您可以简单地先执行select查询，然后再执行insert，也可以在一次行程中执行，如下所示（SQLServer）。您可以使用此处列出的推荐dapper扩展之一

string cmd = @"IF NOT EXISTS (SELECT * FROM Table1 WHERE SupplierId=@SupplierId AND InvoiceId=@InvoiceId)
                    BEGIN 
                        INSERT INTO Table1(SupplierId, InvoiceId) VALUES(@SupplierId, @InvoiceId)
                    END";

using (var dbConn = new SqlConnection("Server=(local);Database=MyDatabase;Integrated Security=true"))
{
    dbConn.Execute(cmd, new { SupplierId = 1, InvoiceId = 2 });
}
Console.ReadKey();

更新mysql，你可以写 INSERT INTO Table1 (SupplierId, InvoiceId) VALUES(@SupplierId, @InvoiceId) ON DUPLICATE KEY SupplierId=SupplierId https://dev.mysql.com/doc/refman/5.7/en/insert-on-duplicate.html