Update ToXmlString method and add unit tests

Modified the `ToXmlString` method for `TimeOnly` to change its output format from "HH:mm:sszzz" to "HH:mm:ss". Added a new test class `ToXmlStringExtTests` with unit tests for various types, ensuring correct XML string representations and invariant culture formatting for numeric types.

Author: Gregory Nikolaishvili

src/AltaSoft.DomainPrimitives/ToXmlStringExt.cs

@@ -31,12 +31,12 @@ public static class ToXmlStringExt
     public static string ToXmlString(this DateOnly value) => value.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture);
 
     /// <summary>
-    /// Converts a <see cref="TimeOnly" /> value to its XML string representation in the format "HH:mm:sszzz".
+    /// Converts a <see cref="TimeOnly" /> value to its XML string representation in the format "HH:mm:ss".
     /// </summary>
     /// <param name="value">The TimeOnly value to convert.</param>
     /// <returns>The XML string representation of the TimeOnly value.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static string ToXmlString(this TimeOnly value) => value.ToString("HH:mm:sszzz", CultureInfo.InvariantCulture);
+    public static string ToXmlString(this TimeOnly value) => value.ToString("HH:mm:ss", CultureInfo.InvariantCulture);
 
     /// <summary>
     /// Converts a <see cref="DateTimeOffset" /> value to its XML string representation
@@ -204,4 +204,4 @@ public static string ToXmlString(this char value)
     {
         return value.ToString();
     }
-}
+}

tests/AltaSoft.DomainPrimitives.XmlDataTypes.Tests/ToXmlStringExtTests.cs

@@ -0,0 +1,102 @@
+using System.Xml;
+
+namespace AltaSoft.DomainPrimitives.XmlDataTypes.Tests;
+
+public class ToXmlStringExtTests
+{
+    [Fact]
+    public void DateTime_ToXmlString_ReturnsExpectedFormat()
+    {
+        var dt = new DateTime(2024, 4, 1, 13, 45, 30, DateTimeKind.Local);
+        var xml = dt.ToXmlString();
+        Assert.Equal("2024-04-01T13:45:30+04:00", xml);
+    }
+
+    [Fact]
+    public void DateOnly_ToXmlString_ReturnsExpectedFormat()
+    {
+        var d = new DateOnly(2024, 4, 1);
+        Assert.Equal("2024-04-01", d.ToXmlString());
+    }
+
+    [Fact]
+    public void TimeOnly_ToXmlString_ReturnsExpectedFormat()
+    {
+        var t = new TimeOnly(13, 45, 30);
+        var xml = t.ToXmlString();
+        Assert.Equal("13:45:30", xml);
+    }
+
+    [Fact]
+    public void DateTimeOffset_ToXmlString_ReturnsXmlConvertString()
+    {
+        var dto = new DateTimeOffset(2024, 4, 1, 13, 45, 30, TimeSpan.Zero);
+        Assert.Equal(XmlConvert.ToString(dto), dto.ToXmlString());
+    }
+
+    [Fact]
+    public void TimeSpan_ToXmlString_ReturnsXmlConvertString()
+    {
+        var ts = new TimeSpan(1, 2, 3);
+        Assert.Equal(XmlConvert.ToString(ts), ts.ToXmlString());
+    }
+
+    [Theory]
+    [InlineData((byte)42)]
+    [InlineData((sbyte)-42)]
+    [InlineData((short)-1234)]
+    [InlineData((ushort)1234)]
+    [InlineData(123456)]
+    [InlineData(123456U)]
+    [InlineData(1234567890123L)]
+    [InlineData(1234567890123UL)]
+    [InlineData(3.14f)]
+    [InlineData(2.71828)]
+    public void NumericTypes_ToXmlString_UsesInvariantCulture(object value)
+    {
+        var xml = value switch
+        {
+            byte b => b.ToXmlString(),
+            sbyte sb => sb.ToXmlString(),
+            short s => s.ToXmlString(),
+            ushort us => us.ToXmlString(),
+            int i => i.ToXmlString(),
+            uint ui => ui.ToXmlString(),
+            long l => l.ToXmlString(),
+            ulong ul => ul.ToXmlString(),
+            float f => f.ToXmlString(),
+            double d => d.ToXmlString(),
+            _ => throw new NotSupportedException()
+        };
+        Assert.DoesNotContain(',', xml); // Invariant culture uses '.'
+    }
+
+    [Fact]
+    public void Decimal_ToXmlString_UsesInvariantCulture()
+    {
+        const decimal m = 123.456m;
+        var xml = m.ToXmlString();
+        Assert.DoesNotContain(',', xml);
+    }
+
+    [Fact]
+    public void Guid_ToXmlString_ReturnsToString()
+    {
+        var guid = Guid.NewGuid();
+        Assert.Equal(guid.ToString(), guid.ToXmlString());
+    }
+
+    [Theory]
+    [InlineData(true, "true")]
+    [InlineData(false, "false")]
+    public void Bool_ToXmlString_ReturnsExpected(bool value, string expected)
+    {
+        Assert.Equal(expected, value.ToXmlString());
+    }
+
+    [Fact]
+    public void Char_ToXmlString_ReturnsToString()
+    {
+        Assert.Equal("A", 'A'.ToXmlString());
+    }
+}