fix: make national-prefix stripping's null handling provable

csharp/PhoneNumbers.Test/TestMetadataFilter.cs

@@ -887,10 +887,26 @@ public void TestIntegrityOfFieldSets()
         }
 
         [Fact]
-        public void TestEquals_WhenNull_ReturnsFalse()
+        public void TestEqualsRejectsNullAndForeignTypesAndAcceptsSameBlacklist()
         {
-            var result = new MetadataFilter(new Dictionary<string, SortedSet<string>>()).Equals(null);
-            Assert.False(result);
+            var filter = new MetadataFilter(new Dictionary<string, SortedSet<string>>());
+
+            // Object.Equals must answer false for a null reference rather than throwing. The null is
+            // held in a variable so the call is a real reference comparison at run time rather than
+            // a literal argument, which reads as a compile-time constant comparison.
+            object? nullReference = null;
+            Assert.False(filter.Equals(nullReference));
+
+            // ... and false for an object that is not a MetadataFilter at all. Held in an object
+            // local for the same reason: passing the string literal straight in is a comparison
+            // between statically incomparable types, which is a real smell everywhere except here.
+            object foreignObject = "not a MetadataFilter";
+            Assert.False(filter.Equals(foreignObject));
+
+            // Equality is by blacklist contents, so a separately built empty filter is equal, and a
+            // filter carrying a non-empty blacklist is not.
+            Assert.True(filter.Equals(MetadataFilter.EmptyFilter()));
+            Assert.False(filter.Equals(MetadataFilter.ForLiteBuild()));
         }
 
         [Fact]

csharp/PhoneNumbers.Test/TestPhoneNumberUtil.cs

@@ -1952,6 +1952,25 @@ public void TestMaybeStripNationalPrefix()
             Assert.Equal(transformedNumber, numberToStrip.ToString());
         }
 
+        [Fact]
+        public void TestMaybeStripNationalPrefixLeavesEmptyNumberAlone()
+        {
+            var metadata = new PhoneMetadata.Builder()
+                .SetNationalPrefixForParsing("34")
+                .SetGeneralDesc(new PhoneNumberDesc.Builder().SetNationalNumberPattern("\\d{4,8}").Build())
+                .BuildPartial();
+            // A zero-length number has no national prefix to strip, and asking must not throw even
+            // though there is nothing to turn into a string.
+            var numberToStrip = new StringBuilder();
+            Assert.False(phoneUtil.MaybeStripNationalPrefixAndCarrierCode(numberToStrip, metadata, null));
+            Assert.Equal("", numberToStrip.ToString());
+
+            // Same for a carrier code being requested.
+            var carrierCode = new StringBuilder();
+            Assert.False(phoneUtil.MaybeStripNationalPrefixAndCarrierCode(numberToStrip, metadata, carrierCode));
+            Assert.Equal("", carrierCode.ToString());
+        }
+
         [Fact]
         public void TestMaybeStripInternationalPrefix()
         {

csharp/PhoneNumbers/PhoneNumberUtil.cs

@@ -2604,14 +2604,30 @@ public bool MaybeStripNationalPrefixAndCarrierCode(StringBuilder number, PhoneMe
         internal bool MaybeStripNationalPrefixAndCarrierCode(StringBuilder number, string numberString, PhoneMetadata metadata, bool getCarrier, out string carrierCode)
         {
             carrierCode = null;
-            var numberLength = numberString?.Length ?? number?.Length ?? 0;
-            if (numberLength == 0 || !metadata.HasNationalPrefixForParsing)
+            if (!metadata.HasNationalPrefixForParsing)
+            {
+                return false;
+            }
+            // Callers supply either form of the number: the ones that already hold a string pass it
+            // so the StringBuilder need not be materialised, while the public overload passes only
+            // the StringBuilder. Branch on which one was supplied rather than folding both into a
+            // single length, so the ToString() below is reached only when the StringBuilder is the
+            // form that is present.
+            if (numberString is null)
+            {
+                // Early return for numbers of zero length.
+                if (number is null || number.Length == 0)
+                {
+                    return false;
+                }
+                // Attempt to parse the first digits as a national prefix.
+                numberString = number.ToString();
+            }
+            else if (numberString.Length == 0)
             {
                 // Early return for numbers of zero length.
                 return false;
             }
-            // Attempt to parse the first digits as a national prefix.
-            numberString ??= number.ToString();
 
             // Whether the groups are needed at all is known before matching: only a transform rule or
             // a requested carrier code reads them. Without either, the length of the prefix is the