fix NRE when binding no args to list type property

Author: jonsequitur

src/System.CommandLine.Tests/Binding/ModelBinderTests.cs

@@ -10,6 +10,7 @@
 using FluentAssertions;
 using Xunit;
 using System.Reflection;
+using System.Threading.Tasks;
 
 namespace System.CommandLine.Tests.Binding
 {
@@ -627,12 +628,7 @@ public void Default_values_from_options_with_the_same_type_are_bound_and_use_the
             second.Should().Be(2);
         }
 
-        public class MyModel
-        {
-            public List<string> Abc { get; set; }
-
-            public string AbcDef { get; set; }
-        }
+      
 
         [Fact]
         public void Binder_does_not_match_on_partial_name()
@@ -642,10 +638,10 @@ public void Binder_does_not_match_on_partial_name()
                 new Option<List<string>>("--abc")
             };
 
-            MyModel boundValue = default;
+            ClassWithOnePropertyNameThatIsSubstringOfAnother boundValue = default;
 
             command.Handler = CommandHandler.Create(
-                (MyModel s) =>
+                (ClassWithOnePropertyNameThatIsSubstringOfAnother s) =>
                 {
                     boundValue = s;
                 }
@@ -660,5 +656,25 @@ public void Binder_does_not_match_on_partial_name()
                       .Should()
                       .Be("1");
         }
+
+        [Fact] 
+        public async Task Empty_input_is_bound_correctly_to_list_type_properties()
+        {
+            ClassWithListTypePropertiesAndDefaultCtor boundInstance = default;
+            
+            var cmd = new RootCommand
+            {
+                Handler = CommandHandler.Create((ClassWithListTypePropertiesAndDefaultCtor value) =>
+                {
+                    boundInstance = value;
+                })
+            };
+
+            var result = cmd.Parse();
+
+            await result.InvokeAsync();
+
+            boundInstance.Should().NotBeNull();
+        }
     }
 }

src/System.CommandLine.Tests/Binding/TestModels.cs

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation and contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.Collections.Generic;
 using System.Threading.Tasks;
 
 namespace System.CommandLine.Tests.Binding
@@ -29,21 +30,11 @@ public class ClassWithMultiLetterSetters
         public bool BoolOption { get; set; }
     }
 
-    public class ClassWithSettersAndCtorParametersWithDifferentNames
+    public class ClassWithListTypePropertiesAndDefaultCtor
     {
-        public ClassWithSettersAndCtorParametersWithDifferentNames(
-            int i = 123,
-            string s = "the default",
-            bool b = false)
-        {
-            IntOption = i;
-            StringOption = s;
-            BoolOption = b;
-        }
+        public List<string> Strings { get; set; }
 
-        public int IntOption { get; set; }
-        public string StringOption { get; set; }
-        public bool BoolOption { get; set; }
+        public IEnumerable<int> BoolOption { get; set; }
     }
 
     public class ClassWithCtorParameter<T>
@@ -92,7 +83,6 @@ public class ClassWithMultipleCtor
     {
         public ClassWithMultipleCtor()
         {
-
         }
 
         public ClassWithMultipleCtor(int intProperty)
@@ -102,4 +92,28 @@ public ClassWithMultipleCtor(int intProperty)
 
         public int IntProperty { get; }
     }
+
+    public class ClassWithSettersAndCtorParametersWithDifferentNames
+    {
+        public ClassWithSettersAndCtorParametersWithDifferentNames(
+            int i = 123,
+            string s = "the default",
+            bool b = false)
+        {
+            IntOption = i;
+            StringOption = s;
+            BoolOption = b;
+        }
+
+        public int IntOption { get; set; }
+        public string StringOption { get; set; }
+        public bool BoolOption { get; set; }
+    }
+
+    public class ClassWithOnePropertyNameThatIsSubstringOfAnother
+    {
+        public List<string> Abc { get; set; }
+
+        public string AbcDef { get; set; }
+    }
 }

src/System.CommandLine/Binding/ModelBinder.cs

@@ -65,24 +65,32 @@ public void BindMemberFromValue(PropertyInfo property, IValueDescriptor valueDes
             return newInstance;
         }
 
-        private (bool success, object? newInstance, bool anyNonDefaults) CreateInstanceInternal(
-                    BindingContext bindingContext)
+        private (bool success, object? newInstance, bool anyNonDefaults) CreateInstanceInternal(BindingContext bindingContext)
         {
             if (DisallowedBindingType())
             {
                 throw new InvalidOperationException($"The type {ModelDescriptor.ModelType} cannot be bound");
             }
+
             if (ShortCutTheBinding())
             {
                 return GetSimpleModelValue(MemberBindingSources, bindingContext);
             }
 
             var constructorAndArgs = GetBestConstructorAndArgs(bindingContext);
-            var constructor = constructorAndArgs.Constructor;
-            var boundValues = constructorAndArgs.BoundValues;
-            var nonDefaultsUsed = constructorAndArgs.NonDefaultsUsed;
 
-            return InstanceFromSpecificConstructor(bindingContext, constructor, boundValues, ref nonDefaultsUsed);
+            if (constructorAndArgs is null)
+            {
+                return GetSimpleModelValue(ConstructorArgumentBindingSources, bindingContext);
+            }
+            else
+            {
+                var constructor = constructorAndArgs.Constructor;
+                var boundValues = constructorAndArgs.BoundValues;
+                var nonDefaultsUsed = constructorAndArgs.NonDefaultsUsed;
+
+                return InstanceFromSpecificConstructor(bindingContext, constructor, boundValues, ref nonDefaultsUsed);
+            }
         }
 
         private bool DisallowedBindingType()
@@ -153,7 +161,7 @@ private bool UpdateInstanceInternalNotifyIfNonDefaultsUsed<T>(T instance, Bindin
             return anyNonDefaults;
         }
 
-        private ConstructorAndArgs GetBestConstructorAndArgs(BindingContext bindingContext)
+        private ConstructorAndArgs? GetBestConstructorAndArgs(BindingContext bindingContext)
         {
             var constructorDescriptors =
                   ModelDescriptor
@@ -184,7 +192,7 @@ private ConstructorAndArgs GetBestConstructorAndArgs(BindingContext bindingConte
                 }
             }
 
-            return bestNonMatching!;
+            return bestNonMatching;
         }
 
         internal static (IReadOnlyList<BoundValue> boundValues, bool anyNonDefaults) GetBoundValues(