Program.cs

// Ref: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-directive#static-modifier

using static System.Console;
using static System.Math;
using static System.String;

class Program
{
    static void Main()
    {
        Write("Enter a circle's radius: ");
        var input = ReadLine();
        if (!IsNullOrEmpty(input) && double.TryParse(input, out var radius))
        {
            var c = new Circle(radius);

            string s = "\nInformation about the circle:\n";
            s = s + Format("   Radius: {0:N2}\n", c.Radius);
            s = s + Format("   Diameter: {0:N2}\n", c.Diameter);
            s = s + Format("   Circumference: {0:N2}\n", c.Circumference);
            s = s + Format("   Area: {0:N2}\n", c.Area);
            WriteLine(s);
        }
        else
        {
            WriteLine("Invalid input...");
        }
    }
}

public class Circle
{
    public Circle(double radius)
    {
        Radius = radius;
    }

    public double Radius { get; set; }

    public double Diameter
    {
        get { return 2 * Radius; }
    }

    public double Circumference
    {
        get { return 2 * Radius * PI; }
    }

    public double Area
    {
        get { return PI * Pow(Radius, 2); }
    }
}
// The example displays the following output:
//       Enter a circle's radius: 12.45
//
//       Information about the circle:
//          Radius: 12.45
//          Diameter: 24.90
//          Circumference: 78.23
//          Area: 486.95