diff --git a/src/NHapi.Base/HL7Exception.cs b/src/NHapi.Base/HL7Exception.cs
index 954f90608..43f26e853 100644
--- a/src/NHapi.Base/HL7Exception.cs
+++ b/src/NHapi.Base/HL7Exception.cs
@@ -50,112 +50,112 @@ public class HL7Exception : Exception
         /// </summary>
         /// <remarks>Deprecated use <see cref="AcknowledgmentCode.AA"/> instead.</remarks>
         [Obsolete("Deprecated use 'AcknowledgementCode.AA' instead.")]
-        public const int ACK_AA = 1;
+        public static readonly int ACK_AA = 1;
 
         /// <summary>
         /// Acknowledgment Application Error.
         /// </summary>
         /// <remarks>Deprecated use <see cref="AcknowledgmentCode.AE"/> instead.</remarks>
         [Obsolete("Deprecated use 'AcknowledgementCode.AE' instead.")]
-        public const int ACK_AE = 2;
+        public static readonly int ACK_AE = 2;
 
         /// <summary>
         /// Acknowledgment Application Reject.
         /// </summary>
         /// <remarks>Deprecated use <see cref="AcknowledgmentCode.AR"/> instead.</remarks>
         [Obsolete("Deprecated use 'AcknowledgmentCode.AR' instead.")]
-        public const int ACK_AR = 3;
+        public static readonly int ACK_AR = 3;
 
         /// <summary>
         /// Message accepted.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.MESSAGE_ACCEPTED"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.MESSAGE_ACCEPTED' instead.")]
-        public const int MESSAGE_ACCEPTED = 0;
+        public static readonly int MESSAGE_ACCEPTED; // Default initialised to 0.
 
         /// <summary>
         /// Segment sequence error.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.SEGMENT_SEQUENCE_ERROR"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.SEGMENT_SEQUENCE_ERROR' instead.")]
-        public const int SEGMENT_SEQUENCE_ERROR = 100;
+        public static readonly int SEGMENT_SEQUENCE_ERROR = 100;
 
         /// <summary>
         /// Required field missing.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.REQUIRED_FIELD_MISSING"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.REQUIRED_FIELD_MISSING' instead.")]
-        public const int REQUIRED_FIELD_MISSING = 101;
+        public static readonly int REQUIRED_FIELD_MISSING = 101;
 
         /// <summary>
         /// Date type error.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.DATA_TYPE_ERROR"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.DATA_TYPE_ERROR' instead.")]
-        public const int DATA_TYPE_ERROR = 102;
+        public static readonly int DATA_TYPE_ERROR = 102;
 
         /// <summary>
         /// Table value not found.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.TABLE_VALUE_NOT_FOUND"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.TABLE_VALUE_NOT_FOUND' instead.")]
-        public const int TABLE_VALUE_NOT_FOUND = 103;
+        public static readonly int TABLE_VALUE_NOT_FOUND = 103;
 
         /// <summary>
         /// Unsupported message type.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.UNSUPPORTED_MESSAGE_TYPE"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.UNSUPPORTED_MESSAGE_TYPE' instead.")]
-        public const int UNSUPPORTED_MESSAGE_TYPE = 200;
+        public static readonly int UNSUPPORTED_MESSAGE_TYPE = 200;
 
         /// <summary>
         /// Unsupported event code.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.UNSUPPORTED_EVENT_CODE"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.UNSUPPORTED_EVENT_CODE' instead.")]
-        public const int UNSUPPORTED_EVENT_CODE = 201;
+        public static readonly int UNSUPPORTED_EVENT_CODE = 201;
 
         /// <summary>
         /// Unsupported processing id.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.UNSUPPORTED_PROCESSING_ID"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.UNSUPPORTED_PROCESSING_ID' instead.")]
-        public const int UNSUPPORTED_PROCESSING_ID = 202;
+        public static readonly int UNSUPPORTED_PROCESSING_ID = 202;
 
         /// <summary>
         /// Unsupported version id.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.UNSUPPORTED_VERSION_ID"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.UNSUPPORTED_VERSION_ID' instead.")]
-        public const int UNSUPPORTED_VERSION_ID = 203;
+        public static readonly int UNSUPPORTED_VERSION_ID = 203;
 
         /// <summary>
         /// Unknown key id.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.MESSAGE_ACCEPTED"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.UNKNOWN_KEY_IDENTIFIER' instead.")]
-        public const int UNKNOWN_KEY_IDENTIFIER = 204;
+        public static readonly int UNKNOWN_KEY_IDENTIFIER = 204;
 
         /// <summary>
         /// Duplicate key id.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.DUPLICATE_KEY_IDENTIFIER"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.DUPLICATE_KEY_IDENTIFIER' instead.")]
-        public const int DUPLICATE_KEY_IDENTIFIER = 205;
+        public static readonly int DUPLICATE_KEY_IDENTIFIER = 205;
 
