使用实体框架即时加载（DataGridView）

Question

大家好，

我正在尝试实现及时加载到虚拟模式DataGridView（Winforms）中的方法，但是我错过了很大的难题或无法正常工作。

我的桌子上约有房子。 150k记录，大约需要2分钟才能加载，这是不可接受的。

下面的代码摘录

 public partial class Customers : Form
{
    private List<Database.Customer> store = new List<Database.Customer>();
    public Customers()
    {
        InitializeComponent();
    }

    private void Customers_Load(object sender, EventArgs e)
    {
        store = Global.db.Customers.ToList();
        dgv_data.VirtualMode = true;
        BindingSource bs = new BindingSource();
        bs.DataSource = Global.db.Customers.Local.ToBindingList();
        dgv_data.DataSource = bs;
    }
    private void dgv_data_CellEndEdit(object sender, DataGridViewCellEventArgs e)
    {
        Global.db.SaveChanges();
    }

    void dgv_data_CellValueNeeded(object sender, DataGridViewCellValueEventArgs e)
    {
        e.Value = store[e.RowIndex].GetType().GetProperties()[e.ColumnIndex].GetValue(store[e.RowIndex], null); ;
    }
}

现在我的理解是CellValueNeeded仅从数据库中提取DataGridView可以显示的信息，因此一次30行，从而将加载时间缩短到可以忽略的持续时间

谁能阐明为什么它不能按预期工作？

Answer 1

诀窍是建立一个内存缓存，一次显示100条左右的行，并在需要其他行时更新。

public partial class Customers : Form
{
    private Cache memoryCache;

    public Customers()
    {
        InitializeComponent();

    }

    private void Customers_Load(object sender, EventArgs e)
    {
        dgv_data.VirtualMode = true;
        Load_Data();
    }
    private void Load_Data()
    {
        try
        {
            DataRetriever retriever =
                new DataRetriever(Global.RetrieverCNS, "*Database Table Name*",null,true);
            memoryCache = new Cache(retriever, 100);
            foreach (DataColumn column in retriever.Columns)
            {
                dgv_data.Columns.Add(
                    column.ColumnName, column.ColumnName);
            }
            this.dgv_data.RowCount = retriever.RowCount;
        }
        catch (SqlException)
        {
            MessageBox.Show("Connection could not be established. " +
                "Verify that the connection string is valid.");
            Application.Exit();
        }
    }

    void dgv_data_CellValueNeeded(object sender, DataGridViewCellValueEventArgs e)
    {
        e.Value = memoryCache.RetrieveElement(e.RowIndex, e.ColumnIndex);
    }
}

数据缓存类（复制和粘贴）

 public interface IDataPageRetriever
    {
        DataTable SupplyPageOfData(int lowerPageBoundary, int rowsPerPage);
    }
    public class DataRetriever : IDataPageRetriever
    {
        private string tableName;
        private SqlCommand command;

        public DataRetriever(string connectionString, string tableName,string sortby,bool desc)
        {
            if (sortby != null)
                columnToSortBy = sortby;
            sortdesc = desc;
            SqlConnection connection = new SqlConnection(connectionString);
            connection.Open();
            command = connection.CreateCommand();
            this.tableName = tableName;
        }

        private int rowCountValue = -1;

        public int RowCount
        {
            get
            {
                // Return the existing value if it has already been determined.
                if (rowCountValue != -1)
                {
                    return rowCountValue;
                }

                // Retrieve the row count from the database.
                command.CommandText = "SELECT COUNT(*) FROM " + tableName;
                rowCountValue = (int)command.ExecuteScalar();
                return rowCountValue;
            }
        }

        private DataColumnCollection columnsValue;

        public DataColumnCollection Columns
        {
            get
            {
                // Return the existing value if it has already been determined.
                if (columnsValue != null)
                {
                    return columnsValue;
                }

