如何：实现 CopyToDataTable<T>，其中泛型类型 T 不是 DataRow

2008-08-22

更新：November 2007

DataTable 对象通常用于数据绑定。CopyToDataTable 方法接收查询结果并将数据复制到 DataTable 中，后者随后会使用该数据进行数据绑定。但是，只在 IEnumerable<T> 源上执行 CopyToDataTable 方法，其中泛型参数 T 的类型为 DataRow。虽然这十分有用，但是它并不允许从标量类型的序列、投影匿名类型的查询或执行表联接的查询创建表。

此主题描述如何实现两个自定义的 CopyToDataTable<T> 扩展方法，其接受类型不是 DataRow 的泛型参数 T。从 IEnumerable<T> 源创建 DataTable 的逻辑包含在 ObjectShredder<T> 类中，该类稍后被包装在两个重载的 CopyToDataTable<T> 扩展方法中。ObjectShredder<T> 类的 Shred 方法返回填充的 DataTable 并接受三个输入参数：一个 IEnumerable<T> 源、一个 DataTable 和一个 LoadOption 枚举。返回的 DataTable 的初始架构是以类型为 T 的架构为基础的。如果现有表作为输入提供，则该架构必须与类型为 T 的架构一致。每个公共属性和类型为 T 的字段将转换为返回的表中的 DataColumn。如果源序列包含从 T 派生的类型，则为任何其他公共属性或字段扩展返回的表的架构。

有关使用自定义的 CopyToDataTable<T> 方法的示例，请参见通过查询创建数据表 (LINQ to DataSet)。

在应用程序中实现自定义的 CopyToDataTable<T> 方法

实现 ObjectShredder<T> 类，以从下面的 IEnumerable<T> 源创建 DataTable：

Public Class ObjectShredder(Of T)
    ' Fields
    Private _fi As FieldInfo()
    Private _ordinalMap As Dictionary(Of String, Integer)
    Private _pi As PropertyInfo()
    Private _type As Type

    ' Constructor 
    Public Sub New()
        Me._type = GetType(T)
        Me._fi = Me._type.GetFields
        Me._pi = Me._type.GetProperties
        Me._ordinalMap = New Dictionary(Of String, Integer)
    End Sub

    Public Function ShredObject(ByVal table As DataTable, ByVal instance As T) As Object()
        Dim fi As FieldInfo() = Me._fi
        Dim pi As PropertyInfo() = Me._pi
        If (Not instance.GetType Is GetType(T)) Then
            ' If the instance is derived from T, extend the table schema
            ' and get the properties and fields.
            Me.ExtendTable(table, instance.GetType)
            fi = instance.GetType.GetFields
            pi = instance.GetType.GetProperties
        End If

        ' Add the property and field values of the instance to an array.
        Dim values As Object() = New Object(table.Columns.Count - 1) {}
        Dim f As FieldInfo
        For Each f In fi
            values(Me._ordinalMap.Item(f.Name)) = f.GetValue(instance)
        Next
        Dim p As PropertyInfo
        For Each p In pi
            values(Me._ordinalMap.Item(p.Name)) = p.GetValue(instance, Nothing)
        Next

        ' Return the property and field values of the instance.
        Return values
    End Function


    ' Summary:           Loads a DataTable from a sequence of objects.
    ' source parameter:  The sequence of objects to load into the DataTable.</param>
    ' table parameter:   The input table. The schema of the table must match that 
    '                    the type T.  If the table is null, a new table is created  
    '                    with a schema created from the public properties and fields 
    '                    of the type T.
    ' options parameter: Specifies how values from the source sequence will be applied to 
    '                    existing rows in the table.
    ' Returns:           A DataTable created from the source sequence.

    Public Function Shred(ByVal source As IEnumerable(Of T), ByVal table As DataTable, ByVal options As LoadOption?) As DataTable

        ' Load the table from the scalar sequence if T is a primitive type.
        If GetType(T).IsPrimitive Then
            Return Me.ShredPrimitive(source, table, options)
        End If

        ' Create a new table if the input table is null.
        If (table Is Nothing) Then
            table = New DataTable(GetType(T).Name)
        End If

        ' Initialize the ordinal map and extend the table schema based on type T.
        table = Me.ExtendTable(table, GetType(T))

        ' Enumerate the source sequence and load the object values into rows.
        table.BeginLoadData()
        Using e As IEnumerator(Of T) = source.GetEnumerator
            Do While e.MoveNext
                If options.HasValue Then
                    table.LoadDataRow(Me.ShredObject(table, e.Current), options.Value)
                Else
                    table.LoadDataRow(Me.ShredObject(table, e.Current), True)
                End If
            Loop
        End Using
        table.EndLoadData()

