Fix 32-bit floating point JSON parsing of maximal values for C#

Fixes #10509.

csharp/src/Google.Protobuf.Test/JsonParserTest.cs

@@ -576,6 +576,10 @@ namespace Google.Protobuf
         [TestCase("-3.402823e38", -3.402823e38f)]
         [TestCase("1.5e1", 15f)]
         [TestCase("15e-1", 1.5f)]
+        [TestCase("3.4028235e38", float.MaxValue)]
+        [TestCase("-3.4028235e38", float.MinValue)]
+        [TestCase("3.4028235e+38", float.MaxValue)]
+        [TestCase("-3.4028235e+38", float.MinValue)]
         public void NumberToFloat_Valid(string jsonValue, float expectedParsedValue)
         {
             string json = "{ \"singleFloat\": " + jsonValue + "}";
@@ -584,8 +588,10 @@ namespace Google.Protobuf
         }
 
         [Test]
-        [TestCase("3.402824e38", typeof(InvalidProtocolBufferException))]
-        [TestCase("-3.402824e38", typeof(InvalidProtocolBufferException))]
+        [TestCase("3.4028236e38", typeof(InvalidProtocolBufferException))]
+        [TestCase("-3.4028236e38", typeof(InvalidProtocolBufferException))]
+        [TestCase("3.4028236e+38", typeof(InvalidProtocolBufferException))]
+        [TestCase("-3.4028236e+38", typeof(InvalidProtocolBufferException))]
         [TestCase("1,0", typeof(InvalidJsonException))]
         [TestCase("1.0.0", typeof(InvalidJsonException))]
         [TestCase("+1", typeof(InvalidJsonException))]

csharp/src/Google.Protobuf/JsonParser.cs

@@ -642,19 +642,15 @@ namespace Google.Protobuf
                             {
                                 return float.NaN;
                             }
-                            if (value > float.MaxValue || value < float.MinValue)
+                            float converted = (float) value;
+                            // If the value is out of range of float, the cast representation will be infinite.
+                            // If the original value was infinite as well, that's fine - we'll return the 32-bit
+                            // version (with the correct sign).
+                            if (float.IsInfinity(converted) && !double.IsInfinity(value))
                             {
-                                if (double.IsPositiveInfinity(value))
-                                {
-                                    return float.PositiveInfinity;
-                                }
-                                if (double.IsNegativeInfinity(value))
-                                {
-                                    return float.NegativeInfinity;
-                                }
                                 throw new InvalidProtocolBufferException($"Value out of range: {value}");
                             }
-                            return (float) value;
+                            return converted;
                         case FieldType.Enum:
                             CheckInteger(value);
                             // Just return it as an int, and let the CLR convert it.