将 CopyToDataTable 与“new {..}”LINQ 查询一起使用时出现异常

Question

从这段代码中，我可以按预期调用 bmwCars.CopyToDataTable()。

var bmwCars = from car in dataTable.AsEnumerable()
                          where car.Field<string>("Make").ToLower().Equals("bmw")
                          select car;

但是当我将一些代码语句更改为下面时，我无法调用 CopyToDataTable()，为什么？

var bmwCars = from car in dataTable.AsEnumerable()
                          where car.Field<string>("Make").ToLower().Equals("bmw")
                          select new
                          {
                              Make = car.Field<string>("Make"),
                              Color = car.Field<string>("Color"),
                              PetName = car.Field<string>("PetName")
                          };

Answer 1

您可以构建自己的 CopyToDataTable，它接受任何类型的 IEnumerable（不仅是 DataRow）并返回一个新的 DataTable：

// following would not compile by default
// because input is not an IEnumerable<DataRow> but an anonymous type
var tblResult = bmwCars.CopyToDataTable(); 

这里是实现（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> :)

Answer 2

根据您对Field<T> 的使用，dataTable 中的对象（我假设其类型为Car）继承DataRow。这是调用CopyToDataTable 扩展方法所必需的。然而，正如所写，您正在返回一个无法继承 DataRow 的匿名类型的枚举。

所以，可能是你的

select new

应该是

select new Car

这样您将返回 IEnumerable<Car> 而不是匿名类型的 IEnumerable<>。

根据Car 类的确切结构，可能需要进行一些小的语法更改。如果Car 具有公共属性Make、Color 和PetName，那么它将按照我的建议工作。相反，如果Car 有一个构造函数，其方法签名大约等于

public Car(string make, string color, string petName)

那么您将不得不将 LINQ 语句更改为

var bmwCars = from car in dataTable.AsEnumerable()
              where car.Field<string>("Make").ToLower().Equals.("bmw")
              select new Car(
                  car.Field<string>("Make"),
                  car.Field<string>("Color"),
                  car.Field<string>("PetName")                          
              );

Answer 3

因为您返回的是一个新的匿名类型，而不是汽车对象本身。