[CSharp] Generate (more) presence checks in accessors.

Previously, some accessors did not check that the acting version (being
decoded) had the field/group/varData being accessed. Therefore, it might
return garbage. But other accessors did check.

In this commit, I've attempted to make more of the accessors (hopefully
all) check the acting version before attempting to read data from the
underlying buffer.

This change should make the C# codecs closer to the Java decoders.

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/csharp/CSharpGenerator.java

@@ -255,6 +255,15 @@ private void generateGroupClassHeader(
                 groupName));
         }
 
+        sb.append("\n")
+            .append(indent).append(INDENT).append("public void NotPresent()\n")
+            .append(indent).append(INDENT).append("{\n")
+            .append(indent).append(TWO_INDENT).append("_count = 0;\n")
+            .append(indent).append(TWO_INDENT).append("_index = 0;\n")
+            .append(indent).append(TWO_INDENT).append("_buffer = null;\n")
+            .append(indent).append(TWO_INDENT).append("_offset = 0;\n")
+            .append(indent).append(INDENT).append("}\n");
+
         sb.append(String.format("\n" +
             indent + INDENT + "public void WrapForDecode(%s parentMessage, DirectBuffer buffer, int actingVersion)\n" +
             indent + INDENT + "{\n" +
@@ -407,15 +416,18 @@ private CharSequence generateGroupProperty(
 
         generateSinceActingDeprecated(sb, indent, toUpperFirstChar(groupName), token);
 
+        final String groupField = "_" + toLowerFirstChar(groupName);
+
         sb.append(String.format("\n" +
             "%1$s" +
             indent + "public %2$sGroup %3$s\n" +
             indent + "{\n" +
             indent + INDENT + "get\n" +
             indent + INDENT + "{\n" +
+            generateGroupNotPresentCondition(token.version(), indent + INDENT + INDENT, groupField) +
             indent + INDENT + INDENT + "_%4$s.WrapForDecode(_parentMessage, _buffer, _actingVersion);\n" +
             generateAccessOrderListenerCall(accessOrderModel, indent + TWO_INDENT, token,
-            "_" + groupName + ".Count", "\"decode\"") +
+            groupField + ".Count", "\"decode\"") +
             indent + INDENT + INDENT + "return _%4$s;\n" +
             indent + INDENT + "}\n" +
             indent + "}\n",
@@ -480,6 +492,7 @@ private CharSequence generateVarData(
                 sb.append(String.format(indent + "\n" +
                     indent + "public int %1$sLength()\n" +
                     indent + "{\n" +
+                    generateArrayFieldNotPresentCondition(token.version(), indent, "0") +
                     accessOrderListenerCall +
                     indent + INDENT + "_buffer.CheckLimit(_parentMessage.Limit + %2$d);\n" +
                     indent + INDENT + "return (int)_buffer.%3$sGet%4$s(_parentMessage.Limit);\n" +
@@ -509,7 +522,7 @@ private CharSequence generateVarData(
                     indent + INDENT + "return bytesCopied;\n" +
                     indent + "}\n",
                     propertyName,
-                    generateArrayFieldNotPresentCondition(token.version(), indent),
+                    generateArrayFieldNotPresentCondition(token.version(), indent, "0"),
                     sizeOfLengthField,
                     lengthTypePrefix,
                     byteOrderStr));
@@ -518,6 +531,7 @@ private CharSequence generateVarData(
                     indent + "// Allocates and returns a new byte array\n" +
                     indent + "public byte[] Get%1$sBytes()\n" +
                     indent + "{\n" +
+                    generateArrayFieldNotPresentCondition(token.version(), indent, "new byte[0]") +
                     accessOrderListenerCall +
                     indent + INDENT + "const int sizeOfLengthField = %2$d;\n" +
                     indent + INDENT + "int limit = _parentMessage.Limit;\n" +
@@ -561,6 +575,7 @@ private CharSequence generateVarData(
                         .append(String.format(
                         indent + "public string Get%1$s()\n" +
                         indent + "{\n" +
+                        generateArrayFieldNotPresentCondition(token.version(), indent, "\"\"") +
                         accessOrderListenerCall +
                         indent + INDENT + "const int sizeOfLengthField = %2$d;\n" +
                         indent + INDENT + "int limit = _parentMessage.Limit;\n" +
@@ -948,16 +963,34 @@ private CharSequence generateFieldNotPresentCondition(
             literal);
     }
 
