Consider this simple class:

class SmsUtil
{
    public bool SendMessage(string from, string to, string message, int retryCount, object attachment)
    {
         // Some code
    }
}

Before C# 3.0 it was:

var result = SmsUtil.SendMessage("Mehran", "Maryam", "Hello there!", 12, null);

you can make this method call even more clear with named arguments:

var result = SmsUtil.SendMessage(
    from: "Mehran",
    to:  "Maryam",
    message "Hello there!",
    retryCount: 12,
    attachment: null);

You can combine named arguments with optional parameters.

Let see this method:

public sealed class SmsUtil
{
    public static bool SendMessage(string from, string to, string message, int retryCount = 5, object attachment = null)
    {
         // Some code
    }
}

When you want to call this method without set retryCount argument :

var result = SmsUtil.SendMessage(
                        from       : "Cihan",
                        to         : "Yakar",
                        message    : "Hello there!",
                        attachment : new object());

You can place named arguments in any order you want.

Sample Method:

public static string Sample(string left, string right)
{
     return string.Join("-",left,right);
}

Call Sample:

Console.WriteLine (Sample(left:"A",right:"B"));
Console.WriteLine (Sample(right:"A",left:"B"));

Results:

A-B
B-A

Always use Named Arguments to optional parameters, to avoid potential bugs when the method is modified.

class Employee
{
    public string Name { get; private set; }

    public string Title { get; set; }

    public Employee(string name = "<No Name>", string title = "<No Title>")
    {
        this.Name = name;
        this.Title = title;
    }
}

var jack = new Employee("Jack", "Associate");   //bad practice in this line

The above code compiles and works fine, until the constructor is changed some day like:

//Evil Code: add optional parameters between existing optional parameters
public Employee(string name = "<No Name>", string department = "intern", string title = "<No Title>")
{
    this.Name = name;
    this.Department = department;
    this.Title = title;
}

//the below code still compiles, but now "Associate" is an argument of "department"
var jack = new Employee("Jack", "Associate");

Best practice to avoid bugs when "someone else in the team" made mistakes:

var jack = new Employee(name: "Jack", title: "Associate");