Workshop 01

Workshop 01 - OOP

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Creating a new C# .Net solution

Install Microsoft Visual Studio Community: MS VS Downloads
Choose: Create New Project\Windows Forms App C#\.NET 8 or .NET 10 and name it UPBDotNet
In Solution Explorer, right click on solution and choose Add\New Project\Class Library C#\.NET 8 or .NET 10 and name it UPBLib
In Solution Explorer, right click on solution and choose Add\New Project\MSTtest Test Project C#\.NET 8 or .NET 10 and name it UPBTest
In Solution Explorer, right click on UPBDotNet\Dependencies Select Add Project Reference and check UpbLib
In Solution Explorer, right click on UPBTest\Dependencies Select Add Project Reference and check UpbLib

Creating a Matrix class

In Solution Explorer, right click on the UPBLib project, choose Add\Class, and name it Matrix.cs
In Matrix.cs, add a private "_matrix" array variable together with 2 overloaded constructors.
Observation: as opposed to private variables, which are visible only in the current class (not just current instance!), protected variables are also visible in derived classes.

private double[,] _matrix;

public Matrix(int rows, int cols)
{
	_matrix = new double[rows, cols];
	Rows = rows;
	Cols = cols;
}

public Matrix(int rows) : this(rows, rows){}

Important! Unlike linked lists, arrays store elements contiguously, allowing fast random access, but making insertions slower because elements need to be shifted.
In agreement with the encapsulation principle, private/protected variables should be accessed through dedicated accessor functions:

private int _data;

public int GetData()
{
	return _data;
}

public void SetData(int value)
{
	_data = value;
}

Thus, it is possible to provide only the "get" methods in order to implement read-only properties. In C# .Net the syntax is significantly simplified:

public int Rows { get; private set; }
public int Cols { get; private set; }

Implement an indexer for accessing matrix elements

public double this[int row, int col]
{
	get
	{
		return _matrix[row, col];
	}
	set
	{
		_matrix[row, col] = value;
	}
}

In .Net C# any class inherits Object by default
Override the ToString() virtual method (for performance reasons, use a StringBuilder class instance for string concatenation):

public override string ToString()
{
    var sb = new StringBuilder();
    string line;
    for (int row = 0; row < Rows; row++)
    {
        line = "";
        var separator = "";
        for (int col = 0; col < Cols; col++)
        {
            line += $"{separator}{this[row, col]}";
            separator = ",";
        }
        sb.AppendLine(line);
    }
    return sb.ToString();
}

Important! Multiple inheritance is not allowed in .Net. However, one could implement any number of interfaces. I would name interfaces "pure abstract classes". While abstract classes may contain both functions with implementation and prototypes, interfaces contain only functions/properties definitions. An interface defines "a contract" that classes must conform to.

Implement matrix multiplication by overloading the * operator. An exception is thrown when incorrect arguments are provided.

public static Matrix operator *(Matrix m1, Matrix m2)
{
	if (m1.Cols != m2.Rows) throw new ArgumentException();
	var m = new Matrix(m1.Rows, m2.Cols);
	double sum;
	for (int row = 0; row < m.Rows; row++)
	{
		for (int col = 0; col < m.Cols; col++)
		{
			sum = 0;
			for (int index = 0; index < m1.Cols; index++)
			{
				sum += m1[row, index] * m2[index, col];
			}
			m[row, col] = sum;
		}
	}
	return m;
}

For implementing a custom exception one could inherit the Exception class:

public class CustomException : Exception
{
	public CustomException(string? message) 
		: base(message) 
	{ }
}

Test the previously created Matrix class

In UPBDotNet project, open the form designer, select Button from Toolbox, and "draw" a new button on the form
Select the previously created button and, in properties window, change its name to buttonOK

Double click on buttonOK in order to write code for its corresponding click event
Add first the necessary using statement at the top of the cs file (as Matrix is defined in the UPBLib class library project): using UPBLib;
Use try/catch for exception handling

 private void buttonOK_Click(object sender, EventArgs e)
 {
	 try
	 {
		 var m = new Matrix(2, 2);
		 m[0, 0] = 15;
		 m[0, 1] = 20;
		 m[1, 0] = 34;
		 m[1, 1] = 75;
		 MessageBox.Show(m.ToString());
	 }
	 catch (Exception ex)
	 {
		 MessageBox.Show(ex.Message);
	 }
 }

Run the UPBDotNet project

References:

Microsoft C# language documentation

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