-    private CharSequence generateArrayFieldNotPresentCondition(final int sinceVersion, final String indent)
+    private CharSequence generateGroupNotPresentCondition(
+        final int sinceVersion,
+        final String indent,
+        final String groupInstanceField)
     {
         if (0 == sinceVersion)
         {
             return "";
         }
 
-        return String.format(
-            indent + INDENT + INDENT + "if (_actingVersion < %d) return 0;\n\n",
-            sinceVersion);
+        return indent + "if (_actingVersion < " + sinceVersion + ")" +
+            indent + "{\n" +
+            indent + INDENT + groupInstanceField + ".NotPresent();\n" +
+            indent + INDENT + "return " + groupInstanceField + ";\n" +
+            indent + "}\n\n";
+    }
+
+    private CharSequence generateArrayFieldNotPresentCondition(
+        final int sinceVersion,
+        final String indent,
+        final String defaultValue)
+    {
+        if (0 == sinceVersion)
+        {
+            return "";
+        }
+
+        return indent + INDENT + "if (_actingVersion < " + sinceVersion + ") return " + defaultValue + ";\n\n";
     }
 
     private CharSequence generateBitSetNotPresentCondition(
@@ -1053,6 +1086,7 @@ private CharSequence generateArrayProperty(
             indent + "{\n" +
             indent + INDENT + "get\n" +
             indent + INDENT + "{\n" +
+            generateArrayFieldNotPresentCondition(fieldToken.version(), indent + INDENT + INDENT, "new %2$s[0]") +
             accessOrderListenerCallDoubleIndent +
             indent + INDENT + INDENT + "return _buffer.AsReadOnlySpan<%2$s>(_offset + %4$s, %3$sLength);\n" +
             indent + INDENT + "}\n" +
@@ -1069,6 +1103,7 @@ private CharSequence generateArrayProperty(
             "%1$s" +
             indent + "public Span<%2$s> %3$sAsSpan()\n" +
             indent + "{\n" +
+            generateArrayFieldNotPresentCondition(fieldToken.version(), indent + INDENT + INDENT, "new %2$s[0]") +
             accessOrderListenerCall +
             indent + INDENT + "return _buffer.AsSpan<%2$s>(_offset + %4$s, %3$sLength);\n" +
             indent + "}\n",
@@ -1087,7 +1122,10 @@ private CharSequence generateArrayProperty(
                 accessOrderListenerCall +
                 indent + INDENT + "return Get%1$s(new Span<byte>(dst, dstOffset, length));\n" +
                 indent + "}\n",
-                propName, fieldLength, generateArrayFieldNotPresentCondition(fieldToken.version(), indent), offset));
+                propName,
+                fieldLength,
+                generateArrayFieldNotPresentCondition(fieldToken.version(), indent, "0"),
+                offset));
 
             sb.append(String.format("\n" +
                 indent + "public int Get%1$s(Span<byte> dst)\n" +
@@ -1102,7 +1140,10 @@ private CharSequence generateArrayProperty(
                 indent + INDENT + "_buffer.GetBytes(_offset + %4$d, dst);\n" +
                 indent + INDENT + "return length;\n" +
                 indent + "}\n",
-                propName, fieldLength, generateArrayFieldNotPresentCondition(fieldToken.version(), indent), offset));
+                propName,
+                fieldLength,
+                generateArrayFieldNotPresentCondition(fieldToken.version(), indent, "0"),
+                offset));
 
             sb.append(String.format("\n" +
                 indent + "public void Set%1$s(byte[] src, int srcOffset)\n" +