        ' Return the table.
        Return table
    End Function


    Public Function ShredPrimitive(ByVal source As IEnumerable(Of T), ByVal table As DataTable, ByVal options As LoadOption?) As DataTable
        ' Create a new table if the input table is null.
        If (table Is Nothing) Then
            table = New DataTable(GetType(T).Name)
        End If
        If Not table.Columns.Contains("Value") Then
            table.Columns.Add("Value", GetType(T))
        End If

        ' Enumerate the source sequence and load the scalar values into rows.
        table.BeginLoadData()
        Using e As IEnumerator(Of T) = source.GetEnumerator
            Dim values As Object() = New Object(table.Columns.Count - 1) {}
            Do While e.MoveNext
                values(table.Columns.Item("Value").Ordinal) = e.Current
                If options.HasValue Then
                    table.LoadDataRow(values, options.Value)
                Else
                    table.LoadDataRow(values, True)
                End If
            Loop
        End Using
        table.EndLoadData()

        ' Return the table.
        Return table
    End Function

    Public Function ExtendTable(ByVal table As DataTable, ByVal type As Type) As DataTable
        ' Extend the table schema if the input table was null or if the value 
        ' in the sequence is derived from type T.
        Dim f As FieldInfo
        Dim p As PropertyInfo

        For Each f In type.GetFields
            If Not Me._ordinalMap.ContainsKey(f.Name) Then
                Dim dc As DataColumn

                ' Add the field as a column in the table if it doesn't exist
                ' already.
                dc = IIf(table.Columns.Contains(f.Name), table.Columns.Item(f.Name), table.Columns.Add(f.Name, f.FieldType))

                ' Add the field to the ordinal map.
                Me._ordinalMap.Add(f.Name, dc.Ordinal)
            End If

        Next

        For Each p In type.GetProperties
            If Not Me._ordinalMap.ContainsKey(p.Name) Then
                ' Add the property as a column in the table if it doesn't exist
                ' already.
                Dim dc As DataColumn
                dc = IIf(table.Columns.Contains(p.Name), table.Columns.Item(p.Name), table.Columns.Add(p.Name, p.PropertyType))

                ' Add the property to the ordinal map.
                Me._ordinalMap.Add(p.Name, dc.Ordinal)
            End If
        Next

        ' Return the table.
        Return table
    End Function

End Class

public class ObjectShredder<T>
{
    private System.Reflection.FieldInfo[] _fi;
    private System.Reflection.PropertyInfo[] _pi;
    private System.Collections.Generic.Dictionary<string, int> _ordinalMap;
    private System.Type _type;

    // ObjectShredder constructor.
    public ObjectShredder()
    {
        _type = typeof(T);
        _fi = _type.GetFields();
        _pi = _type.GetProperties();
        _ordinalMap = new Dictionary<string, int>();
    }

    /// <summary>
    /// Loads a DataTable from a sequence of objects.
    /// </summary>
    /// <param name="source">The sequence of objects to load into the DataTable.</param>
    /// <param name="table">The input table. The schema of the table must match that 
    /// the type T.  If the table is null, a new table is created with a schema 
    /// created from the public properties and fields of the type T.</param>
    /// <param name="options">Specifies how values from the source sequence will be applied to 
    /// existing rows in the table.</param>
    /// <returns>A DataTable created from the source sequence.</returns>
    public DataTable Shred(IEnumerable<T> source, DataTable table, LoadOption? options)
    {
        // Load the table from the scalar sequence if T is a primitive type.
        if (typeof(T).IsPrimitive)
        {
            return ShredPrimitive(source, table, options);
        }

        // Create a new table if the input table is null.
        if (table == null)
        {
            table = new DataTable(typeof(T).Name);
        }

        // Initialize the ordinal map and extend the table schema based on type T.
        table = ExtendTable(table, typeof(T));

        // Enumerate the source sequence and load the object values into rows.
        table.BeginLoadData();
        using (IEnumerator<T> e = source.GetEnumerator())
        {
            while (e.MoveNext())
            {
                if (options != null)
                {
                    table.LoadDataRow(ShredObject(table, e.Current), (LoadOption)options);
                }
                else
                {
                    table.LoadDataRow(ShredObject(table, e.Current), true);
                }
            }
        }
        table.EndLoadData();

        // Return the table.
        return table;
    }

    public DataTable ShredPrimitive(IEnumerable<T> source, DataTable table, LoadOption? options)
    {
        // Create a new table if the input table is null.
        if (table == null)
        {
            table = new DataTable(typeof(T).Name);
        }

        if (!table.Columns.Contains("Value"))
        {
            table.Columns.Add("Value", typeof(T));
        }

