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.

Practice

1. What is the main purpose of a multi-parameter indexer in C#?

easy

A. To allow accessing elements using multiple keys like a table or matrix.

B. To create a method that returns multiple values.

C. To define multiple constructors in a class.

D. To overload operators for custom classes.

Solution

1. Step 1: Understand what an indexer does

   An indexer lets you access elements in a class like an array using keys.

2. Step 2: Identify the role of multiple parameters

   Multi-parameter indexers allow using more than one key, like row and column in a table.

3. Final Answer:

   To allow accessing elements using multiple keys like a table or matrix. -> Option A

4. Quick Check:

   Multi-parameter indexer = multiple keys access [OK]

Hint: Think of multi-parameter indexers as multi-key access [OK]

Common Mistakes:

• Confusing indexers with methods returning multiple values
• Thinking indexers define constructors
• Mixing indexers with operator overloading

2. Which of the following is the correct syntax to declare a multi-parameter indexer in C#?

easy

A. public int this[int x; int y] { get { return 0; } set { } }

B. public int this(int x, int y) { get { return 0; } set { } }

C. public int this[int x][int y] { get { return 0; } set { } }

D. public int this[int x, int y] { get { return 0; } set { } }

Solution

1. Step 1: Recall indexer syntax

   Indexers use the keyword this followed by square brackets with parameters separated by commas.

2. Step 2: Check each option

   public int this[int x, int y] { get { return 0; } set { } } uses square brackets with comma-separated parameters, which is correct syntax.

3. Final Answer:

   public int this[int x, int y] { get { return 0; } set { } } -> Option D

4. Quick Check:

   Indexer syntax uses square brackets with commas [OK]

Hint: Use square brackets with commas for multiple parameters [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Separating parameters with semicolons
• Trying to nest square brackets for parameters

3. What will be the output of this code snippet?

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

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

medium

A. 4

B. 2

C. 3

D. 1

Solution

1. Step 1: Understand the data array

   The 2D array data is {{1,2},{3,4}}. The first index is row, second is column.

2. Step 2: Access element at m[1, 0]

   Row 1, column 0 is the second row, first column, which is 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   m[1,0] = 3 [OK]

Hint: Remember: first index = row, second = column [OK]

Common Mistakes:

• Mixing row and column indices
• Confusing array initialization syntax
• Assuming zero-based indexing is off

4. Identify the error in this multi-parameter indexer declaration:

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

medium

A. Missing return type for the indexer.

B. Using semicolon instead of comma between parameters in indexer.

C. Data array should be one-dimensional for indexers.

D. Indexer must be static.

Solution

1. Step 1: Check parameter separator in indexer

   Parameters in indexers must be separated by commas, not semicolons.

2. Step 2: Verify other parts

   Return type is present, data array can be multi-dimensional, indexers are instance members by default.

3. Final Answer:

   Using semicolon instead of comma between parameters in indexer. -> Option B

4. Quick Check:

   Parameters separated by commas, not semicolons [OK]

Hint: Separate indexer parameters with commas, not semicolons [OK]

Common Mistakes:

• Using semicolons instead of commas
• Thinking indexers must be static
• Confusing array dimensions with indexer parameters

5. You want to create a class that stores grades for students in different subjects using a multi-parameter indexer. Which implementation correctly allows setting and getting grades by student name and subject?

hard

A. public class Grades {
private Dictionary<(string, string), int> data = new();
public int this[string student, string subject] {
get => data[(student, subject)];
set => data[(student, subject)] = value;
}
}

B. public class Grades {
private Dictionary data = new();
public int this[string student, string subject] {
get => data[student + subject];
set => data[student + subject] = value;
}
}

C. public class Grades {
private Dictionary data = new();
public int this[string student] {
get => data[student];
set => data[student] = value;
}
}

D. public class Grades {
private Dictionary<(string, string), int> data = new();
public int this[string student; string subject] {
get => data[(student, subject)];
set => data[(student, subject)] = value;
}
}

Solution

1. Step 1: Understand the data structure needed

   We need a dictionary with a tuple key (student, subject) to store grades.

2. Step 2: Check indexer syntax and dictionary usage

   public class Grades { private Dictionary<(string, string), int> data = new(); public int this[string student, string subject] { get => data[(student, subject)]; set => data[(student, subject)] = value; } } uses a tuple key dictionary and correct indexer syntax with comma-separated parameters.

3. Step 3: Identify errors in other options

   public class Grades { private Dictionary data = new(); public int this[string student, string subject] { get => data[student + subject]; set => data[student + subject] = value; } } concatenates keys as string which can cause collisions; C uses only one parameter; D uses semicolon instead of comma in indexer.

4. Final Answer:

   public class Grades { private Dictionary<(string, string), int> data = new(); public int this[string student, string subject] { get => data[(student, subject)]; set => data[(student, subject)] = value; } } -> Option A

5. Quick Check:

   Tuple keys + comma-separated indexer = correct [OK]

Hint: Use tuple keys and commas in indexer parameters [OK]

Common Mistakes:

• Concatenating keys as strings causing collisions
• Using semicolons instead of commas in indexer
• Using single-parameter indexer for multi-key data