Why does SqlClient use unnecessary XML conversion while passing SqlXml?

Question

I`ve got a question about passing xml datatype to a query from C# code.

First, here`s a table on SQL Server:

CREATE TABLE [dbo].[XmlTable](
    [id] [int] IDENTITY(1,1) NOT NULL,
    [dat] [xml] NOT NULL
)

Second, I have an application which is written on C#. And I insert rows into the XmlTable from the application.

Here`s the code:

XDocument doc = new XDocument(new XElement("root",
     new XElement("abc", "123"),
     new XElement("cba", "321")));

var reader = new XmlTextReader(new StringReader(doc.ToString()));
var xml = new SqlXml(reader);

using (var connection = new SqlConnection(_connectionString))
{
     using (var command = new SqlCommand("insert into XmlTable (dat) values(@data)", connection))
     {
          command.Parameters.Add(new SqlParameter("@data", SqlDbType.Xml){Value = xml});
          connection.Open();
          command.ExecuteNonQuery();
     }
}

Everything works fine, rows are inserted. But if I run the SQL Profiler and inspect the insert command, I see following:

declare @p3 xml
set @p3=convert(xml,N'<root><abc>123</abc><cba>321</cba></root>')
exec sp_executesql N'insert into XmlTable (dat) values(@data)',N'@data xml',@data=@p3

We can see that in spite of passing SqlXml type into a query SqlClient performs unnecessary conversion from string to XML. XML parser does waste work.

The question is: why does SqlClient do this way? And how can I make it do without extra conversion?

Answer 1

There is no converssion occuring in reallity. What you see is an artifact of Profiler/Trace having to somehow represent the TDS RPC call as a T-SQL batch you can copy paste and execute in SSMS. The real text executed is just insert into XmlTable (dat) values(@data) and @data type/value comes from the ParamData portion of the request. As with any wire protocol, serialization and encoding has to occur in the client and decoding and deserialization has to occur at the server. As a side note, there is no call to sp_execute_sql either, that is again an artifact of how Profiler/Trace displays an RPC call.

this code is used in high-load scenario and additional convertation makes it slower

That is speculation. You should base performance analysis on measurements and observations.