C# Online Compiler | .NET Fiddle

using System;
using System.Threading.Tasks;
using System.IO;
					
public class Program
{
	public static void Main()
	{
		//HandlingExceptionFromSingleTask();
		//UsingExceptionProperty();
		//HandlingExceptionFromMultipleTasks();
		//HandlingExceptionFromAttachedNestedTask();
		//FlatteningExceptionFromAttachedNestedTask();
		//HandlingExceptionFromDetachedNestedTask();
		UsingAggregateExceptionHandle();
	}
	
	private static void HandlingExceptionFromSingleTask() 
	{
		var task = Task.Run(() => throw new Exception("An exception occured"));
		try
		{
			task.Wait();
		}
		catch(AggregateException ex)
		{
			Console.WriteLine(ex.Message);
			Console.WriteLine(ex.InnerExceptions[0].Message);
		}
	}
	
	private static void UsingExceptionProperty() 
	{
		var task = Task.Run(() => throw new Exception("An exception occured"));
		while(!task.IsCompleted) {}
		
		if(task.Status == TaskStatus.Faulted) 
		{
			Console.WriteLine(task.Exception.InnerExceptions[0].Message);
		}
	}
	
	private static void HandlingExceptionFromMultipleTasks() 
	{
		Task[] taskArray = {
			Task.Run(() => throw new Exception("An exception occured from task 1")),
			Task.Run(() => throw new Exception("An exception occured from task 2")),
			Task.Run(() => throw new Exception("An exception occured from task 3"))
		};
		
		try
		{
			Task.WaitAll(taskArray);
		}
		catch(AggregateException ex)
		{
			foreach(var e in ex.InnerExceptions) {
				Console.WriteLine(e.Message);
			}
		}
	}
	
	private static void HandlingExceptionFromAttachedNestedTask() 
	{
		var task = Task.Factory.StartNew(() => {
			var childTask1 = Task.Factory.StartNew(()=> {
				var childTask2 = Task.Factory.StartNew(() =>  throw new Exception("Exception from child2 task"), TaskCreationOptions.AttachedToParent);
				
				throw new Exception("Exception from child1 task");
			}, TaskCreationOptions.AttachedToParent);
		});
		
		try
		{
			task.Wait();
		}
		catch(AggregateException ex)
		{
			Console.WriteLine(ex.InnerExceptions[0].Message);
			//child1
			Console.WriteLine(((AggregateException)ex.InnerExceptions[0]).InnerExceptions[0].Message);
		}
	}
	
	private static void FlatteningExceptionFromAttachedNestedTask() 
	{
		var task = Task.Factory.StartNew(() => {
			var childTask1 = Task.Factory.StartNew(()=> {
				var childTask2 = Task.Factory.StartNew(() =>  throw new Exception("Exception from child2 task"), TaskCreationOptions.AttachedToParent);
				
				throw new Exception("Exception from child1 task");
			}, TaskCreationOptions.AttachedToParent);
		});
		
		try
		{
			task.Wait();
		}
		catch(AggregateException ex)
		{
			foreach(var e in ex.Flatten().InnerExceptions) {
				Console.WriteLine(e.Message);
			}
		}
	}
	
	private static void HandlingExceptionFromDetachedNestedTask() 
	{
		var task = Task.Factory.StartNew(() => {
			var childTask1 = Task<int>.Factory.StartNew(()=> {
				throw new Exception("Exception from child1 task");
			});
			
			//To propagate the exception access childTask1.Wait or childTask1.Result
			int result = childTask1.Result;
		});
		
		try
		{
			task.Wait();
		}
		catch(AggregateException ex)
		{
			foreach(var e in ex.Flatten().InnerExceptions) {
				Console.WriteLine(e.Message);
			}
		}
	}
	
	private static void UsingAggregateExceptionHandle()
	{
		try
		{
			var files = GetAllFiles(@"C:\temp");
			foreach(var file in files)
				Console.WriteLine(file);
			
		}
		catch(AggregateException ae)
		{
			foreach(var ex in ae.InnerExceptions)
				Console.WriteLine("Tasks Exception: {0} {1}", ex.GetType().Name, ex.Message);
		}
		
		try
		{
			var files = GetAllFiles(null);
			foreach(var file in files)
				Console.WriteLine(file);
			
		}
		catch(AggregateException ae)
		{
			foreach(var ex in ae.InnerExceptions)
				Console.WriteLine("Tasks Exception: {0} {1}", ex.GetType().Name, ex.Message);
		}
	}
	
	private static string[] GetAllFiles(string path)
	{
		var task = Task.Run(() => Directory.GetFiles(path, "*.txt"));
		try
		{
			return task.Result;
		}
		catch(AggregateException ae)
		{
			ae.Handle(x => {
				if(x is DirectoryNotFoundException)
				{
					Console.WriteLine("directory not found exception handled");
				}
				return x is DirectoryNotFoundException;
			});
			return Array.Empty<String>();
		}
	}
}

                var childTask2 = Task.Factory.StartNew(() =>  throw new Exception("Exception from child2 task"), TaskCreationOptions.AttachedToParent);
 
using System;
using System.Threading.Tasks;
using System.IO;
                    
public class Program
{
    public static void Main()
    {
        //HandlingExceptionFromSingleTask();
        //UsingExceptionProperty();
        //HandlingExceptionFromMultipleTasks();
        //HandlingExceptionFromAttachedNestedTask();
        //FlatteningExceptionFromAttachedNestedTask();
        //HandlingExceptionFromDetachedNestedTask();
        UsingAggregateExceptionHandle();
    }
    
    private static void HandlingExceptionFromSingleTask() 
    {
        var task = Task.Run(() => throw new Exception("An exception occured"));
        try
        {
            task.Wait();
        }

Cached Result

directory not found exception handled
Tasks Exception: ArgumentNullException Value cannot be null. (Parameter 'path')