fix #1360

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

@@ -17,7 +17,7 @@ namespace System.CommandLine.Tests.Binding
     public partial class ModelBindingCommandHandlerTests
     {
         [Theory]
-        [InlineData(typeof(string), "")]
+        [InlineData(typeof(string), null)]
         [InlineData(typeof(FileInfo), null)]
         [InlineData(typeof(int), 0)]
         [InlineData(typeof(int?), null)]

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

@@ -11,7 +11,7 @@ namespace System.CommandLine.Tests.Binding
     public class ParameterDescriptorTests
     {
         [Theory]
-        [InlineData(typeof(string), "")]
+        [InlineData(typeof(string), null)]
         [InlineData(typeof(int), 0)]
         [InlineData(typeof(int?), null)]
         public void GetDefaultValue_returns_the_default_for_the_type(Type type, object defaultValue)

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

@@ -11,7 +11,7 @@ namespace System.CommandLine.Tests.Binding
     public class PropertyDescriptorTests
     {
         [Theory]
-        [InlineData(typeof(string), "")]
+        [InlineData(typeof(string), null)]
         [InlineData(typeof(int), 0)]
         [InlineData(typeof(int?), null)]
         public void GetDefaultValue_returns_the_default_for_the_type(Type type, object defaultValue)

src/System.CommandLine.Tests/CommandTests.cs

@@ -330,7 +330,7 @@ public void Command_argument_of_string_defaults_to_empty_when_not_specified()
             var result = command.Parse("mycommand");
             result.GetValueForArgument(argument)
                   .Should()
-                  .BeEmpty();
+                  .BeNull();
         }
 
         [Fact]

src/System.CommandLine.Tests/Invocation/CommandHandlerTests.cs

@@ -142,7 +142,7 @@ void Execute(string name, int age)
 
             await command.InvokeAsync("command", _console);
 
-            boundName.Should().Be("");
+            boundName.Should().BeNull();
             boundAge.Should().Be(0);
         }
 

src/System.CommandLine.Tests/OptionTests.cs

@@ -321,7 +321,7 @@ public void Option_T_default_value_is_validated()
         }
 
         [Fact]
-        public void Option_of_string_defaults_to_empty_when_not_specified()
+        public void Option_of_string_defaults_to_null_when_not_specified()
         {
             var option = new Option<string>("-x");
 
@@ -331,7 +331,7 @@ public void Option_of_string_defaults_to_empty_when_not_specified()
                 .BeFalse();
             result.GetValueForOption(option)
                 .Should()
-                .BeEmpty();
+                .BeNull();
         }
 
         [Fact]

src/System.CommandLine/Binding/Binder.cs

@@ -53,11 +53,6 @@ internal static bool CanBeBoundFromScalarValue(this Type type)
 
         internal static object? GetDefaultValue(Type type)
         {
-            if (type == typeof(string))
-            {
-                return "";
-            }
-
             if (GetItemTypeIfEnumerable(type) is { } itemType)
             {
                 if (type.IsArray)