fix: preserve explicit zero country-code presence

csharp/PhoneNumbers.Test/TestPhoneNumberUtil.cs

@@ -1092,6 +1092,20 @@ public void TestFormatAuShortCodeNumber()
             Assert.Equal("+675000", phoneUtil.Format(pgShortCodeNumber, PhoneNumberFormat.E164));
         }
 
+        [Fact]
+        public void TestFormatExplicitZeroCountryCode()
+        {
+            var number = PhoneNumber.CreateBuilder().SetCountryCode(0).SetNationalNumber(0).Build();
+            Assert.Equal("+00", phoneUtil.Format(number, PhoneNumberFormat.E164));
+
+            number = number.ToBuilder().SetRawInput("000")
+                .SetCountryCodeSource(PhoneNumber.Types.CountryCodeSource.FROM_DEFAULT_COUNTRY).Build();
+            Assert.Equal("+0000", phoneUtil.Format(number, PhoneNumberFormat.E164));
+
+            number = number.ToBuilder().ClearCountryCode().Build();
+            Assert.Equal("000", phoneUtil.Format(number, PhoneNumberFormat.E164));
+        }
+
         [Fact]
         public void TestFormatNumberWithExtension()
         {

csharp/PhoneNumbers.Test/TestPhonenumber.cs

@@ -20,6 +20,60 @@ namespace PhoneNumbers.Test
 {
     public class TestPhonenumber
     {
+        [Fact]
+        public void TestCountryCodePresenceForExplicitZero()
+        {
+            var builder = PhoneNumber.CreateBuilder();
+            Assert.False(builder.HasCountryCode);
+            Assert.False(PhoneNumber.DefaultInstance.HasCountryCode);
+
+            builder.CountryCode = 0;
+            Assert.True(builder.HasCountryCode);
+            var number = builder.SetNationalNumber(1).Build();
+            Assert.True(number.HasCountryCode);
+            Assert.True(number.IsInitialized);
+            Assert.Equal(0, number.CountryCode);
+            Assert.True(number.ToBuilder().Build().HasCountryCode);
+            Assert.True(PhoneNumber.CreateBuilder(number).Build().HasCountryCode);
+        }
+
+        [Fact]
+        public void TestCountryCodePresenceSurvivesMergeAndClone()
+        {
+            var zero = PhoneNumber.CreateBuilder().SetCountryCode(0).Build();
+            var builder = PhoneNumber.CreateBuilder().SetCountryCode(44).MergeFrom(zero);
+            Assert.Equal(0, builder.CountryCode);
+            Assert.True(builder.HasCountryCode);
+            Assert.True(builder.Clone().Build().HasCountryCode);
+
+            builder.MergeFrom(PhoneNumber.DefaultInstance);
+            Assert.True(builder.HasCountryCode);
+            Assert.Equal(0, builder.CountryCode);
+        }
+
+        [Fact]
+        public void TestCountryCodePresenceClearedIndependentlyOfValue()
+        {
+            var builder = PhoneNumber.CreateBuilder().SetCountryCode(0);
+            builder.ClearCountryCode();
+            Assert.False(builder.HasCountryCode);
+            Assert.Equal(0, builder.CountryCode);
+            var target = PhoneNumber.CreateBuilder().SetCountryCode(44).MergeFrom(builder.Build());
+            Assert.Equal(44, target.CountryCode);
+            Assert.True(target.HasCountryCode);
+            target.Clear();
+            Assert.False(target.HasCountryCode);
+            Assert.Equal(0, target.CountryCode);
+        }
+
+        [Fact]
+        public void TestCountryCodePresenceDoesNotAffectEquality()
+        {
+            var zero = PhoneNumber.CreateBuilder().SetCountryCode(0).Build();
+            Assert.Equal(PhoneNumber.DefaultInstance, zero);
+            Assert.Equal(PhoneNumber.DefaultInstance.GetHashCode(), zero.GetHashCode());
+        }
+
         [Fact]
         public void TestEqualSimpleNumber()
         {

csharp/PhoneNumbers/Phonenumber.cs

@@ -24,9 +24,19 @@ public sealed class PhoneNumber : IEquatable<PhoneNumber>
 
         public PhoneNumber DefaultInstanceForType => DefaultInstance;
 
-        public bool HasCountryCode => CountryCode != 0;
+        public bool HasCountryCode => _hasCountryCode;
 
-        public int CountryCode { get; internal set; }
+        private bool _hasCountryCode;
+        private int _countryCode;
+        public int CountryCode
+        {
+            get => _countryCode;
+            internal set
+            {
+                _hasCountryCode = true;
+                _countryCode = value;
+            }
+        }
 
         public bool HasNationalNumber => NationalNumber != 0;
 
@@ -253,7 +263,8 @@ public Builder SetCountryCode(int value)
 
             public Builder ClearCountryCode()
             {
-                MessageBeingBuilt.CountryCode = 0;
+                MessageBeingBuilt._hasCountryCode = false;
+                MessageBeingBuilt._countryCode = 0;
                 return this;
             }
 

docs/api-differences-from-java.md

@@ -146,9 +146,11 @@ ordinary mutable class), but every setter is `internal`:
 
 ```csharp
 public int CountryCode { get; internal set; }
-public bool HasCountryCode => CountryCode != 0;
+public bool HasCountryCode => _hasCountryCode;
 ```
 
+`HasCountryCode` tracks whether the field was set, including an explicit zero, as in Java.
+
 That means an object initializer that looks perfectly reasonable to a C# developer —
 
 ```csharp