Multi-parameter indexers in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a multi-parameter indexer that returns an integer.

C Sharp (C#)

public int this[int [1], int col] { get { return data[row, col]; } }

Drag options to blanks, or click blank then click option'

Arow

Bindex

Ci

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name that does not match the usage inside the getter.

Using a single parameter instead of two.

The indexer uses row as the first parameter name to access the data array.

2fill in blank

medium

Complete the code to set a value using the multi-parameter indexer.

C Sharp (C#)

public int this[int row, int [1]] { set { data[row, col] = value; } }

Drag options to blanks, or click blank then click option'

Acol

Bindex

Cpos

Dcolumn

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name that does not match the setter's code.

Using a single parameter instead of two.

The second parameter is named col to match the setter's usage.

3fill in blank

hard

Fix the error in the indexer declaration by completing the parameter list.

C Sharp (C#)

public string this[[1]] { get { return matrix[row, col]; } }

Drag options to blanks, or click blank then click option'

Aint index

Bint row, int col

Cstring key

Dint x, int y, int z

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one parameter for a 2D indexer.

Using incorrect parameter types.

The indexer requires two integer parameters to access the 2D matrix.

4fill in blank

hard

Fill both blanks to complete the multi-parameter indexer that sets a value in a 2D array.

C Sharp (C#)

public double this[[1], [2]] { set { grid[row, col] = value; } }

Drag options to blanks, or click blank then click option'

Aint row

Bint index

Cint col

Ddouble val

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-integer parameter types.

Using parameter names that do not match the setter.

The indexer parameters must be int row and int col to match the grid indices.

5fill in blank

hard

Fill all three blanks to complete the multi-parameter indexer with get and set accessors for a 3D array.

C Sharp (C#)

public char this[[1], [2], [3]] { get { return cube[x, y, z]; } set { cube[x, y, z] = value; } }

Drag options to blanks, or click blank then click option'

Aint x

Bint y

Cint z

Dchar c

Attempts:

3 left

💡 Hint

Common Mistakes

Using fewer than three parameters.

Using incorrect parameter types or names.

The indexer requires three integer parameters x, y, and z to access the 3D array.

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