﻿using System;
using System.Collections.Generic;
using System.Reflection;
using Microsoft.Extensions.DependencyModel;

namespace Serilog.Settings.Configuration.Assemblies
{
    abstract class AssemblyFinder
    {
        public abstract IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind);

        protected static bool IsCaseInsensitiveMatch(string text, string textToFind)
        {
            return text != null && text.ToLowerInvariant().Contains(textToFind.ToLowerInvariant());
        }

        public static AssemblyFinder Auto()
        {
            if (Assembly.GetEntryAssembly() != null)
            {
                return new DependencyContextAssemblyFinder(DependencyContext.Default);
            }
            return new DllScanningAssemblyFinder();
        }

        public static AssemblyFinder ForSource(ConfigurationAssemblySource configurationAssemblySource)
        {
            switch (configurationAssemblySource)
            {
                case ConfigurationAssemblySource.UseLoadedAssemblies:
                    return Auto();
                case ConfigurationAssemblySource.AlwaysScanDllFiles:
                    return new DllScanningAssemblyFinder();
                default:
                    throw new ArgumentOutOfRangeException(nameof(configurationAssemblySource), configurationAssemblySource, null);
            }
        }

        public static AssemblyFinder ForDependencyContext(DependencyContext dependencyContext)
        {
            return new DependencyContextAssemblyFinder(dependencyContext);
        }
    }
}

﻿using System;
using System.Collections.Generic;
using System.Reflection;
using Microsoft.Extensions.DependencyModel;

namespace Serilog.Settings.Configuration.Assemblies
{
    abstract class AssemblyFinder
    {
        public abstract IReadOnlyList<AssemblyName> FindAssembliesContainingName(string nameToFind);

        protected static bool IsCaseInsensitiveMatch(string text, string textToFind)
        {
            return text != null && text.ToLowerInvariant().Contains(textToFind.ToLowerInvariant());
        }

        public static AssemblyFinder Auto()
        {
            // Need to check `Assembly.GetEntryAssembly()` first because 
            // `DependencyContext.Default` throws an exception when `Assembly.GetEntryAssembly()` returns null
            if (Assembly.GetEntryAssembly() != null && DependencyContext.Default != null)
            {
                return new DependencyContextAssemblyFinder(DependencyContext.Default);
            }
            return new DllScanningAssemblyFinder();
        }

        public static AssemblyFinder ForSource(ConfigurationAssemblySource configurationAssemblySource)
        {
            switch (configurationAssemblySource)
            {
                case ConfigurationAssemblySource.UseLoadedAssemblies:
                    return Auto();
                case ConfigurationAssemblySource.AlwaysScanDllFiles:
                    return new DllScanningAssemblyFinder();
                default:
                    throw new ArgumentOutOfRangeException(nameof(configurationAssemblySource), configurationAssemblySource, null);
            }
        }

        public static AssemblyFinder ForDependencyContext(DependencyContext dependencyContext)
        {
            return new DependencyContextAssemblyFinder(dependencyContext);
        }
    }
}

Add comments explaining behavior of DependencyContext.Default
Add comments explaining behavior of  DependencyContext.Default

and protect against null