Moved Fetcher to DB from core as its fetching from DB

src/Comparer/PropertyEqualityComparer.cs

@@ -0,0 +1,58 @@
+using System.Reflection;
+
+namespace DbSyncKit.DB.Comparer
+{
+    /// <summary>
+    /// Compares instances of data contracts based on specified properties, which can be either key or comparable properties.
+    /// </summary>
+    public class PropertyEqualityComparer<T> : IEqualityComparer<T>
+    {
+        /// <summary>
+        /// Gets the array of <see cref="PropertyInfo"/> objects representing properties used for equality comparison.
+        /// These properties can serve as either key or comparable properties.
+        /// </summary>
+        public readonly PropertyInfo[] properties;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PropertyEqualityComparer{T}"/> class.
+        /// </summary>
+        /// <param name="Properties">An array of <see cref="PropertyInfo"/> objects representing properties used for equality comparison. These can be either key or comparable properties.</param>
+        public PropertyEqualityComparer(PropertyInfo[] Properties)
+        {
+            properties = Properties;
+        }
+
+        /// <summary>
+        /// Determines whether two instances of the data contract are equal based on the specified properties.
+        /// </summary>
+        /// <param name="x">The first instance to compare.</param>
+        /// <param name="y">The second instance to compare.</param>
+        /// <returns><c>true</c> if the instances are equal; otherwise, <c>false</c>.</returns>
+        public bool Equals(T? x, T? y)
+        {
+            return properties.All(prop => Equals(prop.GetValue(x), prop.GetValue(y)));
+        }
+
+        /// <summary>
+        /// Returns a hash code for the specified instance of the data contract based on the specified properties.
+        /// </summary>
+        /// <param name="obj">The instance for which to get the hash code.</param>
+        /// <returns>A hash code for the specified instance.</returns>
+        public int GetHashCode(T obj)
+        {
+            unchecked
+            {
+                int hash = 17;
+
+                foreach (var prop in properties)
+                {
+                    var value = prop.GetValue(obj);
+                    hash = hash ^ ((value?.GetHashCode() ?? 0) + 23);
+                }
+
+                return hash;
+            }
+        }
+    }
+
+}

