?

C Sharp (C#)

using System;
class Program {
    static void PrintTypeAndValue<T>(T value) {
        Console.WriteLine($"Type: {typeof(T)}, Value: {value}");
    }
    static void Main() {
        PrintTypeAndValue(123);
        PrintTypeAndValue("hello");
    }
}

A Type: System.Object, Value: 123\nType: System.Object, Value: hello

B Type: T, Value: 123\nType: T, Value: hello

C Compilation error due to missing generic constraints

D Type: System.Int32, Value: 123\nType: System.String, Value: hello

💡 Hint

Remember that the generic type T is inferred from the argument type.

The generic method PrintTypeAndValue infers T from the argument type. For 123, T is int (System.Int32). For "hello", T is string (System.String). So the output shows the actual types and values.

🧠 Conceptual

intermediate

Understanding generic method constraints

Which statement about generic method constraints in C# is true?

A Generic method constraints can only specify that T must be a value type.

B You can constrain a generic method type parameter to be a reference type using 'where T : class'.

C Constraints cannot be combined; you can only use one constraint per generic parameter.

D You must always specify a constraint for every generic method parameter.

💡 Hint

Think about how you restrict types to reference types or value types.

In C#, you can use 'where T : class' to restrict T to reference types. You can also use 'where T : struct' for value types. Multiple constraints can be combined. Constraints are optional.

❓ Predict Output

advanced

Output of generic method with multiple type parameters

What is the output of this C# program using a generic method with two type parameters?

C Sharp (C#)

using System;
class Program {
    static void Swap<T, U>(ref T a, ref U b) {
        Console.WriteLine($"Before swap: a={a}, b={b}");
        var temp = a;
        a = (T)(object)b;
        b = (U)(object)temp;
        Console.WriteLine($"After swap: a={a}, b={b}");
    }
    static void Main() {
        int x = 5;
        string y = "10";
        Swap(ref x, ref y);
    }
}

A Runtime InvalidCastException

B Compilation error due to invalid cast

C Before swap: a=5, b=10\nAfter swap: a=5, b=10

D Before swap: a=5, b=10\nAfter swap: a=10, b=5

💡 Hint

Casting between unrelated types using (T)(object) can cause runtime errors.

The cast (T)(object)b tries to cast string "10" to int, which causes a runtime InvalidCastException.

🔧 Debug

advanced

Identify the error in generic method declaration

What error does this C# code produce?

C Sharp (C#)

class Program {
    static void PrintValue<T>() {
        Console.WriteLine(value);
    }
    static void Main() {
        PrintValue<int>();
    }
}

A Runtime error: NullReferenceException

B Compilation error: Missing generic type constraint

C Compilation error: 'value' does not exist in the current context

D No error, prints default value

💡 Hint

Check if the variable 'value' is declared or passed.

The method tries to print 'value' but no variable named 'value' is declared or passed, causing a compilation error.

🧠 Conceptual

expert

Generic method type inference behavior

Given the generic method declaration

void Display(T item) { Console.WriteLine(item); }

and the call

Display(null);

What happens when you compile this code in C#?

A Compilation error: Cannot infer type for T because null is ambiguous

B Compiles and prints an empty line

C Compiles and prints 'null'

D Runtime error: NullReferenceException

💡 Hint

Think about how the compiler infers generic types from null arguments.

The compiler cannot infer the type parameter T from a null literal alone because null can be any reference type, so it causes a compilation error.