diff --git a/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs b/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs
new file mode 100644
index 00000000..c4abd822
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.Commons/Types/AttributeValueCodec.cs
@@ -0,0 +1,99 @@
+using System.Collections;
+using System.Globalization;
+using System.Text.Json;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+
+namespace ZB.MOM.WW.ScadaBridge.Commons.Types;
+
+/// <summary>
+/// Canonical, round-trippable codec for attribute values. Scalars encode to an
+/// invariant-culture string (identical to the historical representation); List
+/// attributes encode to a JSON array. Used wherever a value is stored or
+/// transmitted (DB Value column, site SQLite, gRPC wire). <see cref="ValueFormatter"/>
+/// remains a separate, display-only (comma-joined) formatter.
+/// </summary>
+public static class AttributeValueCodec
+{
+    private static readonly JsonSerializerOptions JsonOpts = new() { WriteIndented = false };
+
+    /// <summary>Encodes a value to its canonical string form.</summary>
+    public static string? Encode(object? value)
+    {
+        switch (value)
+        {
+            case null: return null;
+            case string s: return s;                       // already canonical
+            case IFormattable f: return f.ToString(null, CultureInfo.InvariantCulture);
+            case IEnumerable e:
+                var items = e.Cast<object?>()
+                             .Select(x => x is IFormattable xf
+                                 ? xf.ToString(null, CultureInfo.InvariantCulture)
+                                 : x?.ToString());
+                return JsonSerializer.Serialize(items, JsonOpts);
+            default: return value.ToString();
+        }
+    }
+
+    /// <summary>
+    /// Decodes a canonical string. For <see cref="DataType.List"/> returns a typed
+    /// <c>List&lt;T&gt;</c>; for scalars returns the string unchanged. Throws
+    /// <see cref="FormatException"/> on malformed list JSON or an un-parseable element.
+    /// </summary>
+    public static object? Decode(string? value, DataType dataType, DataType? elementType)
+    {
+        if (dataType != DataType.List) return value;        // scalar: unchanged
+        if (string.IsNullOrEmpty(value)) return null;
+        if (elementType is null)
+            throw new FormatException("List attribute requires an element type.");
+
+        string?[] raw;
+        try { raw = JsonSerializer.Deserialize<string?[]>(value) ?? []; }
+        catch (JsonException ex) { throw new FormatException("Malformed list JSON.", ex); }
+
+        var clrType = ElementClrType(elementType.Value);
+        var listType = typeof(List<>).MakeGenericType(clrType);
+        var result = (IList)Activator.CreateInstance(listType)!;
+        foreach (var item in raw)
+            result.Add(ParseScalar(item, elementType.Value));
+        return result;
+    }
+
+    private static Type ElementClrType(DataType t) => t switch
+    {
+        DataType.String => typeof(string),
+        DataType.Int32 => typeof(int),
+        DataType.Float => typeof(float),
+        DataType.Double => typeof(double),
+        DataType.Boolean => typeof(bool),
+        DataType.DateTime => typeof(DateTime),
+        _ => throw new FormatException($"Unsupported list element type '{t}'.")
+    };
+
+    private static object? ParseScalar(string? s, DataType t)
+    {
+        if (s is null) throw new FormatException("List elements may not be null.");
+        var c = CultureInfo.InvariantCulture;
+        try
+        {
+            return t switch
+            {
+                DataType.String => s,
+                DataType.Int32 => int.Parse(s, c),
+                DataType.Float => float.Parse(s, c),
+                DataType.Double => double.Parse(s, c),
+                DataType.Boolean => bool.Parse(s),
+                DataType.DateTime => DateTime.Parse(s, c, DateTimeStyles.RoundtripKind),
+                _ => throw new FormatException($"Unsupported list element type '{t}'.")
+            };
+        }
+        catch (Exception ex) when (ex is FormatException or OverflowException)
+        {
+            throw new FormatException($"List element '{s}' is not a valid {t}.", ex);
+        }
+    }
+
+    /// <summary>True if the type may be a List element scalar.</summary>
+    public static bool IsValidElementType(DataType t) =>
+        t is DataType.String or DataType.Int32 or DataType.Float
+          or DataType.Double or DataType.Boolean or DataType.DateTime;
+}
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs
new file mode 100644
index 00000000..ac4dce9e
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/AttributeValueCodecTests.cs
@@ -0,0 +1,161 @@
+using System.Globalization;
+using ZB.MOM.WW.ScadaBridge.Commons.Types;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+
+namespace ZB.MOM.WW.ScadaBridge.Commons.Tests.Types;
+
+/// <summary>
+/// Tests for <see cref="AttributeValueCodec"/> — the canonical, round-trippable
+/// codec used wherever an attribute value is stored or transmitted. Scalars must
+/// encode to their historical invariant-culture string; List attributes encode to
+/// a JSON array and decode back to a typed <c>List&lt;T&gt;</c>.
