event.cs

using System;
class Account
{
    public delegate void AccountHandler(string message);
    public event AccountHandler Notify;              // 1.Определение события
    public Account(int sum)
    {
        Sum = sum;
    }
    public int Sum { get; private set;}
    public void Put(int sum)    
    {
        Sum += sum;
        Notify?.Invoke($"На счет поступило: {sum}");   // 2.Вызов события 
    }
    public void Take(int sum)
    {
        if (Sum >= sum)
        {
            Sum -= sum;
            Notify?.Invoke($"Со счета снято: {sum}");   // 2.Вызов события
        }
        else
        {
            Notify?.Invoke($"Недостаточно денег на счете. Текущий баланс: {Sum}"); ;
        }
    }
}

​event.decompiled.cs

using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Security;
using System.Security.Permissions;
using System.Threading;

[assembly: CompilationRelaxations(8)]
[assembly: RuntimeCompatibility(WrapNonExceptionThrows = true)]
[assembly: Debuggable(DebuggableAttribute.DebuggingModes.Default | DebuggableAttribute.DebuggingModes.DisableOptimizations | DebuggableAttribute.DebuggingModes.IgnoreSymbolStoreSequencePoints | DebuggableAttribute.DebuggingModes.EnableEditAndContinue)]
[assembly: SecurityPermission(SecurityAction.RequestMinimum, SkipVerification = true)]
[assembly: AssemblyVersion("0.0.0.0")]
[module: UnverifiableCode]
internal class Account
{
    public delegate void AccountHandler(string message);

    [CompilerGenerated]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private AccountHandler m_Notify;

    [CompilerGenerated]
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    private int <Sum>k__BackingField;

    public int Sum
    {
        [CompilerGenerated]
        get
        {
            return <Sum>k__BackingField;
        }
        [CompilerGenerated]
        private set
        {
            <Sum>k__BackingField = value;
        }
    }

    public event AccountHandler Notify
    {
        [CompilerGenerated]
        add
        {
            AccountHandler accountHandler = this.Notify;
            while (true)
            {
                AccountHandler accountHandler2 = accountHandler;
                AccountHandler value2 = (AccountHandler)Delegate.Combine(accountHandler2, value);
                accountHandler = Interlocked.CompareExchange(ref this.Notify, value2, accountHandler2);
                if ((object)accountHandler == accountHandler2)
                {
                    break;
                }
            }
        }
        [CompilerGenerated]
        remove
        {
            AccountHandler accountHandler = this.Notify;
            while (true)
            {
                AccountHandler accountHandler2 = accountHandler;
                AccountHandler value2 = (AccountHandler)Delegate.Remove(accountHandler2, value);
                accountHandler = Interlocked.CompareExchange(ref this.Notify, value2, accountHandler2);
                if ((object)accountHandler == accountHandler2)
                {
                    break;
                }
            }
        }
    }

    public Account(int sum)
    {
        Sum = sum;
    }

    public void Put(int sum)
    {
        Sum += sum;
        AccountHandler notify = this.Notify;
        if (notify != null)
        {
            notify(string.Format("На счет поступило: {0}", sum));
        }
    }

    public void Take(int sum)
    {
        if (Sum >= sum)
        {
            Sum -= sum;
            AccountHandler notify = this.Notify;
            if (notify != null)
            {
                notify(string.Format("Со счета снято: {0}", sum));
            }
        }
        else
        {
            AccountHandler notify2 = this.Notify;
            if (notify2 != null)
            {
                notify2(string.Format("Недостаточно денег на счете. Текущий баланс: {0}", Sum));
            }
        }
    }
}

​​.sharplab.json

{
    "version": 1,
    "target": "C#",
    "mode": "Debug"
}