class NamedExample
{
    static void Main(string[] args)
    {
        // The usual way
        PrintOrderDetails("Gift Shop", 31, "Red Mug");

        // Named arguments can be supplied for the parameters in any order.
        PrintOrderDetails(orderNum: 31, productName: "Red Mug", sellerName: "Gift Shop");

        // Named arguments mixed with positional arguments are valid
        // as long as they are used in their correct position.
        PrintOrderDetails("Gift Shop", 31, productName: "Red Mug");
        PrintOrderDetails(sellerName: "Gift Shop", 31, productName: "Red Mug"); // C# 7.2+
        PrintOrderDetails("Gift Shop", orderNum: 31, "Red Mug");                // C# 7.2+

        // The following statements will cause a compiler error.
        // PrintOrderDetails(productName: "Red Mug", 31, "Gift Shop");
    }

    static void PrintOrderDetails(string sellerName, int orderNum, string productName)
    {
        // some code here
    }
}

class OptionalExample
{
    static void Main(string[] args)
    {
        PrintOrderDetails("FIRMA s.r.o.", 31);
        
        PrintOrderDetails("FIRMA s.r.o.", 31, "Fakturujeme Vam na zaklade dohody");
    }

    static void PrintOrderDetails(string companyName, int orderNum, 
                                  string additionalInfo = "Fakturujeme Vam na zaklade objednavky:")
    {
        // some code here
    }
}

public delegate void MessageDelegate(string message);

// Create a method for a delegate.
public static void DelegateMethod(string message)
{
    Console.WriteLine(message);
}

// Instantiate the delegate.
Del handler = DelegateMethod;

// Call the delegate.
handler("Hello World");


// Usual usecase
public static void MethodWithCallback(int param1, int param2, Del callback)
{
    callback("The number is: " + (param1 + param2).ToString());
}

// And one nice usage with delegate arithmetics

var obj = new MethodClass();
Del d1 = obj.Method1;
Del d2 = obj.Method2;
Del d3 = DelegateMethod;

//Both types of assignment are valid.
Del allMethodsDelegate = d1 + d2;
allMethodsDelegate += d3;

Func<interest,double> calcSI = SIObj =>(SIObj.P*SIObj.T*SIObj.R)/100;

Interest obj = new Interest() { P = 120, T = 1, R = 3.25};
// Consuming delegate
var si = calcSI(obj);

Action<string> MyAction = y => Console.Write(y);
MyAction("Hello");

Predicate<string> checkValidDate = d => IsDate(d);

class Example
{
    static void Main()
    {
        string contents = GetText(@"C:\temp", "example.txt");
        Console.WriteLine("Contents of the file:\n" + contents);
    }
   
    private static string GetText(string path, string filename)
    {
         var sr = File.OpenText(AppendPathSeparator(path) + filename);
         var text = sr.ReadToEnd();
         return text;
         
         // Declare a local function.
         string AppendPathSeparator(string filepath)
         {
            if (! filepath.EndsWith(@"\"))
               filepath += @"\";

            return filepath;   
         }
    } 
}

public event EventHandler<FileListArgs> Progress;


EventHandler<FileListArgs> onProgress = (sender, eventArgs) =>
    Console.WriteLine(eventArgs.FoundFile);

fileLister.Progress += onProgress;

public class FileFoundArgs : EventArgs
{
    public string FoundFile { get; }

    public FileFoundArgs(string fileName)
    {
        FoundFile = fileName;
    }
}

public class FileSearcher
{
    public event EventHandler<FileFoundArgs> FileFound;

    public void Search(string directory, string searchPattern)
    {
        foreach (var file in Directory.EnumerateFiles(directory, searchPattern))
        {
            FileFound?.Invoke(this, new FileFoundArgs(file));
        }
    }
}

EventHandler<FileFoundArgs> onFileFound = (sender, eventArgs) =>
{
    Console.WriteLine(eventArgs.FoundFile);
    eventArgs.CancelRequested = true;
};

namespace ExtensionMethods
{
    public static class MyExtensions
    {
        public static int WordCount(this String str)
        {
            return str.Split(new char[] { ' ', '.', '?' }, 
                             StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }   
}

string s = "Hello Extension Methods";  
int i = s.WordCount();