         /// <summary>
         /// Application record locked.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.APPLICATION_RECORD_LOCKED"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.APPLICATION_RECORD_LOCKED' instead.")]
-        public const int APPLICATION_RECORD_LOCKED = 206;
+        public static readonly int APPLICATION_RECORD_LOCKED = 206;
 
         /// <summary>
         /// Application error.
         /// </summary>
         /// <remarks>Deprecated use <see cref="ErrorCode.APPLICATION_INTERNAL_ERROR"/> instead.</remarks>
         [Obsolete("Deprecated use 'ErrorCode.APPLICATION_INTERNAL_ERROR' instead.")]
-        public const int APPLICATION_INTERNAL_ERROR = 207;
+        public static readonly int APPLICATION_INTERNAL_ERROR = 207;
 
         /// <summary> Creates an <see cref="HL7Exception" />.
         /// <param name="message">The error message</param>
diff --git a/src/NHapi.Base/Parser/PipeParser.cs b/src/NHapi.Base/Parser/PipeParser.cs
index bf4c9880a..8c2f4baf3 100644
--- a/src/NHapi.Base/Parser/PipeParser.cs
+++ b/src/NHapi.Base/Parser/PipeParser.cs
@@ -46,7 +46,7 @@ namespace NHapi.Base.Parser
     /// <author>Bryan Tripp (bryan_tripp@sourceforge.net).</author>
     public class PipeParser : ParserBase
     {
-        public const string SegmentDelimiter = "\r"; // see section 2.8 of spec
+        public static readonly string SegmentDelimiter = "\r"; // see section 2.8 of spec
 
         private static readonly IHapiLog Log;
 
diff --git a/tests/NHapi.NUnit/ValidationTests.cs b/tests/NHapi.NUnit/ValidationTests.cs
index 3e1ec2f99..7444b8b1d 100644
--- a/tests/NHapi.NUnit/ValidationTests.cs
+++ b/tests/NHapi.NUnit/ValidationTests.cs
@@ -13,7 +13,7 @@ namespace NHapi.NUnit
     [TestFixture]
     public class ValidationTests
     {
-        public const string MessageSINegativeNumber =
+        private const string MessageSINegativeNumber =
             @"MSH|^~\&|XPress Arrival||||200610120839||ORU^R01|EBzH1711114101206|P|2.5.1|||AL|||ASCII
 PID|1||1711114||Appt^Test||19720501||||||||||||001020006
 ORC|||||F
@@ -42,35 +42,35 @@ public void TestStrictValidation_NegativeNumber()
                 $"Strict validation should throw a {nameof(DataTypeException)} when parsing a SI field with a negative value");
         }
 
-        public const string MessageNMAlpha =
+        private const string MessageNMAlpha =
             @"MSH|^~\&|XPress Arrival||||200610120839||ORU^R01|EBzH1711114101206|P|2.5.1|||AL|||ASCII
 PID|1||1711114||Appt^Test||19720501||||||||||||001020006
 ORC|||||F
 OBR|1|||ehipack^eHippa Acknowlegment|||200610120839|||||||||00002^eProvider^Electronic|||||||||F
 OBX|1|NM|||Hello||||||F";
 
-        public const string MessageNMChar =
+        private const string MessageNMChar =
             @"MSH|^~\&|XPress Arrival||||200610120839||ORU^R01|EBzH1711114101206|P|2.5.1|||AL|||ASCII
 PID|1||1711114||Appt^Test||19720501||||||||||||001020006
 ORC|||||F
 OBR|1|||ehipack^eHippa Acknowlegment|||200610120839|||||||||00002^eProvider^Electronic|||||||||F
 OBX|1|NM|||!@#$||||||F";
 
-        public const string MessageNMNumber =
+        private const string MessageNMNumber =
             @"MSH|^~\&|XPress Arrival||||200610120839||ORU^R01|EBzH1711114101206|P|2.5.1|||AL|||ASCII
 PID|1||1711114||Appt^Test||19720501||||||||||||001020006
 ORC|||||F
 OBR|1|||ehipack^eHippa Acknowlegment|||200610120839|||||||||00002^eProvider^Electronic|||||||||F
 OBX|1|NM|||10||||||F";
 
-        public const string MessageNMNegativeNumber =
+        private const string MessageNMNegativeNumber =
             @"MSH|^~\&|XPress Arrival||||200610120839||ORU^R01|EBzH1711114101206|P|2.5.1|||AL|||ASCII
 PID|1||1711114||Appt^Test||19720501||||||||||||001020006
 ORC|||||F
 OBR|1|||ehipack^eHippa Acknowlegment|||200610120839|||||||||00002^eProvider^Electronic|||||||||F
 OBX|1|NM|||-1||||||F";
 
-        public const string MessageNMDecimal =
+        private const string MessageNMDecimal =
             @"MSH|^~\&|XPress Arrival||||200610120839||ORU^R01|EBzH1711114101206|P|2.5.1|||AL|||ASCII
 PID|1||1711114||Appt^Test||19720501||||||||||||001020006
 ORC|||||F