Why selecting rowversion using outer join never returns null

Question

Using SQL Server 2014.

Given table with rowversion column, other table to join with and a select like this:

 CREATE TABLE dbo.FooTable(
 [Id] [int] IDENTITY(1,1) NOT NULL,
 [RowVersion] [rowversion] NULL,
 CONSTRAINT [PK_FooTable] PRIMARY KEY CLUSTERED 
(
 [Id] ASC
)) 
 CREATE TABLE dbo.BarTable(
 [Id] [int] IDENTITY(1,1) NOT NULL
 CONSTRAINT [PK_BarTable] PRIMARY KEY CLUSTERED 
(
 [Id] ASC
)) 
Insert into BarTable default values
GO
SELECT *
 FROM FooTable ft
 FULL OUTER JOIN BarTable bt on ft.Id = Bt.Id

Result of select is:

Id RowVersion Id
NULL 0x 1

Where there was no match with BarTable the RowVersion column is not null, but 0x.

This confuses Dapper, which we use to deserialize result on app server, into thinking, it should construct an object for FooTable part of row - an invalid one, containing just an empty byte array.

Is there any reason for this rather unexpected behaviour?

Are there any smarter solutions than translating the value back to null using CASE in the sql statement or dealing with it in application code?

• Martin Smith: SELECT CAST(NULL AS ROWVERSION) would be a simpler repro

Answer 1

This appears to be a bug in .NET's SqlClient. The column is being returned as an empty byte[] even though it's marked as null in the result.

EG

using System;
using System.Data.Odbc;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using System.Threading;
using System.Threading.Tasks;
using System.Transactions;
namespace ConsoleApp14
{
 class Program
 {
 static void Main(string[] args)
 {
 using (var con = new SqlConnection("Server=localhost;database=tempdb;Integrated Security=true"))
 {
 con.Open();
 var cmd = con.CreateCommand();
 cmd.CommandText = "select cast(null as rowversion) rv";
 using (var rdr = cmd.ExecuteReader())
 {
 rdr.Read();
 var allowDbNull = rdr.GetColumnSchema()[0].AllowDBNull;
 var isNull = rdr.IsDBNull(0);
 var val = rdr[0];
 Console.WriteLine($"SqlClient: AllowDbNull {allowDbNull} IsDbNull: {isNull} {val.GetType().Name} {val}");
 }
 }
 using (var con = new SqlConnection("Server=localhost;database=tempdb;Integrated Security=true"))
 {
 con.Open();
 var cmd = con.CreateCommand();
 cmd.CommandText = "select @val = cast(null as rowversion) ";
 var p = cmd.Parameters.Add(new SqlParameter("@val", System.Data.SqlDbType.Timestamp));
 p.Direction = System.Data.ParameterDirection.Output;
 cmd.ExecuteNonQuery();
 {
 SqlBinary val = (SqlBinary) p.SqlValue;
 Console.WriteLine($"SqlClient (parameter): IsDbNull: {val.IsNull} {val.GetType().Name} {val}");
 }
 }
 using (var con = new OdbcConnection("Driver={ODBC Driver 17 for SQL Server};Server=localhost;Trusted_Connection=yes"))
 {
 con.Open();
 var cmd = con.CreateCommand();
 cmd.CommandText = "select cast(null as rowversion) rv";
 using (var rdr = cmd.ExecuteReader())
 {
 rdr.Read();
 var allowDbNull = rdr.GetSchemaTable().Rows[0]["AllowDBNull"];
 var isNull = rdr.IsDBNull(0);
 var val = rdr[0];
 Console.WriteLine($"ODBC: AllowDbNull {allowDbNull} IsDbNull: {isNull} {val.GetType().Name} {val}");
 }
 }
 }
 }
}

Outputs

SqlClient: AllowDbNull True IsDbNull: False Byte[] System.Byte[]
SqlClient (parameter): IsDbNull: True SqlBinary Null
ODBC: AllowDbNull True IsDbNull: True DBNull

I've opened an issue at https://github.com/dotnet/SqlClient/issues/255 But it's likely to be closed as WontFix. According to notes in the source

// Dev10 Bug #479607 - this should have been the same as SqlDbType.Binary, but it's a rejected breaking change

The issue has been raised, but not fixed as a breaking change. It may get fixed in .NET Core, which is full of breaking changes anyway, and left as-is in .NET Framework.

• Martin Smith: from the comment in the code here looks like this is a known issue github.com/dotnet/SqlClient/blob/master/src/… - and also here github.com/dotnet/SqlClient/blob/master/src/…
• Paul White: Fixed in github.com/dotnet/SqlClient/pull/998 for Microsoft.Data.SqlClient

Answer 2

Except the correct and accepted answer being "it's a bug", what you can do about is what ypercube suggests in comment: Cast it to VARBINARY

SELECT ft.[Id], 
 CAST(ft.[RowVersion] AS varbinary(8)),
 bt.[Id] 
FROM FooTable ft 
 FULL OUTER JOIN BarTable bt
 ON ft.Id = Bt.Id ;