diff --git a/src/Pharmacist.Console/Program.cs b/src/Pharmacist.Console/Program.cs
index 47f77f5..2d60e34 100644
--- a/src/Pharmacist.Console/Program.cs
+++ b/src/Pharmacist.Console/Program.cs
@@ -49,17 +49,26 @@ public static async Task<int> Main(string[] args)
                 {
                     try
                     {
-                        string referenceAssembliesLocation;
-                        if (!string.IsNullOrWhiteSpace(options.ReferenceAssemblies))
+                        var referenceAssembliesLocation = !string.IsNullOrWhiteSpace(options.ReferenceAssemblies)
+                            ? options.ReferenceAssemblies!
+                            : ReferenceLocator.GetReferenceLocation();
+
+                        if (string.IsNullOrWhiteSpace(options.OutputPath))
                         {
-                            referenceAssembliesLocation = options.ReferenceAssemblies;
+                            throw new Exception("Invalid output path for the event generation.");
                         }
-                        else
+
+                        if (string.IsNullOrWhiteSpace(options.OutputPrefix))
+                        {
+                            throw new Exception("Invalid output prefix for the event generation.");
+                        }
+
+                        if (options.Platforms == null)
                         {
-                            referenceAssembliesLocation = ReferenceLocator.GetReferenceLocation();
+                            throw new Exception("Invalid platforms for the event generation.");
                         }
 
-                        await ObservablesForEventGenerator.ExtractEventsFromPlatforms(options.OutputPath, options.OutputPrefix, ".cs", referenceAssembliesLocation, options.Platforms).ConfigureAwait(false);
+                        await ObservablesForEventGenerator.ExtractEventsFromPlatforms(options.OutputPath!, options.OutputPrefix!, ".cs", referenceAssembliesLocation, options.Platforms).ConfigureAwait(false);
 
                         return ExitCode.Success;
                     }
@@ -75,9 +84,24 @@ public static async Task<int> Main(string[] args)
                     {
                         using (var writer = new StreamWriter(Path.Combine(options.OutputPath, options.OutputPrefix + ".cs")))
                         {
-                            await ObservablesForEventGenerator.WriteHeader(writer, options.Assemblies).ConfigureAwait(false);
+                            if (options.Assemblies == null)
+                            {
+                                throw new Exception("Invalid specified assemblies for observable generation.");
+                            }
 
-                            await ObservablesForEventGenerator.ExtractEventsFromAssemblies(writer, options.Assemblies, options.SearchDirectories, options.TargetFramework).ConfigureAwait(false);
+                            if (options.SearchDirectories == null)
+                            {
+                                throw new Exception("Invalid search directories specified for observable generation.");
+                            }
+
+                            if (string.IsNullOrWhiteSpace(options.TargetFramework))
+                            {
+                                throw new Exception("Invalid target framework for the event generation.");
+                            }
+
+                            await ObservablesForEventGenerator.WriteHeader(writer, options.Assemblies!).ConfigureAwait(false);
+
+                            await ObservablesForEventGenerator.ExtractEventsFromAssemblies(writer, options.Assemblies!, options.SearchDirectories!, options.TargetFramework!).ConfigureAwait(false);
                         }
 
