Fluent interface with extensions in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple fluent interface chain

What is the output of this C# code using a fluent interface?

C Sharp (C#)

public class Builder {
    private string text = "";
    public Builder AddHello() {
        text += "Hello";
        return this;
    }
    public Builder AddSpace() {
        text += " ";
        return this;
    }
    public Builder AddWorld() {
        text += "World";
        return this;
    }
    public override string ToString() => text;
}

var result = new Builder().AddHello().AddSpace().AddWorld().ToString();
System.Console.WriteLine(result);

A Hello World

B HelloWorld

C dlroW olleH

D Hello World

💡 Hint

Look at how spaces are added in the chain.

The methods AddHello, AddSpace, and AddWorld add strings and return the same object, allowing chaining. The space is added explicitly by AddSpace, so the output is "Hello World" exactly.

🧠 Conceptual

intermediate

Purpose of extension methods in fluent interfaces

Why are extension methods useful when building fluent interfaces in C#?

A They automatically generate UI elements for the fluent interface.

B They allow adding new chainable methods to existing classes without modifying them.

C They prevent the fluent interface from being used outside the assembly.

D They convert synchronous methods into asynchronous ones.

💡 Hint

Think about how you can add methods to a class you cannot change.

Extension methods let you add new methods to existing classes, enabling fluent chaining without changing the original class code.

🔧 Debug

advanced

Identify the error in this fluent interface extension method

What error will this code cause when compiling?

public static class BuilderExtensions {
    public static Builder AddExclamation(this Builder builder) {
        builder.ToString() += "!";
        return builder;
    }
}

A Cannot assign to method call result (ToString() returns string, which is immutable).

B Extension methods cannot return the original object.

C Missing semicolon after return statement.

D Builder class must be static to use extension methods.

💡 Hint

Consider what ToString() returns and if it can be assigned to.

ToString() returns a string value, which is immutable and cannot be assigned to. The code tries to append "!" to the result of ToString(), which is invalid.

📝 Syntax

advanced

Correct syntax for a fluent extension method

Which option shows the correct syntax for a fluent extension method that adds a period at the end of the Builder text?

A static Builder AddPeriod(Builder builder) { builder.text += "."; return builder; }

B public Builder AddPeriod(this Builder builder) { builder.text += "."; return builder; }

C public static Builder AddPeriod(this Builder builder) { builder.text += "."; return builder; }

D public static void AddPeriod(this Builder builder) { builder.text += "."; }

💡 Hint

Remember extension methods must be static and inside static classes.

Extension methods must be static methods inside static classes, with the first parameter preceded by 'this'. The method should return the builder to allow chaining.

🚀 Application

expert

Result of chaining multiple fluent extension methods

Given the following code, what is the final output?

public class Builder {
    private string text = "Start";
    public Builder Append(string s) {
        text += s;
        return this;
    }
    public override string ToString() => text;
}

public static class BuilderExtensions {
    public static Builder AddDash(this Builder b) => b.Append("-");
    public static Builder AddStar(this Builder b) => b.Append("*");
    public static Builder AddPlus(this Builder b) => b.Append("+");
}

var result = new Builder()
    .AddDash()
    .AddStar()
    .AddPlus()
    .AddDash()
    .ToString();
System.Console.WriteLine(result);

A Start-*+-

B Start-+*-

C Start-*+

D Start-*-+

💡 Hint

Follow the chain step by step, appending each symbol in order.

The Builder starts with "Start". Each extension method appends its symbol in the order called: AddDash adds '-', AddStar adds '*', AddPlus adds '+', then AddDash adds '-' again. So the final string is "Start-*+-".