異常使用CopyToDataTable與「新{..}」LINQ查詢

Question

從此代碼我可以調用bmwCars.CopyToDataTable（），如我所料。

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

但是，當我有改變的代碼的一些語句下面，我不能叫CopyToDataTable（），爲什麼？

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

根據您對Field<T>的使用，dataTable（我假設其類型爲Car）中的對象繼承DataRow。這是調用CopyToDataTable擴展方法所必需的。但正如所寫的，您將返回一個不能繼承DataRow的匿名類型的枚舉。

所以，可能是你的

select new 

應該

select new Car 

讓你返回一個IEnumerable<Car>而不是匿名類型的IEnumerable<>的。

根據您的Car類的確切結構，可能需要做一些小的合成變化。如果Car有公共財產，Make，Color和PetName那麼它將按我的建議工作。相反，如果Car與方法簽名的構造大致等於

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

那麼你將不得不改變LINQ語句是

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 2

因爲您要返回一個新的匿名類型，而不是汽車對象本身。

Answer 3

你可以建立自己的CopyToDataTable即採取任何形式的IEnumerable的（不僅DataRow）並返回一個新DataTable：

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

這裏是實現（與MSDN幫助）：

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; 
    } 
} 

現在你可以添加這些擴展：

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！現在CopyToDataTable適用於任何種類的IEnumerable<T> :)