                // Retrieve the column information from the database.
                command.CommandText = "SELECT * FROM " + tableName;
                SqlDataAdapter adapter = new SqlDataAdapter();
                adapter.SelectCommand = command;
                DataTable table = new DataTable();
                table.Locale = System.Globalization.CultureInfo.InvariantCulture;
                adapter.FillSchema(table, SchemaType.Source);
                columnsValue = table.Columns;
                return columnsValue;
            }
        }

        private string commaSeparatedListOfColumnNamesValue = null;

        private string CommaSeparatedListOfColumnNames
        {
            get
            {
                // Return the existing value if it has already been determined.
                if (commaSeparatedListOfColumnNamesValue != null)
                {
                    return commaSeparatedListOfColumnNamesValue;
                }

                // Store a list of column names for use in the
                // SupplyPageOfData method.
                System.Text.StringBuilder commaSeparatedColumnNames =
                    new System.Text.StringBuilder();
                bool firstColumn = true;
                foreach (DataColumn column in Columns)
                {
                    if (!firstColumn)
                    {
                        commaSeparatedColumnNames.Append(", ");
                    }
                    commaSeparatedColumnNames.Append("[" + column.ColumnName + "]");
                    firstColumn = false;
                }

                commaSeparatedListOfColumnNamesValue =
                    commaSeparatedColumnNames.ToString();
                return commaSeparatedListOfColumnNamesValue;
            }
        }

        // Declare variables to be reused by the SupplyPageOfData method.
        private string columnToSortBy;
        private bool sortdesc = false;
        private SqlDataAdapter adapter = new SqlDataAdapter();

        public DataTable SupplyPageOfData(int lowerPageBoundary, int rowsPerPage)
        {
            // Store the name of the ID column. This column must contain unique 
            // values so the SQL below will work properly.
            if (columnToSortBy == null)
            {
                columnToSortBy = this.Columns[0].ColumnName;
            }

            if (!this.Columns[columnToSortBy].Unique)
            {
                throw new InvalidOperationException(String.Format(
                    "Column {0} must contain unique values.", columnToSortBy));
            }

            // Retrieve the specified number of rows from the database, starting
            // with the row specified by the lowerPageBoundary parameter.

            //DataTable Results = Global.db.Customers.Select<>
            if (sortdesc == false)
                command.CommandText = "Select Top " + rowsPerPage + " " +
                    CommaSeparatedListOfColumnNames + " From " + tableName +
                    " WHERE " + columnToSortBy + " NOT IN (SELECT TOP " +
                    lowerPageBoundary + " " + columnToSortBy + " From " +
                    tableName + " Order By " + columnToSortBy +
                    ") Order By " + columnToSortBy;

            else
            {
                command.CommandText = "Select Top " + rowsPerPage + " " +
                    CommaSeparatedListOfColumnNames + " From " + tableName +
                    " WHERE " + columnToSortBy + " NOT IN (SELECT TOP " +
                    lowerPageBoundary + " " + columnToSortBy + " From " +
                    tableName + " Order By " + columnToSortBy + " DESC" +
                    ") Order By " + columnToSortBy + " DESC";
            }
            adapter.SelectCommand = command;

            DataTable table = new DataTable();
            table.Locale = System.Globalization.CultureInfo.InvariantCulture;
            adapter.Fill(table);
            return table;
        }

    }
    public class Cache
    {
        private static int RowsPerPage;

