Allow field names that begin with underscore and number '_0'

protos/extest/unittest_issues.proto

@@ -80,3 +80,9 @@ message B {
 message AB {
     optional int32 a_b = 1;
 }
+
+// Similar issue with numberic names
+message NumberField {
+    optional int32 _01 = 1;
+}
+

src/ProtoGen/ExtensionGenerator.cs

@@ -75,6 +75,9 @@ namespace Google.ProtocolBuffers.ProtoGen
 
         public void Generate(TextGenerator writer)
         {
+            if (Descriptor.File.CSharpOptions.ClsCompliance && GetFieldConstantName(Descriptor).StartsWith("_"))
+                writer.WriteLine("[global::System.CLSCompliant(false)]");
+
             writer.WriteLine("public const int {0} = {1};", GetFieldConstantName(Descriptor), Descriptor.FieldNumber);
 
             if (UseLiteRuntime)

src/ProtoGen/MessageGenerator.cs

@@ -246,6 +246,9 @@ namespace Google.ProtocolBuffers.ProtoGen
 
             foreach (FieldDescriptor fieldDescriptor in Descriptor.Fields)
             {
+                if (Descriptor.File.CSharpOptions.ClsCompliance && GetFieldConstantName(fieldDescriptor).StartsWith("_"))
+                    writer.WriteLine("[global::System.CLSCompliant(false)]");
+
                 // Rats: we lose the debug comment here :(
                 writer.WriteLine("public const int {0} = {1};", GetFieldConstantName(fieldDescriptor),
                                  fieldDescriptor.FieldNumber);

src/ProtocolBuffers/Descriptors/FieldDescriptor.cs

@@ -352,7 +352,12 @@ namespace Google.ProtocolBuffers.Descriptors
 
         public bool IsCLSCompliant
         {
-            get { return mappedType != MappedType.UInt32 && mappedType != MappedType.UInt64; }
+            get 
+            { 
+                return mappedType != MappedType.UInt32 && 
+                    mappedType != MappedType.UInt64 &&
+                    !NameHelpers.UnderscoresToPascalCase(Name).StartsWith("_");
+            }
         }
 
         public int FieldNumber

src/ProtocolBuffers/NameHelpers.cs

@@ -67,12 +67,22 @@ namespace Google.ProtocolBuffers
         private static string UnderscoresToPascalOrCamelCase(string input, bool pascal)
         {
             string name = Transform(input, pascal ? UnderlineToPascal : UnderlineToCamel, x => x.Value.TrimStart('_').ToUpper());
-            if (!pascal && name.Length > 0 && Char.IsUpper(name[0]))
+
+            if (name.Length == 0)
+                throw new ArgumentException(String.Format("The field name '{0}' is invalid.", input));
+
+            // Pascal case always begins with lower-case letter
+            if (!pascal && Char.IsUpper(name[0]))
             {
                 char[] chars = name.ToCharArray();
                 chars[0] = char.ToLower(chars[0]);
                 return new string(chars);
             }
+
+            // Fields can not start with a number
+            if (Char.IsNumber(name[0]))
+                name = '_' + name;
+
             return name;
         }