SqlClient: SqlClient fails to convert decimal(38,20) to .Net decimal for values within the range of .Net's decimal type

This program

        static void Main(string[] args)
        {
            using (var con = new SqlConnection("server=.;database=tempdb;Integrated Security=true"))
            {
                con.Open();
                var cmd = new SqlCommand("select cast(4210862852.86 as decimal(38,20))  val", con);
                using (SqlDataReader rdr = cmd.ExecuteReader())
                {
                    rdr.Read();

                    var val = rdr.GetDecimal(0);
                    Console.WriteLine(val);
                }
            }
        }

fails with

System.OverflowException: ‘Conversion overflows.’

at SqlBuffer.cs line 254:

                if (StorageType.Decimal == _type)
                {
                    if (_value._numericInfo.data4 != 0 || _value._numericInfo.scale > 28)
                    {
                        throw new OverflowException(SQLResource.ConversionOverflowMessage);
                    }

This value is well within the range of a .NET decimal, and it’s a value that the SqlClient can write using a SqlParameter. So it should be converted to .NET decimal.

The byte pattern in SQL and TDS for this value is:

0x261400010000D877FB4DEE8B51699A5005000000

And it looks like SqlBuffer refuses to convert any value with a non-zero value in the last 4 bytes. But a .NET Decimal is stored as a 12-byte integer, along with a sign and a scaling factor.

So the apparent intent is to determine if the SqlDecimal uses a larger-than-12 byte integer and trigger the overflow.

So this conversion has a bug. It looks like the SqlClient is confused on the byte ordering SQL Server is using, and the last 4 bytes of this buffer are not the 4 least-significant bytes of the 16-byte integer embedded in the decimal.

This number requires uses 12 (base-10) decimal digits to store, so it requires fewer than 12 (base-256) bytes to store.

https://docs.microsoft.com/en-us/dotnet/api/system.decimal?redirectedfrom=MSDN&view=netframework-4.8