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strong text感谢 Tim 在另一篇文章中为我提供了以下代码：Join/merge two excel files in asp.net and display them in gridview。

我在这篇新文章中的问题是：如何重用或将 linq 结果添加到数据集或数据表中，以便我可以查询新表而不是两个单独的表？join 关键字使我可以连接两个表。现在我想使用这个连接表来运行其他查询。

我希望我的问题是有道理的。如果您需要更多信息，请告诉我。

DataSet ds = new DataSet("Stock");
using (var dbConnection = new OleDbConnection(connString))
using (var dbCommand = new OleDbCommand("SELECT * FROM [Sheet1$]", dbConnection))
using (var da = new OleDbDataAdapter(dbCommand))
{
    da.Fill(ds, "HWTypes");
}

using (var dbConnection = new OleDbConnection(stockConn))
using (var dbCommand = new OleDbCommand("SELECT * FROM [Stock_voorlopig$]", dbConnection))
using (var da = new OleDbDataAdapter(dbCommand))
{
    da.Fill(ds, "Stock");
}

var joined = from rType in ds.Tables["HWTypes"].AsEnumerable()
             join rStock in ds.Tables["Stock"].AsEnumerable()
             on rType.Field<string>("ProductID") equals rStock.Field<string>("Partno")
             select new
             {
                 ProductID = rType.Field<string>("ProductID")
                 // add the other columns you need here
             };


GridView1.DataSource = joined;
GridView1.DataBind();

编辑1：

我正在尝试做这样的事情： 

private static DataTable JoinDataTablesWithLinq()
{

 //rest of the code from above
 return joined.CopyToDataTable();
}

但我收到此错误：

The type 'AnonymousType#1' cannot be used as type parameter 'T' in the generic type or method 'System.Data.DataTableExtensions.CopyToDataTable<T>(System.Collections.Generic.IEnumerable<T>)'. There is no implicit reference conversion from 'AnonymousType#1' to 'System.Data.DataRow'.

有什么帮助吗？

4

1 回答 1

1

我正在尝试做这样的事情：

private static DataTable JoinDataTablesWithLinq()
{

 //rest of the code from above
 return joined.CopyToDataTable();
}

您不能创建DataTable“即时”。您选择了一个IEnumerable<DataRow>不需要使用的匿名类型CopyToDataTable。因此，您要么必须使用自定义类并填充 aList<MyClass>所需的所有属性来持久保存它，要么必须创建 aDataTable具有所有必要DataColumns的匿名类型并填充它。

你会发现这并不容易。不过，有一种动态方法。但这需要反思，因此它根本没有效率。

我们走吧……（摘自我的其他答案）

您可以构建自己的CopyToDataTable，它采用任何类型的 IEnumerable（不仅DataRow）并返回一个新的DataTable：

这是实现（在MSDN的帮助下）：

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;
    }
}

现在您可以添加这些扩展：

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);
    }  
}

瞧！现在CopyToDataTable适用于任何类型的IEnumerable<T>:)

于 2012-12-19T12:13:06.890 回答