Override the IErrorHandler

This example shows how to override the IErrorHandler in the ApodClient for your needs. You can use this guide as a reference for overloading any other interface in the library. Intermediate knowledge in C# is assumed.

The project targets .NET Core 3.0 but will work with any platform version that supports .NET Standard 2.0 (small changes may be required for different target frameworks).

Creating the project

Start by creating a .NET Core (or .NET framework) Console App. Read this quickstart guide if you are unsure how to do this.

Installing APOD.Net

There are multiple ways of installing a NuGet package. See Adding APOD.Net to your project in the README for other examples.

1. Open your new project in Visual Studio and open the Package Manager Console (Tools > NuGet Package Manager > Package Manager Console).
2. In the Package Manager Console window, write the following command to add the latest version of APOD.Net to your project.

   Install-Package APOD.Net
   

Once the install finishes you are ready to start using APOD.Net.

Implementation

Setup

To get started with APOD.Net and to verify the installation, add the using directive for the Apod namespace in your Program.cs.

using Apod;

If you get errors, run dotnet restore, restart visual studio and try again. If it still doesn't work, feel free to open an issue.

Since APOD.Net is an asynchronous library we are going to make our Main() method asynchronous, which is a language feature from C# 7.1. If you don't have access to C# 7.1, read this article for workarounds.

using System;
using System.Threading.Tasks;
using Apod;

namespace Example
{
    public class Program
    {
        public static async Task Main()
        {
            Console.WriteLine("Hello World!");
        }
    }
}

Note that we added another using directive for System.Threading.Tasks.

Creating the custom IErrorHandler

To override the IErrorHandler in the ApodClient, we'll create a new class that implements IErrorHandler. In Visual Studio, add a new class (Project > Add Class...) and name it something suitable for your use case. We'll go with CustomErrorHandler.cs for this example.

First off, add the using directive for Apod.Logic.Errors.

using Apod.Logic.Errors;

Next, imlpement IErrorHandler (which should add the following code for you).

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Apod;
using Apod.Logic.Errors;

namespace Example
{
    public class CustomErrorHandler : IErrorHandler
    {
        public ApodError ValidateCount(int count)
        {
            throw new NotImplementedException();
        }

        public ApodError ValidateDate(DateTime dateTime)
        {
            throw new NotImplementedException();
        }

        public ApodError ValidateDateRange(DateTime startDate, DateTime endDate = default)
        {
            throw new NotImplementedException();
        }

        public ValueTask<ApodError> ValidateHttpResponseAsync(HttpResponseMessage httpResponse)
        {
            throw new NotImplementedException();
        }
    }
}

Now we can start writing our custom behaviour for the methods. It's very useful to use the source code of the default implementation of IErrorHandler as a reference if you're not sure about what a method is supposed to do. It can also be useful to look how the IErrorHandler is used in the source code of the ApodClient.

For this example, let's say we need ValidateCount to depend on the current day of the month. The users can only get 5 or less APODs on the 5th, 14 or less APODs on the 14th, and so on.

public ApodError ValidateCount(int count)
{
    var today = DateTime.Today;
    var countIsValid = today.Day <= count;

    if (countIsValid) 
    {
        return new ApodError(ApodErrorCode.None);
    }

    if (DateTime.DaysInMonth(today.Year, today.Month) < count)
    {
        return new ApodError(ApodErrorCode.CountOutOfRange, 
            $"There aren't {count} days in {today.ToString("MMMM")}");
    }

    return new ApodError(ApodErrorCode.CountOutOfRange, 
        $"You have to wait until the {count} {today.ToString("MMMM")} to get that many APODs.");
}

Next, we'll implement ValidateHttpResponseAsync. For this example, we don't care which type of error it was, if any. We just want to know if there was an error or not, which we can do synchronously.

public ValueTask<ApodError> ValidateHttpResponseAsync(HttpResponseMessage httpResponse)
{
    if (httpResponse.IsSuccessStatusCode) 
    {
        var noError = new ApodError(ApodErrorCode.None);
        return new ValueTask<ApodError>(noError);
    }

    var error = new ApodError(ApodErrorCode.BadRequest, "An error occured.");
    return new ValueTask<ApodError>(error);
}

We'll assume ValidateDate() and ValidateDateRange() are never going to be called and leave them as is.