src/Fetcher/DataContractFetcher.cs

@@ -0,0 +1,142 @@
+using DbSyncKit.DB.Comparer;
+using DbSyncKit.DB.Factory;
+using DbSyncKit.DB.Interface;
+
+namespace DbSyncKit.DB.Fetcher
+{
+    /// <summary>
+    /// Represents a callback function for filtering a list of data entities of type T.
+    /// </summary>
+    /// <typeparam name="T">The type of data entities to filter.</typeparam>
+    /// <param name="data">The list of data entities to filter.</param>
+    /// <returns>A filtered list of data entities.</returns>
+    /// <remarks>
+    /// This delegate defines a callback function that takes a list of data entities as input,
+    /// performs filtering operations on the data, and returns the filtered list.
+    /// </remarks>
+    public delegate List<T> FilterCallback<T>(List<T> data);
+
+    /// <summary>
+    /// Utility class for fetching data from a database using data contracts.
+    /// </summary>
+    public class DataContractFetcher
+    {
+        #region Properties
+
+        /// <summary>
+        /// Gets the query generator factory instance used for creating query generators.
+        /// </summary>
+        private readonly QueryGeneratorFactory Factory;
+
+        /// <summary>
+        /// Gets or sets the QueryGenerationManager instance for generating queries for the destination database.
+        /// </summary>
+        /// <remarks>
+        /// This property is set by the <see cref="RetrieveDataFromDatabases{T}(IDatabase, IDatabase, string, List{string}, PropertyEqualityComparer{T}, FilterCallback{T}?, out HashSet{T}, out HashSet{T})"/> method.
+        /// It allows the reuse of the QueryGenerationManager instance when the source and destination databases share the same provider.
+        /// </remarks>
+        public IQueryGenerator? DestinationQueryGenerationManager { get; private set; }
+
+        #endregion
+
+        #region Constructor
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DataContractFetcher"/> class with a custom query generator factory.
+        /// </summary>
+        /// <param name="factory">The custom query generator factory instance.</param>
+        public DataContractFetcher(QueryGeneratorFactory factory)
+        {
+            Factory = factory;
+        }
+
+        #endregion
+
+        #region Methods
+
+        /// <summary>
+        /// Retrieves data from source and destination databases for a specified table and column list, using a specified data contract type.
+        /// </summary>
+        /// <typeparam name="T">The type of data entities to retrieve.</typeparam>
+        /// <param name="source">The source database.</param>
+        /// <param name="destination">The destination database.</param>
+        /// <param name="tableName">The name of the table from which to retrieve data.</param>
+        /// <param name="ColumnList">A list of column names to retrieve from the table.</param>
+        /// <param name="ComparablePropertyEqualityComparer">An equality comparer for identifying properties used in data comparison.</param>
+        /// <param name="filterCallback">A callback function for filtering the retrieved data.</param>
+        /// <param name="sourceList">An output parameter that receives the retrieved data from the source database.</param>
+        /// <param name="destinationList">An output parameter that receives the retrieved data from the destination database.</param>
+        /// <remarks>
+        /// The method uses a QueryGenerationManager to generate queries for retrieving data based on the specified table and columns.
+        /// If the providers of the source and destination databases differ, a new QueryGenerationManager is created for the destination database.
+        /// The retrieved data can be optionally filtered using the provided filter callback function.
+        /// </remarks>
+        public void RetrieveDataFromDatabases<T>(
+            IDatabase source,
+            IDatabase destination,
+            string tableName,
+            List<string> ColumnList,
+            PropertyEqualityComparer<T> ComparablePropertyEqualityComparer,
+            FilterCallback<T>? filterCallback,
+            out HashSet<T> sourceList,
+            out HashSet<T> destinationList)
+        {
+            var sourceQueryGenerationManager = new QueryGenerationManager(Factory.GetQueryGenerator(source.Provider));
+            sourceList = GetDataFromDatabase<T>(tableName, source, sourceQueryGenerationManager, ColumnList, ComparablePropertyEqualityComparer, filterCallback);
+
+            if (source.Provider != destination.Provider)
+            {
+                sourceQueryGenerationManager.Dispose();
+                DestinationQueryGenerationManager = new QueryGenerationManager(Factory.GetQueryGenerator(destination.Provider));
+            }
+            else
+            {
+                DestinationQueryGenerationManager = sourceQueryGenerationManager;
+            }
+
+            destinationList = GetDataFromDatabase<T>(tableName, destination, DestinationQueryGenerationManager, ColumnList, ComparablePropertyEqualityComparer, filterCallback);
+        }
+
+        /// <summary>
+        /// Retrieves data from a database for a specified table, columns, and data contract type.
+        /// </summary>
+        /// <typeparam name="T">The type of data entities to retrieve.</typeparam>
+        /// <param name="tableName">The name of the table from which to retrieve data.</param>
+        /// <param name="connection">The database connection.</param>
+        /// <param name="manager">The query generation manager for creating the SELECT query.</param>
+        /// <param name="columns">A list of column names to retrieve from the table.</param>
+        /// <param name="ComparablePropertyEqualityComparer">An equality comparer for identifying properties used in data comparison.</param>
+        /// <param name="filterCallback">A callback function for filtering the retrieved data.</param>
+        /// <returns>
+        /// A HashSet of data entities of type T retrieved from the specified table and columns in the database.
+        /// </returns>
+        /// <remarks>
+        /// The method generates a SELECT query using the provided query generation manager and executes it using a DatabaseManager.
+        /// The resulting data is converted to a HashSet using the specified property equality comparer for data comparison.
+        /// The retrieved data can be optionally filtered using the provided filter callback function.
+        /// </remarks>
+        public HashSet<T> GetDataFromDatabase<T>(
+            string tableName,
+            IDatabase connection,
+            IQueryGenerator manager,
+            List<string> columns,
+            PropertyEqualityComparer<T> ComparablePropertyEqualityComparer,
+            FilterCallback<T>? filterCallback)
+        {
+            var query = manager.GenerateSelectQuery<T>(tableName, columns, string.Empty);
+
+            using var DBManager = new DatabaseManager<IDatabase>(connection);
+
+            var data = DBManager.ExecuteQuery<T>(query, tableName);
+
+            if (filterCallback != null)
+            {
+                data = filterCallback(data);
+            }
+
+            return data.ToHashSet(ComparablePropertyEqualityComparer);
+        }
+
+        #endregion
+    }
+}