Program.cs

using System.Reflection;

namespace Delegates;

class Program
{
    static void Main()
    {
    }

    public static int GetIndexOfFirstNonEmptyBin(int[] bins)
        => Array.FindIndex(bins, IsNonZero);

    private static bool IsNonZero(int value) => value != 0;

    // These examples illustrates how various types and members declared. Since these are all
    // defined by .NET, we don't in fact need to define them ourselves, hence the #if false.
#if false
    public static int FindIndex<T>(
          T[] array,
          Predicate<T> match)

    public delegate bool Predicate<in T>(T obj);

    public delegate void Action();
    public delegate void Action<in T1>(T1 arg1);
    public delegate void Action<in T1, in T2 >(T1 arg1, T2 arg2);
    public delegate void Action<in T1, in T2, in T3>(T1 arg1, T2 arg2, T3 arg3);

    public delegate TResult Func<out TResult>();
    public delegate TResult Func<in T1, out TResult>(T1 arg1);
    public delegate TResult Func<in T1, in T2, out TResult>(T1 arg1, T2 arg2);
    public delegate TResult Func<in T1, in T2, in T3, out TResult>(
        T1 arg1, T2 arg2, T3 arg3);

    public Predicate(object target, IntPtr method);

    public bool Invoke(T obj);

    public IAsyncResult BeginInvoke(T obj, AsyncCallback callback, object state);
    public bool EndInvoke(IAsyncResult result);
#endif

    public static void CreatingADelegate()
    {
        var p = IsNonZero;

        Console.WriteLine(p(42));
    }

    public static void ConstructingADelegate()
    {
        var p = new Predicate<int>(IsNonZero);

        Console.WriteLine(p(42));
    }

    public static void ImplicitDelegateCreation()
    {
        Predicate<int> p = IsNonZero;

        Console.WriteLine(p(42));
    }

    public static void ExplicitInstance()
    {
        var zeroThreshold = new ThresholdComparer { Threshold = 0 };
        var tenThreshold = new ThresholdComparer { Threshold = 10 };
        var hundredThreshold = new ThresholdComparer { Threshold = 100 };

        Predicate<int> greaterThanZero = zeroThreshold.IsGreaterThan;
        Predicate<int> greaterThanTen = tenThreshold.IsGreaterThan;
        Predicate<int> greaterThanOneHundred = hundredThreshold.IsGreaterThan;

        Console.WriteLine(greaterThanZero(42));
        Console.WriteLine(greaterThanTen(42));
        Console.WriteLine(greaterThanOneHundred(42));

        Predicate<int> megaPredicate1 =
            greaterThanZero + greaterThanTen + greaterThanOneHundred;

        Predicate<int> megaPredicate2 = greaterThanZero;
        megaPredicate2 += greaterThanTen;
        megaPredicate2 += greaterThanOneHundred;
    }

    public static void UsingCreateDelegate()
    {
        var zeroThreshold = new ThresholdComparer { Threshold = 0 };

        MethodInfo m = typeof(ThresholdComparer).GetMethod("IsGreaterThan")!;
        var greaterThanZero = m.CreateDelegate<Predicate<int>>(zeroThreshold);

        Console.WriteLine(greaterThanZero(42));
    }

    public static void CallMeRightBack(Predicate<int> userCallback)
    {
        bool result = userCallback(42);
        Console.WriteLine(result);
    }
}

public delegate void Log(string message, string? source = "");