@DonaldJamesParker , Welcome to Microsoft Q&A, based on my test, I find that I can used System.Data.SqlClient and Microsoft.Data.SqlClient normally both.
SqlClient is only a namespace and not a class.
Please try the following steps to use the reference in .net 6.0 project.
First , Please install nuget-package System.Data.SqlClient.
Second, Please add the using statement:
using System.Data.SqlClient;
Finally, you could try the following code to check if the code works for you.
SqlConnection connection = new SqlConnection("connstr");
connection.Open();
If the answer is the right solution, please click "Accept Answer" and upvote it.If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.