类映射错误：“T”必须是一个具有公共无参数构造函数的非抽象types
虽然映射类我得到错误“T”必须是一个非抽象types与公共无参数构造函数为了使用它作为genericstypes或方法中的参数“T”。
下面是我的SqlReaderBase类
public abstract class SqlReaderBase<T> : ConnectionProvider { #region Abstract Methods protected abstract string commandText { get; } protected abstract CommandType commandType { get; } protected abstract Collection<IDataParameter> GetParameters(IDbCommand command); **protected abstract MapperBase<T> GetMapper();** #endregion #region Non Abstract Methods /// <summary> /// Method to Execute Select Queries for Retrieveing List of Result /// </summary> /// <returns></returns> public Collection<T> ExecuteReader() { //Collection of Type on which Template is applied Collection<T> collection = new Collection<T>(); // initializing connection using (IDbConnection connection = GetConnection()) { try { // creates command for sql operations IDbCommand command = connection.CreateCommand(); // assign connection to command command.Connection = connection; // assign query command.CommandText = commandText; //state what type of query is used, text, table or Sp command.CommandType = commandType; // retrieves parameter from IDataParameter Collection and assigns it to command object foreach (IDataParameter param in GetParameters(command)) command.Parameters.Add(param); // Establishes connection with database server connection.Open(); // Since it is designed for executing Select statements that will return a list of results // so we will call command's execute reader method that return a Forward Only reader with // list of results inside. using (IDataReader reader = command.ExecuteReader()) { try { // Call to Mapper Class of the template to map the data to its // respective fields MapperBase<T> mapper = GetMapper(); collection = mapper.MapAll(reader); } catch (Exception ex) // catch exception { throw ex; // log errr } finally { reader.Close(); reader.Dispose(); } } } catch (Exception ex) { throw ex; } finally { connection.Close(); connection.Dispose(); } } return collection; } #endregion } 我想要做的是，我执行一些命令并dynamic地填充我的class级。 这个课程如下：
namespace FooZo.Core { public class Restaurant { #region Private Member Variables private int _restaurantId = 0; private string _email = string.Empty; private string _website = string.Empty; private string _name = string.Empty; private string _address = string.Empty; private string _phone = string.Empty; private bool _hasMenu = false; private string _menuImagePath = string.Empty; private int _cuisine = 0; private bool _hasBar = false; private bool _hasHomeDelivery = false; private bool _hasDineIn = false; private int _type = 0; private string _restaurantImagePath = string.Empty; private string _serviceAvailableTill = string.Empty; private string _serviceAvailableFrom = string.Empty; public string Name { get { return _name; } set { _name = value; } } public string Address { get { return _address; } set { _address = value; } } public int RestaurantId { get { return _restaurantId; } set { _restaurantId = value; } } public string Website { get { return _website; } set { _website = value; } } public string Email { get { return _email; } set { _email = value; } } public string Phone { get { return _phone; } set { _phone = value; } } public bool HasMenu { get { return _hasMenu; } set { _hasMenu = value; } } public string MenuImagePath { get { return _menuImagePath; } set { _menuImagePath = value; } } public string RestaurantImagePath { get { return _restaurantImagePath; } set { _restaurantImagePath = value; } } public int Type { get { return _type; } set { _type = value; } } public int Cuisine { get { return _cuisine; } set { _cuisine = value; } } public bool HasBar { get { return _hasBar; } set { _hasBar = value; } } public bool HasHomeDelivery { get { return _hasHomeDelivery; } set { _hasHomeDelivery = value; } } public bool HasDineIn { get { return _hasDineIn; } set { _hasDineIn = value; } } public string ServiceAvailableFrom { get { return _serviceAvailableFrom; } set { _serviceAvailableFrom = value; } } public string ServiceAvailableTill { get { return _serviceAvailableTill; } set { _serviceAvailableTill = value; } } #endregion public Restaurant() { } } }
为了dynamic填充我的类属性，我有另外一个名为MapperBase类的类，它具有以下方法：
public abstract class MapperBase<T> where T : new() { protected T Map(IDataRecord record) { T instance = new T(); string fieldName; PropertyInfo[] properties = typeof(T).GetProperties(); for (int i = 0; i < record.FieldCount; i++) { fieldName = record.GetName(i); foreach (PropertyInfo property in properties) { if (property.Name == fieldName) { property.SetValue(instance, record[i], null); } } } return instance; } public Collection<T> MapAll(IDataReader reader) { Collection<T> collection = new Collection<T>(); while (reader.Read()) { collection.Add(Map(reader)); } return collection; } }
还有另一个inheritanceSqlreaderBaseClass的类，名为DefaultSearch。 代码如下
public class DefaultSearch: SqlReaderBase<Restaurant> { protected override string commandText { get { return "Select Name from vw_Restaurants"; } } protected override CommandType commandType { get { return CommandType.Text; } } protected override Collection<IDataParameter> GetParameters(IDbCommand command) { Collection<IDataParameter> parameters = new Collection<IDataParameter>(); parameters.Clear(); return parameters; } protected override MapperBase<Restaurant> GetMapper() { MapperBase<Restaurant> mapper = new RMapper(); return mapper; } }
但是，无论何时我试图构build，我得到错误“T”必须是一个非抽象types与公共无参数构造函数，以便在genericstypes或方法中使用它作为参数“T”。 即使在这里T是餐厅有一个无参数公共构造函数。
问题是你试图使用SqlReaderBase的T作为SqlReaderBase的types参数 – 但是你没有任何对T约束。
尝试改变你的SqlReaderBase声明：
public abstract class SqlReaderBase<T> : ConnectionProvider where T : new()
下面是一个简短的例子，它演示了同样的问题：
class Foo<T> { Bar<T> bar; } class Bar<T> where T : new() { }
解决方法是将Foo<T>的声明更改为：
class Foo<T> where T : new()
然后编译器会知道Foo的T是Bar的有效types参数。
约束条件必须适用于链中的每一种types; 因此您需要：
public abstract class SqlReaderBase<T> : ConnectionProvider where T : new()
没有这个，你不能满足T的约束：
protected abstract MapperBase<T> GetMapper();
要么
MapperBase<T> mapper = GetMapper();
因为MapperBase<> 只有在T有: new()
我遇到过同样的问题。 在使用Google之前，我应该先阅读这条消息。 我需要添加一个无参数的构造函数… 🙂
public MyClass() { //stuff }
