CalculateBMI(weight: 123, height: 64);
CalculateBMI(height: 64, weight: 123);

CalculateBMI(123, height: 64);

CalculateBMI(weight: 123, 64);

class NamedExample
{
  static void Main(string[] args)
  {
    // The method can be called in the normal way, by using positional arguments.
    Console.WriteLine(CalculateBMI(123, 64));

    // Named arguments can be supplied for the parameters in either order.
    Console.WriteLine(CalculateBMI(weight: 123, height: 64));
    Console.WriteLine(CalculateBMI(height: 64, weight: 123));

    // Positional arguments cannot follow named arguments.
    // The following statement causes a compiler error.
    //Console.WriteLine(CalculateBMI(weight: 123, 64));

    // Named arguments can follow positional arguments.
    Console.WriteLine(CalculateBMI(123, height: 64));
  }

  static int CalculateBMI(int weight, int height)
  {
    return (weight * 703) / (height * height);
  }
}

public void ExampleMethod(int required, string optionalstr = "default string",
  int optionalint = 10)

anExample.ExampleMethod(3, ,4);

anExample.ExampleMethod(3, optionalint: 4);

namespace OptionalNamespace
{
  class OptionalExample
  {
    static void Main(string[] args)
    {
      // Instance anExample does not send an argument for the constructor's
      // optional parameter.
      ExampleClass anExample = new ExampleClass();
      anExample.ExampleMethod(1, "One", 1);
      anExample.ExampleMethod(2, "Two");
      anExample.ExampleMethod(3);

      // Instance anotherExample sends an argument for the constructor's
      // optional parameter.
      ExampleClass anotherExample = new ExampleClass("Provided name");
      anotherExample.ExampleMethod(1, "One", 1);
      anotherExample.ExampleMethod(2, "Two");
      anotherExample.ExampleMethod(3);

      // The following statements produce compiler errors.

      // An argument must be supplied for the first parameter, and it
      // must be an integer.
      //anExample.ExampleMethod("One", 1);
      //anExample.ExampleMethod();

      // You cannot leave a gap in the provided arguments. 
      //anExample.ExampleMethod(3, ,4);
      //anExample.ExampleMethod(3, 4);

      // You can use a named parameter to make the previous 
      // statement work.
      anExample.ExampleMethod(3, optionalint: 4);
    }
  }

  class ExampleClass
  {
    private string _name;

    // Because the parameter for the constructor, name, has a default
    // value assigned to it, it is optional.
    public ExampleClass(string name = "Default name")
    {
      _name = name;
    }

    // The first parameter, required, has no default value assigned
    // to it. Therefore, it is not optional. Both optionalstr and 
    // optionalint have default values assigned to them. They are optional.
    public void ExampleMethod(int required, string optionalstr = "default string",
      int optionalint = 10)
    {
      Console.WriteLine("{0}: {1}, {2}, and {3}.", _name, required, optionalstr,
        optionalint);
    }
  }
}

Default name: 1, One, and 1.
Default name: 2, Two, and 10.
Default name: 3, default string, and 10.
Provided name: 1, One, and 1.
Provided name: 2, Two, and 10.
Provided name: 3, default string, and 10.
Default name: 3, default string, and 4.

// In C# 3.0 and earlier versions, you need to supply an argument for
// every parameter. The following call specifies a value for the first
// parameter, and sends a placeholder value for the other six. The
// default values are used for those parameters.
var excelApp = new Microsoft.Office.Interop.Excel.Application();
excelApp.Workbooks.Add();
excelApp.Visible = true;

var myFormat = 
  Microsoft.Office.Interop.Excel.XlRangeAutoFormat.xlRangeAutoFormatAccounting1;

excelApp.get_Range("A1", "B4").AutoFormat(myFormat, Type.Missing, 
  Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);

// The following code shows the same call to AutoFormat in C# 4.0. Only
// the argument for which you want to provide a specific value is listed.
excelApp.Range["A1", "B4"].AutoFormat( Format: myFormat );