Web-Based Calculator

The PHP frontend builds the expression string and calls PHP's system() to spawn a compiled C# .exe as a subprocess. The .exe evaluates the expression using ADO.NET's DataTable column expression parser — a rarely-seen trick that turns SQL expression evaluation into a math engine.

// PHP (jQuery) — build expression and POST to backend
$.post("index.php", { var_value: path + " " + field.text() });

// PHP backend — execute the compiled C# .exe as a subprocess
$result = system($_POST['var_value']);

// C# — evaluate the expression using ADO.NET's DataTable parser
public static double Evaluate(string expression)
{
    DataTable table = new DataTable();
    table.Columns.Add("expression", typeof(string), expression);
    DataRow row = table.NewRow();
    table.Rows.Add(row);
    return double.Parse((string)row["expression"]);
}

The Windows Presentation Foundation version uses INotifyPropertyChanged for two-way data binding between the button grid and the display label — the same MVVM binding pattern used in enterprise .NET desktop applications.

public string LabelText
{
    get { return labelText; }
    set
    {
        labelText = value;
        RaisePropertyChanged("LabelText");
    }
}

public event PropertyChangedEventHandler PropertyChanged;

private void RaisePropertyChanged(string property)
{
    if (PropertyChanged != null)
        PropertyChanged(this, new PropertyChangedEventArgs(property));
}

This demo reimplements the same calculator as a React client component, evaluating the expression with JavaScript's Function constructor in a try/catch to safely handle invalid input.

function evaluate(expr: string): string {
  try {
    const result = Function('return ' + expr)()
    return String(result)
  } catch {
    return 'Error'
  }
}