Fix FbZonedDateTime invalid cast (#1194)

Author: willibrandon

src/FirebirdSql.Data.FirebirdClient.Tests/FbZonedDateTimeTypeTests.cs

@@ -47,6 +47,14 @@ public void EqualityFalse(FbZonedDateTime expected, FbZonedDateTime actual)
 		Assert.AreNotEqual(expected, actual);
 	}
 
+	[Test]
+	public void ConvertToDateTimeShouldNotThrow()
+	{
+		var fbZonedDateTime = new FbZonedDateTime(new DateTime(2020, 12, 4, 10, 38, 0, DateTimeKind.Utc), "UTC");
+		
+		Assert.DoesNotThrow(() => Convert.ChangeType(fbZonedDateTime, typeof(DateTime)));
+	}
+
 	public void DateTimeShouldBeUtc()
 	{
 		Assert.Throws<ArgumentException>(() =>

src/FirebirdSql.Data.FirebirdClient/Types/FbZonedDateTime.cs

@@ -21,7 +21,7 @@
 namespace FirebirdSql.Data.Types;
 
 [StructLayout(LayoutKind.Auto)]
-public readonly struct FbZonedDateTime : IEquatable<FbZonedDateTime>
+public readonly struct FbZonedDateTime : IEquatable<FbZonedDateTime>, IConvertible
 {
 	public DateTime DateTime { get; }
 	public string TimeZone { get; }
@@ -72,8 +72,43 @@ public override int GetHashCode()
 		}
 	}
 
-	public bool Equals(FbZonedDateTime other) => DateTime.Equals(other.DateTime) && TimeZone.Equals(other.TimeZone, StringComparison.OrdinalIgnoreCase);
-
+	public bool Equals(FbZonedDateTime other) => DateTime.Equals(other.DateTime) && TimeZone.Equals(other.TimeZone, StringComparison.OrdinalIgnoreCase);
+
+	TypeCode IConvertible.GetTypeCode() => TypeCode.Object;
+
+	DateTime IConvertible.ToDateTime(IFormatProvider provider) => DateTime;
+
+	string IConvertible.ToString(IFormatProvider provider) => ToString();
+
+	object IConvertible.ToType(Type conversionType, IFormatProvider provider)
+		=> ReferenceEquals(conversionType, typeof(FbZonedDateTime)) ? this : throw new InvalidCastException(conversionType?.FullName);
+
+	bool IConvertible.ToBoolean(IFormatProvider provider) => throw new InvalidCastException(nameof(Boolean));
+
+	byte IConvertible.ToByte(IFormatProvider provider) => throw new InvalidCastException(nameof(Byte));
+
+	char IConvertible.ToChar(IFormatProvider provider) => throw new InvalidCastException(nameof(Char));
+
+	decimal IConvertible.ToDecimal(IFormatProvider provider) => throw new InvalidCastException(nameof(Decimal));
+
+	double IConvertible.ToDouble(IFormatProvider provider) => throw new InvalidCastException(nameof(Double));
+
+	short IConvertible.ToInt16(IFormatProvider provider) => throw new InvalidCastException(nameof(Int16));
+
+	int IConvertible.ToInt32(IFormatProvider provider) => throw new InvalidCastException(nameof(Int32));
+
+	long IConvertible.ToInt64(IFormatProvider provider) => throw new InvalidCastException(nameof(Int64));
+
+	sbyte IConvertible.ToSByte(IFormatProvider provider) => throw new InvalidCastException(nameof(SByte));
+
+	float IConvertible.ToSingle(IFormatProvider provider) => throw new InvalidCastException(nameof(Single));
+
+	ushort IConvertible.ToUInt16(IFormatProvider provider) => throw new InvalidCastException(nameof(UInt16));
+
+	uint IConvertible.ToUInt32(IFormatProvider provider) => throw new InvalidCastException(nameof(UInt32));
+
+	ulong IConvertible.ToUInt64(IFormatProvider provider) => throw new InvalidCastException(nameof(UInt64));
+
 	public static bool operator ==(FbZonedDateTime lhs, FbZonedDateTime rhs) => lhs.Equals(rhs);
 
 	public static bool operator !=(FbZonedDateTime lhs, FbZonedDateTime rhs) => lhs.Equals(rhs);