Fix NullReferenceException when creating a default immutable instance (#495)

* Fix NullReferenceException when creating a default immutable instance

* Improve exception message when immutable type can't be created

Author: 0xced

src/CommandLine/Core/InstanceBuilder.cs

@@ -167,7 +167,7 @@ private static T BuildImmutable<T>(Type typeInfo, Maybe<Func<T>> factory, IEnume
 
             if(ctor == null)
             {
-                throw new InvalidOperationException($"Type appears to be immutable, but no constructor found for type {typeInfo.FullName} to accept values.");
+                throw new InvalidOperationException($"Type {typeInfo.FullName} appears to be immutable, but no constructor found to accept values.");
             }
 
             var values =

src/CommandLine/Infrastructure/ReflectionHelper.cs

@@ -91,6 +91,11 @@ public static T CreateDefaultImmutableInstance<T>(Type[] constructorTypes)
         public static object CreateDefaultImmutableInstance(Type type, Type[] constructorTypes)
         {
             var ctor = type.GetTypeInfo().GetConstructor(constructorTypes);
+            if (ctor == null)
+            {
+                throw new InvalidOperationException($"Type {type.FullName} appears to be immutable, but no constructor found to accept values.");
+            }
+
             var values = (from prms in ctor.GetParameters()
                           select prms.ParameterType.CreateDefaultForImmutable()).ToArray();
             return ctor.Invoke(values);

tests/CommandLine.Tests/Unit/Core/InstanceBuilderTests.cs

@@ -1063,7 +1063,17 @@ public void OptionClass_IsImmutable_HasNoCtor()
         {
             Action act = () => InvokeBuild<ValueWithNoSetterOptions>(new string[] { "Test" }, false, false);
 
-            act.Should().Throw<InvalidOperationException>();
+            act.Should().Throw<InvalidOperationException>()
+                .Which.Message.Should().Be("Type CommandLine.Tests.Unit.Core.InstanceBuilderTests+ValueWithNoSetterOptions appears to be immutable, but no constructor found to accept values.");
+        }
+
+        [Fact]
+        public void OptionClass_IsImmutable_HasNoCtor_HelpRequested()
+        {
+            Action act = () => InvokeBuild<ValueWithNoSetterOptions>(new string[] { "--help" });
+
+            act.Should().Throw<InvalidOperationException>()
+                .Which.Message.Should().Be("Type CommandLine.Tests.Unit.Core.InstanceBuilderTests+ValueWithNoSetterOptions appears to be immutable, but no constructor found to accept values.");
         }
 
         [Fact]