        // Represents one page of data.  
        public struct DataPage
        {
            public DataTable table;
            private int lowestIndexValue;
            private int highestIndexValue;

            public DataPage(DataTable table, int rowIndex)
            {
                this.table = table;
                lowestIndexValue = MapToLowerBoundary(rowIndex);
                highestIndexValue = MapToUpperBoundary(rowIndex);
                System.Diagnostics.Debug.Assert(lowestIndexValue >= 0);
                System.Diagnostics.Debug.Assert(highestIndexValue >= 0);
            }

            public int LowestIndex
            {
                get
                {
                    return lowestIndexValue;
                }
            }

            public int HighestIndex
            {
                get
                {
                    return highestIndexValue;
                }
            }

            public static int MapToLowerBoundary(int rowIndex)
            {
                // Return the lowest index of a page containing the given index.
                return (rowIndex / RowsPerPage) * RowsPerPage;
            }

            private static int MapToUpperBoundary(int rowIndex)
            {
                // Return the highest index of a page containing the given index.
                return MapToLowerBoundary(rowIndex) + RowsPerPage - 1;
            }
        }

        private DataPage[] cachePages;
        private IDataPageRetriever dataSupply;

        public Cache(IDataPageRetriever dataSupplier, int rowsPerPage)
        {
            dataSupply = dataSupplier;
            Cache.RowsPerPage = rowsPerPage;
            LoadFirstTwoPages();
        }

        // Sets the value of the element parameter if the value is in the cache.
        private bool IfPageCached_ThenSetElement(int rowIndex,
            int columnIndex, ref string element)
        {
            if (IsRowCachedInPage(0, rowIndex))
            {
                element = cachePages[0].table
                    .Rows[rowIndex % RowsPerPage][columnIndex].ToString();
                return true;
            }
            else if (IsRowCachedInPage(1, rowIndex))
            {
                element = cachePages[1].table
                    .Rows[rowIndex % RowsPerPage][columnIndex].ToString();
                return true;
            }

            return false;
        }

        public string RetrieveElement(int rowIndex, int columnIndex)
        {
            string element = null;

            if (IfPageCached_ThenSetElement(rowIndex, columnIndex, ref element))
            {
                return element;
            }
            else
            {
                return RetrieveData_CacheIt_ThenReturnElement(
                    rowIndex, columnIndex);
            }
        }

        private void LoadFirstTwoPages()
        {
            cachePages = new DataPage[]{
            new DataPage(dataSupply.SupplyPageOfData(
                DataPage.MapToLowerBoundary(0), RowsPerPage), 0),
            new DataPage(dataSupply.SupplyPageOfData(
                DataPage.MapToLowerBoundary(RowsPerPage),
                RowsPerPage), RowsPerPage)};
        }

        private string RetrieveData_CacheIt_ThenReturnElement(
            int rowIndex, int columnIndex)
        {
            // Retrieve a page worth of data containing the requested value.
            DataTable table = dataSupply.SupplyPageOfData(
                DataPage.MapToLowerBoundary(rowIndex), RowsPerPage);

            // Replace the cached page furthest from the requested cell
            // with a new page containing the newly retrieved data.
            cachePages[GetIndexToUnusedPage(rowIndex)] = new DataPage(table, rowIndex);

            return RetrieveElement(rowIndex, columnIndex);
        }

        // Returns the index of the cached page most distant from the given index
        // and therefore least likely to be reused.
        private int GetIndexToUnusedPage(int rowIndex)
        {
            if (rowIndex > cachePages[0].HighestIndex &&
                rowIndex > cachePages[1].HighestIndex)
            {
                int offsetFromPage0 = rowIndex - cachePages[0].HighestIndex;
                int offsetFromPage1 = rowIndex - cachePages[1].HighestIndex;
                if (offsetFromPage0 < offsetFromPage1)
                {
                    return 1;
                }
                return 0;
            }
            else
            {
                int offsetFromPage0 = cachePages[0].LowestIndex - rowIndex;
                int offsetFromPage1 = cachePages[1].LowestIndex - rowIndex;
                if (offsetFromPage0 < offsetFromPage1)
                {
                    return 1;
                }
                return 0;
            }

        }

        // Returns a value indicating whether the given row index is contained
        // in the given DataPage. 
        private bool IsRowCachedInPage(int pageNumber, int rowIndex)
        {
            return rowIndex <= cachePages[pageNumber].HighestIndex &&
                rowIndex >= cachePages[pageNumber].LowestIndex;
        }