+/// </summary>
+public class AttributeValueCodecTests
+{
+    [Fact]
+    public void Encode_StringList_ProducesJsonArray() =>
+        Assert.Equal("[\"WO-1\",\"WO-2\"]",
+            AttributeValueCodec.Encode(new List<string> { "WO-1", "WO-2" }));
+
+    [Fact]
+    public void Encode_Scalar_String_ReturnedAsIs() =>
+        Assert.Equal("hello", AttributeValueCodec.Encode("hello"));
+
+    [Fact]
+    public void Encode_Scalar_Double_IsInvariant()
+    {
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = new CultureInfo("de-DE");
+            Assert.Equal("1.5", AttributeValueCodec.Encode(1.5));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
+    [Fact]
+    public void Encode_Null_ReturnsNull() =>
+        Assert.Null(AttributeValueCodec.Encode(null));
+
+    [Fact]
+    public void Encode_EmptyList_IsBracketPair() =>
+        Assert.Equal("[]", AttributeValueCodec.Encode(new List<string>()));
+
+    [Fact]
+    public void Encode_StringWithComma_IsEscaped() =>
+        Assert.Equal("[\"ACME, Inc.\"]",
+            AttributeValueCodec.Encode(new List<string> { "ACME, Inc." }));
+
+    [Fact]
+    public void Encode_DoubleList_IsInvariant()
+    {
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = new CultureInfo("de-DE");
+            Assert.Equal("[\"1.5\",\"2.5\"]",
+                AttributeValueCodec.Encode(new List<double> { 1.5, 2.5 }));
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
+    [Fact]
+    public void RoundTrip_Int32List()
+    {
+        var json = AttributeValueCodec.Encode(new List<int> { 1, 2, 3 });
+        var back = (IList<int>)AttributeValueCodec.Decode(json, DataType.List, DataType.Int32)!;
+        Assert.Equal(new[] { 1, 2, 3 }, back);
+    }
+
+    [Fact]
+    public void RoundTrip_DoubleList_IsCultureInvariant()
+    {
+        var original = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = new CultureInfo("de-DE");
+            var json = AttributeValueCodec.Encode(new List<double> { 1.5, 2.5 });
+            var back = (IList<double>)AttributeValueCodec.Decode(json, DataType.List, DataType.Double)!;
+            Assert.Equal(new[] { 1.5, 2.5 }, back);
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = original;
+        }
+    }
+
+    [Fact]
+    public void RoundTrip_BoolList()
+    {
+        var json = AttributeValueCodec.Encode(new List<bool> { true, false, true });
+        var back = (IList<bool>)AttributeValueCodec.Decode(json, DataType.List, DataType.Boolean)!;
+        Assert.Equal(new[] { true, false, true }, back);
+    }
+
+    [Fact]
+    public void RoundTrip_DateTimeList_Iso8601()
+    {
+        var a = new DateTime(2026, 6, 15, 13, 45, 30, DateTimeKind.Utc);
+        var b = new DateTime(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        var json = AttributeValueCodec.Encode(new List<DateTime> { a, b });
+        var back = (IList<DateTime>)AttributeValueCodec.Decode(json, DataType.List, DataType.DateTime)!;
+        Assert.Equal(new[] { a, b }, back);
+    }
+
+    [Fact]
+    public void Decode_StringArray_ProducesListOfString()
+    {
+        var back = (IList<string>)AttributeValueCodec.Decode("[\"a\",\"b\"]", DataType.List, DataType.String)!;
+        Assert.Equal(new[] { "a", "b" }, back);
+    }
+
+    [Fact]
+    public void Decode_Scalar_ReturnsString() =>
+        Assert.Equal("42", AttributeValueCodec.Decode("42", DataType.Int32, null));
+
+    [Fact]
+    public void Decode_List_NullValue_ReturnsNull() =>
+        Assert.Null(AttributeValueCodec.Decode(null, DataType.List, DataType.String));
+
+    [Fact]
+    public void Decode_List_EmptyValue_ReturnsNull() =>
+        Assert.Null(AttributeValueCodec.Decode("", DataType.List, DataType.String));
+
+    [Fact]
+    public void Decode_MalformedJson_Throws() =>
+        Assert.Throws<FormatException>(() =>
+            AttributeValueCodec.Decode("not json", DataType.List, DataType.String));
+
+    [Fact]
+    public void Decode_UnparseableElement_Throws() =>
+        Assert.Throws<FormatException>(() =>
+            AttributeValueCodec.Decode("[\"abc\"]", DataType.List, DataType.Int32));
+
+    [Fact]
+    public void Decode_NullElement_Throws() =>
+        Assert.Throws<FormatException>(() =>
+            AttributeValueCodec.Decode("[null]", DataType.List, DataType.String));
+
+    [Fact]
+    public void Decode_List_WithoutElementType_Throws() =>
+        Assert.Throws<FormatException>(() =>
+            AttributeValueCodec.Decode("[\"a\"]", DataType.List, null));
+
+    [Theory]
+    [InlineData(DataType.String, true)]
+    [InlineData(DataType.Int32, true)]
+    [InlineData(DataType.Float, true)]
+    [InlineData(DataType.Double, true)]
+    [InlineData(DataType.Boolean, true)]
+    [InlineData(DataType.DateTime, true)]
+    [InlineData(DataType.Binary, false)]
+    [InlineData(DataType.List, false)]
+    public void IsValidElementType_MatchesScalarSet(DataType t, bool expected) =>
+        Assert.Equal(expected, AttributeValueCodec.IsValidElementType(t));
+}