Program.cs

using System.Reflection;

namespace Delegates;

class Program
{
    static void Main()
    {
    }

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

    private static bool IsGreaterThanZero(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 = IsGreaterThanZero;

        Console.WriteLine(p(42));
    }

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

        Console.WriteLine(p(42));
    }

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

        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 = (Predicate<int>)m.CreateDelegate(
            typeof(Predicate<int>), zeroThreshold);

        Console.WriteLine(greaterThanZero(42));
    }

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

    public static void TestForMajority(Predicate<int> userCallbacks)
    {
        int trueCount = 0;
        int falseCount = 0;
        foreach (Predicate<int> p in userCallbacks.GetInvocationList())
        {
            bool result = p(42);
            if (result)
            {
                trueCount += 1;
            }
            else
            {
                falseCount += 1;
            }
        }
        if (trueCount > falseCount)
        {
            Console.WriteLine("The majority returned true");
        }
        else if (falseCount > trueCount)
        {
            Console.WriteLine("The majority returned false");
        }
        else
        {
            Console.WriteLine("It's a tie");
        }
    }
}