cherry pick 853eff3 from PR785

Make possible to explicitly define the assembly that will be used to read metadata (assembly attributes) to produce the help.

commandlineparser#785

Author: mnivet

src/CommandLine/Infrastructure/ReflectionHelper.cs

@@ -16,6 +16,14 @@ static class ReflectionHelper
         /// </summary>
         [ThreadStatic] private static IDictionary<Type, Attribute> _overrides;
 
+        private static Assembly _programAssembly;
+
+        public static Assembly ProgramAssembly
+        {
+            get => _programAssembly ?? GetExecutingOrEntryAssembly();
+            set => _programAssembly = value;
+        }
+
         /// <summary>
         /// Assembly attribute overrides for testing.
         /// </summary>
@@ -51,12 +59,10 @@ public static Maybe<TAttribute> GetAttribute<TAttribute>()
                         Maybe.Nothing<TAttribute>();
             }
 
-            var assembly = GetExecutingOrEntryAssembly();
-
 #if NET40
-            var attributes = assembly.GetCustomAttributes(typeof(TAttribute), false);
+            var attributes = ProgramAssembly.GetCustomAttributes(typeof(TAttribute), false);
 #else
-            var attributes = assembly.GetCustomAttributes<TAttribute>().ToArray();
+            var attributes = ProgramAssembly.GetCustomAttributes<TAttribute>().ToArray();
 #endif
 
             return attributes.Length > 0
@@ -66,14 +72,12 @@ public static Maybe<TAttribute> GetAttribute<TAttribute>()
 
         public static string GetAssemblyName()
         {
-            var assembly = GetExecutingOrEntryAssembly();
-            return assembly.GetName().Name;
+            return ProgramAssembly.GetName().Name;
         }
 
         public static string GetAssemblyVersion()
         {
-            var assembly = GetExecutingOrEntryAssembly();
-            return assembly.GetName().Version.ToStringInvariant();
+            return ProgramAssembly.GetName().Version.ToStringInvariant();
         }
 
         public static bool IsFSharpOptionType(Type type)

src/CommandLine/Text/HelpText.cs

@@ -305,6 +305,17 @@ public SentenceBuilder SentenceBuilder
             get { return sentenceBuilder; }
         }
 
+        /// <summary>
+        /// Gets or sets the <see cref="Assembly"/> that will be used to look for meta data (assembly attributes) to auto build the help.
+        /// By default the entry assembly is automatically selected, but this may not match what is really expected, in particular when
+        /// running unit tests.
+        /// </summary>
+        public static Assembly AutoBuildMetadataAssembly
+        {
+            get => ReflectionHelper.ProgramAssembly;
+            set => ReflectionHelper.ProgramAssembly = value;
+        }
+
         /// <summary>
         /// Creates a new instance of the <see cref="CommandLine.Text.HelpText"/> class using common defaults.
         /// </summary>

tests/CommandLine.Tests/Unit/Text/HelpTextTests.cs

@@ -22,6 +22,7 @@ public class HelpTextTests : IDisposable
         public void Dispose()
         {
             ReflectionHelper.SetAttributeOverride(null);
+            HelpText.AutoBuildMetadataAssembly = null;
         }
 
         [Fact]
@@ -716,6 +717,21 @@ public void AutoBuild_with_assembly_company_attribute_only()
             actualResult.Copyright.Should().Be(string.Format("Copyright (C) {0} {1}", DateTime.Now.Year, expectedCompany));
         }
 
+        [Fact]
+        public void AutoBuild_with_AutoBuildMetadataAssembly_defined()
+        {
+            HelpText.AutoBuildMetadataAssembly = typeof(HelpText).Assembly;
+            string expectedHeading = "CommandLine 0.0.0";
+            string expectedCopyright = "Copyright (c) 2005 - 2020 Giacomo Stelluti Scala & Contributors";
+
+            ParserResult<Simple_Options> fakeResult = new NotParsed<Simple_Options>(
+                TypeInfo.Create(typeof(Simple_Options)), new Error[0]);
+            HelpText actualResult = HelpText.AutoBuild(fakeResult, ht => ht, ex => ex);
+
+            actualResult.Heading.Should().Be(expectedHeading);
+            actualResult.Copyright.Should().Be(expectedCopyright);
+        }
+
         [Fact]
         public void Add_line_with_two_empty_spaces_at_the_end()
         {