TweetDuck/lib/TweetLib.Core/App.cs

using System;
using TweetLib.Core.Application;

namespace TweetLib.Core{
    public sealed class App{
        #pragma warning disable CS8618 // Non-nullable field is uninitialized. Consider declaring as nullable.
        public static IAppErrorHandler ErrorHandler { get; private set; }
        public static IAppLockHandler LockHandler { get; private set; }
        public static IAppSystemHandler SystemHandler { get; private set; }
        public static IAppResourceHandler ResourceHandler { get; private set; }
        #pragma warning restore CS8618 // Non-nullable field is uninitialized. Consider declaring as nullable.

        // Builder

        public sealed class Builder{
            public IAppErrorHandler? ErrorHandler { get; set; }
            public IAppLockHandler? LockHandler { get; set; }
            public IAppSystemHandler? SystemHandler { get; set; }
            public IAppResourceHandler? ResourceHandler { get; set; }

            // Validation

            internal void Initialize(){
                App.ErrorHandler = Validate(ErrorHandler, nameof(ErrorHandler))!;
                App.LockHandler = Validate(LockHandler, nameof(LockHandler))!;
                App.SystemHandler = Validate(SystemHandler, nameof(SystemHandler))!;
                App.ResourceHandler = Validate(ResourceHandler, nameof(ResourceHandler))!;
            }

            private T Validate<T>(T obj, string name){
                if (obj == null){
                    throw new InvalidOperationException("Missing property " + name + " on the provided App.");
                }

                return obj;
            }
        }
    }
}