Multi-parameter indexers in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of multi-parameter indexer access

What is the output of the following C# code using a multi-parameter indexer?

C Sharp (C#)

class Matrix {
    private int[,] data = new int[2, 2] { {1, 2}, {3, 4} };
    public int this[int row, int col] {
        get { return data[row, col]; }
        set { data[row, col] = value; }
    }
}

var m = new Matrix();
Console.WriteLine(m[1, 0]);

A 2

B 4

C 1

D 3

💡 Hint

Remember that the first index is the row and the second is the column.

The matrix is initialized as {{1, 2}, {3, 4}}. Accessing m[1, 0] means row 1, column 0, which is 3.

🧠 Conceptual

intermediate

Understanding multi-parameter indexer syntax

Which of the following correctly declares a multi-parameter indexer in C# that takes two integers and returns a string?

A public string this[int x, int y] { get { return ""; } }

B public string this[int x; int y] { get { return ""; } }

C public string this[int x][int y] { get { return ""; } }

D public string this(int x, int y) { get { return ""; } }

💡 Hint

Indexers use square brackets with comma-separated parameters.

Option A uses the correct syntax: this[int x, int y]. Option A uses invalid syntax with multiple brackets. Option A uses semicolon instead of comma. Option A uses parentheses which is invalid for indexers.

🔧 Debug

advanced

Identify the error in multi-parameter indexer implementation

What error will the following code produce?

C Sharp (C#)

class Grid {
    private int[,] values = new int[3,3];
    public int this[int x, int y] {
        get { return values[x]; }
        set { values[x, y] = value; }
    }
}

A No error, code compiles and runs

B Runtime IndexOutOfRangeException

C Compile-time error: Cannot convert int[] to int

D Compile-time error: Missing set accessor

💡 Hint

Check the getter's return statement and the type of values.

The getter tries to return values[x], but values is a 2D array and values[x] returns an int[], which cannot be converted to int. This causes a compile-time error.

❓ Predict Output

advanced

Output of multi-parameter indexer with set and get

What will be printed after running this code?

C Sharp (C#)

class Table {
    private string[,] data = new string[2, 2];
    public string this[int r, int c] {
        get => data[r, c];
        set => data[r, c] = value.ToUpper();
    }
}

var t = new Table();
t[0, 1] = "hello";
Console.WriteLine(t[0, 1]);

A hello

B HELLO

C Hello

D null

💡 Hint

Look at how the setter modifies the value before storing.

The setter converts the string to uppercase before storing it. So when we get t[0,1], it returns "HELLO".

🧠 Conceptual

expert

Behavior of multi-parameter indexers with different parameter types

Given the following indexer declaration, which statement is true about its usage? public int this[string key, int index] { get { return index + key.Length; } }

A You can access it like obj["abc", 2] and it returns 5

B You must pass two strings as parameters

C It only accepts integer parameters

D It cannot be used because indexers require parameters of the same type

💡 Hint

Check the parameter types and how the indexer is accessed.

The indexer takes a string and an int. Accessing obj["abc", 2] passes "abc" and 2. The getter returns 2 + 3 (length of "abc") = 5.