Task.WhenAll Exception Handling in C# — Capturing All Exceptions (.NET 8)

A .NET 8 console application demonstrating how to catch every exception from Task.WhenAll in C#, not just the first one, when several concurrent tasks fail.

The Problem

When Task.WhenAll is awaited and multiple tasks throw, only the first exception is rethrown into the catch block. The remaining exceptions are silently discarded if you only inspect the caught Exception.

The Solution

After catching, iterate over each task in your list and collect their Exception.InnerExceptions:

catch (Exception ex)
{
    var exceptions = tasks.SelectMany(
        task => task.Exception?.InnerExceptions ?? Enumerable.Empty<Exception>());

    foreach (var exception in exceptions)
    {
        Console.WriteLine(exception.Message);
    }
}

Demo

The application runs two async methods concurrently:

Method Delay Exception Message
MethodOneAsync 1 second "Custom Error Method One"
MethodTwoAsync 2 seconds "Custom Error Method Two"

Both tasks are stored in a List<Task> before Task.WhenAll is awaited. After the catch block, the code collects inner exceptions from all tasks and prints both error messages.

Requirements

Build and Run

cd TaskErrors
dotnet run

Expected Output

Custom Error Method One
Custom Error Method Two

Project Structure

TaskErrors/
├── Program.cs          # Two throwing async methods + WhenAll error capture pattern
├── TaskErrors.csproj
└── TaskErrors.sln

The expected output above was confirmed by running the project.

FAQ

Why does await Task.WhenAll(...) only throw one exception? Task.WhenAll returns a task whose Exception is an AggregateException containing every failure, but await unwraps it and rethrows only the first inner exception. Keeping a reference to the tasks (or to the WhenAll task) lets you read all of them afterwards.

Is there a shorter alternative? You can store the WhenAll task in a variable, await it inside try, and in the catch read whenAllTask.Exception.InnerExceptions — it contains the same exceptions as collecting them from each task.

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