Refactoring in C#

TopQuery.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace BeforeRefactoring
{
    public class TopQuery
    {
        public const int DB_ACCESS = 1;
        public const int DB_ORACLE = 2;
        public const int DB_SQL_SERVER = 3;
        public const int DB_SYBASE = 4;
        public const int DB_OTHER = 5;

        private int dbType = -1;

        public readonly static int NO_MAXIMUM_LIMIT = -1;
        private int maxResults = NO_MAXIMUM_LIMIT;

        private bool IsValidDBType(int db)
        {
            switch (db)
            {
                case DB_ACCESS: return true;
                case DB_ORACLE: return true;
                case DB_SQL_SERVER: return true;
                case DB_SYBASE: return true;
                case DB_OTHER: return true;
                default: return false;
            }
        }

        public int MaxResults
        {
            set
            {
                this.maxResults = (value < 0) ? NO_MAXIMUM_LIMIT : value;
            }

            get
            {
                return maxResults;
            }
        }

        public int GetDbType()
        {
            return dbType;
        }

        public void SetDbType(int dbType)
        {
            if (IsValidDBType(dbType))
                this.dbType = dbType;
        }

        public String Query(String query)
        {
            switch (dbType)
            {
                case DB_ACCESS: return (maxResults == NO_MAXIMUM_LIMIT) ? query
                                        : "SELECT TOP " + maxResults +
                                          " * FROM (" + query + ")";

                case DB_SYBASE:
                case DB_SQL_SERVER: if (maxResults == NO_MAXIMUM_LIMIT)
                        return query;
                    else
                    {
                        query = query.ToUpper();
                        const String SELECT = "SELECT ";
                        int index = query.IndexOf(SELECT);
                        if (index != -1)
                        {
                            String modifiedQuery = query.Substring(index + SELECT.Length);
                            return modifiedQuery.Insert(0, "SELECT TOP " + maxResults + " ");
                        }
                        else
                            return query;
                    }

                case DB_ORACLE: return (maxResults == NO_MAXIMUM_LIMIT) ? query
                                  : "SELECT * FROM (" + query +
                                    ") WHERE rownum <= " + maxResults;
                case DB_OTHER:
                default: return query;
            }
        }
    }
}

TopQuerySpecs.cs

using System;
using System.Text;
using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Specs = Microsoft.VisualStudio.TestTools.UnitTesting.TestClassAttribute;
using Specification = Microsoft.VisualStudio.TestTools.UnitTesting.TestMethodAttribute;
using Setup = Microsoft.VisualStudio.TestTools.UnitTesting.TestInitializeAttribute;
using Assert = NUnit.Framework.Assert;
using NUnit.Framework;


namespace BeforeRefactoring
{
    [Specs]
    public class TopQueryTest
    {
        private const String query = "select * from employee";
        private TopQuery topQuery;
        private int maxResults = 50;

        [Setup]
        public void setUp()
        {
            topQuery = new TopQuery();
            topQuery.MaxResults = maxResults;
        }

        [Specification]
        public void SetMaximumResultsToNoLimitsWhenNegativeNumberIsPassed()
        {
            topQuery.MaxResults = -100;
            Assert.AreEqual(TopQuery.NO_MAXIMUM_LIMIT, topQuery.MaxResults);
        }

        [Specification]
        public void SetsMaximumResultsToNumberOfResultsRequired()
        {
            Assert.AreEqual(maxResults, topQuery.MaxResults);
        }

        [Specification]
        public void CreatesAccessTopQuery()
        {

            topQuery.SetDbType(TopQuery.DB_ACCESS);
            String expectedQuery = "SELECT TOP " + maxResults + " * FROM (" + query + ")";
            String actualQuery = topQuery.Query(query);
            Assert.AreEqual(expectedQuery, actualQuery);
        }

        [Specification]
        public void CreatesSybaseTopQuery()
        {
            topQuery.SetDbType(TopQuery.DB_SYBASE);
            String expectedQuery = "SELECT TOP " + maxResults + " * FROM EMPLOYEE";
            String actualQuery = topQuery.Query(query);
            Assert.AreEqual(expectedQuery, actualQuery);
        }

        [Specification]
        public void CreatesSQLServerTopQuery()
        {
            topQuery.SetDbType(TopQuery.DB_SQL_SERVER);
            String expectedQuery = "SELECT TOP " + maxResults + " * FROM EMPLOYEE";
            String actualQuery = topQuery.Query(query);
            Assert.AreEqual(expectedQuery, actualQuery);
        }

        [Specification]
        public void CreatesOracleTopQuery()
        {
            topQuery.SetDbType(TopQuery.DB_ORACLE);
            String expectedQuery = "SELECT * FROM (" + query + ") WHERE rownum <= " + maxResults;
            String actualQuery = topQuery.Query(query);
            Assert.AreEqual(expectedQuery, actualQuery);
        }

        [Specification]
        public void CreatesOtherTopQuery()
        {
            topQuery.SetDbType(TopQuery.DB_OTHER);
            String actualQuery = topQuery.Query(query);
            Assert.AreEqual(query, actualQuery);
        }
    }
}