        // Enumerate the source sequence and load the scalar values into rows.
        table.BeginLoadData();
        using (IEnumerator<T> e = source.GetEnumerator())
        {
            Object[] values = new object[table.Columns.Count];
            while (e.MoveNext())
            {
                values[table.Columns["Value"].Ordinal] = e.Current;

                if (options != null)
                {
                    table.LoadDataRow(values, (LoadOption)options);
                }
                else
                {
                    table.LoadDataRow(values, true);
                }
            }
        }
        table.EndLoadData();

        // Return the table.
        return table;
    }        

    public object[] ShredObject(DataTable table, T instance)
    {

        FieldInfo[] fi = _fi;
        PropertyInfo[] pi = _pi;

        if (instance.GetType() != typeof(T))
        {
            // If the instance is derived from T, extend the table schema
            // and get the properties and fields.
            ExtendTable(table, instance.GetType());
            fi = instance.GetType().GetFields();
            pi = instance.GetType().GetProperties();
        }

        // Add the property and field values of the instance to an array.
        Object[] values = new object[table.Columns.Count];
        foreach (FieldInfo f in fi)
        {
            values[_ordinalMap[f.Name]] = f.GetValue(instance);
        }

        foreach (PropertyInfo p in pi)
        {
            values[_ordinalMap[p.Name]] = p.GetValue(instance, null);
        }

        // Return the property and field values of the instance.
        return values;
    }

    public DataTable ExtendTable(DataTable table, Type type)
    {
        // Extend the table schema if the input table was null or if the value 
        // in the sequence is derived from type T.            
        foreach (FieldInfo f in type.GetFields())
        {
            if (!_ordinalMap.ContainsKey(f.Name))
            {
                // Add the field as a column in the table if it doesn't exist
                // already.
                DataColumn dc = table.Columns.Contains(f.Name) ? table.Columns[f.Name]
                    : table.Columns.Add(f.Name, f.FieldType);

                // Add the field to the ordinal map.
                _ordinalMap.Add(f.Name, dc.Ordinal);
            }
        }
        foreach (PropertyInfo p in type.GetProperties())
        {
            if (!_ordinalMap.ContainsKey(p.Name))
            {
                // Add the property as a column in the table if it doesn't exist
                // already.
                DataColumn dc = table.Columns.Contains(p.Name) ? table.Columns[p.Name]
                    : table.Columns.Add(p.Name, p.PropertyType);

                // Add the property to the ordinal map.
                _ordinalMap.Add(p.Name, dc.Ordinal);
            }
        }

        // Return the table.
        return table;
    }
}

在下面类中实现自定义 CopyToDataTable<T> 扩展方法：

Public Module CustomLINQtoDataSetMethods
    <Extension()> _
    Public Function CopyToDataTable(Of T)(ByVal source As IEnumerable(Of T)) As DataTable
        Return New ObjectShredder(Of T)().Shred(source, Nothing, Nothing)
    End Function

    <Extension()> _
    Public Function CopyToDataTable(Of T)(ByVal source As IEnumerable(Of T), ByVal table As DataTable, ByVal options As LoadOption?) As DataTable
        Return New ObjectShredder(Of T)().Shred(source, table, options)
    End Function

End Module

public static class CustomLINQtoDataSetMethods
{
    public static DataTable CopyToDataTable<T>(this IEnumerable<T> source)
    {
        return new ObjectShredder<T>().Shred(source, null, null);
    }

    public static DataTable CopyToDataTable<T>(this IEnumerable<T> source,
                                                DataTable table, LoadOption? options)
    {
        return new ObjectShredder<T>().Shred(source, table, options);
    }

}

在应用程序中添加 ObjectShredder<T> 类和 CopyToDataTable<T> 扩展方法。

Module Module1
    Sub Main()
    ' Your application code using CopyToDataTable<T>.
    End Sub
End Module

Public Module CustomLINQtoDataSetMethods
…
End Module

Public Class ObjectShredder(Of T)
…
End Class

class Program
{
        static void Main(string[] args)
        {
           // Your application code using CopyToDataTable<T>.
        }
}
public static class CustomLINQtoDataSetMethods
{
…
}
public class ObjectShredder<T>
{
…
}

通过

如何：实现 CopyToDataTable<T>，其中泛型类型 T 不是 DataRow

在应用程序中实现自定义的 CopyToDataTable<T> 方法

请参见

概念

其他资源

其他资源