CustomErrorHandler.cs result:

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Apod;
using Apod.Logic.Errors;

namespace Example
{
    public class CustomErrorHandler : IErrorHandler
    {
        public ApodError ValidateCount(int count)
        {
            var today = DateTime.Today;
            var countIsValid = count <= today.Day;

            if (countIsValid) 
            {
                return new ApodError(ApodErrorCode.None);
            }

            if (DateTime.DaysInMonth(today.Year, today.Month) < count)
            {
                return new ApodError(ApodErrorCode.CountOutOfRange, 
                    $"There aren't {count} days in {today.ToString("MMMM")}.");
            }

            return new ApodError(ApodErrorCode.CountOutOfRange, 
                $"You have to wait until {today.ToString("MMMM")} {count} to get that many APODs.");
        }

        public ApodError ValidateDate(DateTime dateTime)
        {
            throw new NotImplementedException();
        }

        public ApodError ValidateDateRange(DateTime startDate, DateTime endDate = default)
        {
            throw new NotImplementedException();
        }

        public ValueTask<ApodError> ValidateHttpResponseAsync(HttpResponseMessage httpResponse)
        {
            if (httpResponse.IsSuccessStatusCode) 
            {
                var noError = new ApodError(ApodErrorCode.None);
                return new ValueTask<ApodError>(noError);
            }

            var error = new ApodError(ApodErrorCode.BadRequest, "An error occured.");
            return new ValueTask<ApodError>(error);
        }
    }
}

Using our custom error handler

Overriding the error handler in the client is easy. Back in our Program.cs, we create a new instance of our CustomErrorHandler and then inject it with the ApodClient(String, IHttpRequester, IHttpResponseParser, IErrorHandler) constructor.

public static async Task Main()
{
    var errorHandler = new CustomErrorHandler();
    using var client = new ApodClient("DEMO_KEY", errorHandler: errorHandler);
}

The client will now use our custom IErrorHandler to validate the requests.

Full example code

Below is an example project that uses the CustomErrorHandler. We won't go into detail how it's built, but it's similar to a very simplified version of Download all APODs between two dates.

// Program.cs
using System;
using System.Threading.Tasks;
using Apod;

namespace Example
{
    public class Program
    {
        public static async Task Main()
        {
            var errorHandler = new CustomErrorHandler();
            using var client = new ApodClient("DEMO_KEY", errorHandler: errorHandler);

            var amount = GetValidInt("How many astronomy pictures would you like?");
            var response = await client.FetchApodAsync(amount);

            if (response.StatusCode != ApodStatusCode.OK)
            {
                Console.WriteLine(response.Error.ErrorMessage);
                return;
            }

            foreach (var apod in response.AllContent)
            {
                Console.WriteLine(apod.ContentUrl);
            }
        }

        private static int GetValidInt(string prompt)
        {
            Console.WriteLine(prompt);

            int result;
            while (!int.TryParse(Console.ReadLine(), out result))
            {
                Console.WriteLine("That is not a valid integer. Try again.");
            }

            return result;
        }
    }
}

// CustomErrorHandler.cs
using System;
using System.Net.Http;
using System.Threading.Tasks;
using Apod;
using Apod.Logic.Errors;

namespace Example
{
    public class CustomErrorHandler : IErrorHandler
    {
        public ApodError ValidateCount(int count)
        {
            var today = DateTime.Today;
            var countIsValid = count <= today.Day;

            if (countIsValid) 
            {
                return new ApodError(ApodErrorCode.None);
            }

            if (DateTime.DaysInMonth(today.Year, today.Month) < count)
            {
                return new ApodError(ApodErrorCode.CountOutOfRange, 
                    $"There aren't {count} days in {today.ToString("MMMM")}.");
            }

            return new ApodError(ApodErrorCode.CountOutOfRange,
                $"You have to wait until {today.ToString("MMMM")} {count} to get that many APODs.");
        }

        public ApodError ValidateDate(DateTime dateTime)
        {
            throw new NotImplementedException();
        }

        public ApodError ValidateDateRange(DateTime startDate, DateTime endDate = default)
        {
            throw new NotImplementedException();
        }

        public ValueTask<ApodError> ValidateHttpResponseAsync(HttpResponseMessage httpResponse)
        {
            if (httpResponse.IsSuccessStatusCode) 
            {
                var noError = new ApodError(ApodErrorCode.None);
                return new ValueTask<ApodError>(noError);
            }

            var error = new ApodError(ApodErrorCode.BadRequest, "An error occured.");
            return new ValueTask<ApodError>(error);
        }
    }
}