                         return ExitCode.Success;
@@ -94,8 +118,13 @@ public static async Task<int> Main(string[] args)
                     {
                         using (var writer = new StreamWriter(Path.Combine(options.OutputPath, options.OutputPrefix + ".cs")))
                         {
+                            if (string.IsNullOrWhiteSpace(options.TargetFramework))
+                            {
+                                throw new Exception("Invalid target framework for the event generation.");
+                            }
+
                             var packageIdentity = new[] { new LibraryRange(options.NugetPackageName, VersionRange.Parse(options.NugetVersion), LibraryDependencyTarget.Package) };
-                            var nugetFramework = options.TargetFramework.ToFrameworks();
+                            var nugetFramework = options.TargetFramework!.ToFrameworks();
                             await ObservablesForEventGenerator.WriteHeader(writer, packageIdentity).ConfigureAwait(false);
                             await ObservablesForEventGenerator.ExtractEventsFromNuGetPackages(writer, packageIdentity, nugetFramework, options.PackageFolder).ConfigureAwait(false);
                         }
diff --git a/src/Pharmacist.Core/Generation/ReflectionExtensions.cs b/src/Pharmacist.Core/Generation/ReflectionExtensions.cs
index 84e5045..9699edc 100644
--- a/src/Pharmacist.Core/Generation/ReflectionExtensions.cs
+++ b/src/Pharmacist.Core/Generation/ReflectionExtensions.cs
@@ -5,7 +5,6 @@
 
 using System.Collections.Concurrent;
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Linq;
 using System.Text;
 
@@ -21,7 +20,7 @@ namespace Pharmacist.Core.Generation
     /// </summary>
     internal static class ReflectionExtensions
     {
-        private static readonly ConcurrentDictionary<ICompilation, ImmutableDictionary<string, ImmutableList<ITypeDefinition>>> _typeNameMapping = new ConcurrentDictionary<ICompilation, ImmutableDictionary<string, ImmutableList<ITypeDefinition>>>();
+        private static readonly ConcurrentDictionary<ICompilation, Dictionary<string, List<ITypeDefinition>>> _typeNameMapping = new ConcurrentDictionary<ICompilation, Dictionary<string, List<ITypeDefinition>>>();
         private static readonly ConcurrentDictionary<ICompilation, IEnumerable<ITypeDefinition>> _publicNonGenericTypeMapping = new ConcurrentDictionary<ICompilation, IEnumerable<ITypeDefinition>>();
         private static readonly ConcurrentDictionary<ICompilation, IEnumerable<ITypeDefinition>> _publicEventsTypeMapping = new ConcurrentDictionary<ICompilation, IEnumerable<ITypeDefinition>>();
 
@@ -57,9 +56,14 @@ public static IEnumerable<ITypeDefinition> GetPublicTypesWithStaticEvents(this I
         /// <returns>The name of the items.</returns>
         public static IReadOnlyCollection<ITypeDefinition> GetReferenceTypeDefinitionsWithFullName(this ICompilation compilation, string name)
         {
-            var map = _typeNameMapping.GetOrAdd(compilation, comp => comp.ReferencedModules.Concat(compilation.Modules).SelectMany(x => x.TypeDefinitions).GroupBy(x => x.ReflectionName).ToImmutableDictionary(x => x.Key, x => x.ToImmutableList()));
+            var map = _typeNameMapping.GetOrAdd(compilation, comp => comp.ReferencedModules.Concat(compilation.Modules).SelectMany(x => x.TypeDefinitions).GroupBy(x => x.ReflectionName).ToDictionary(x => x.Key, x => x.ToList()));
 
-            return map.GetValueOrDefault(name) ?? ImmutableList<ITypeDefinition>.Empty;
+            if (map.TryGetValue(name, out var value))
+            {
+                return value;
+            }
+
+            return Array.Empty<ITypeDefinition>();
         }
 
         /// <summary>
diff --git a/src/Pharmacist.Core/Pharmacist.Core.csproj b/src/Pharmacist.Core/Pharmacist.Core.csproj
index 8ec540b..67c0fe3 100644
--- a/src/Pharmacist.Core/Pharmacist.Core.csproj
+++ b/src/Pharmacist.Core/Pharmacist.Core.csproj
@@ -18,7 +18,6 @@
     <PackageReference Include="NuGet.LibraryModel" Version="5.4.0" />
     <PackageReference Include="polly" Version="7.2.0" />
     <PackageReference Include="splat" Version="9.3.1" />
-    <PackageReference Include="system.collections.immutable" Version="1.7.0" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.0" />
   </ItemGroup>