关于Dapper的使用笔记3

1、默认的类型映射，直接以实体属性(或字段)映射到SQL中字段，（名称可以忽略大小写）

2、如果字段带下划线的，设置Dapper.DefaultTypeMap.MatchNamesWithUnderscores = true; 即可。

3、正因为有这个DefaultTypeMap默认的映射策略，所以，一般情况下，可以直接通过SQL查询，生成实体List.

4、Dapper还是以数据库为中心的思考问题模式，不同于Entity framework和nhibernate，提倡以实体到DB的思考问题方式。

5、Dapper在处理实体关联方面算是硬伤了，只能程序员自已根据DTO的模型去自已动手组装了。

附上代码

    /// <summary>
    /// 表示Dapper默认的类型映射策略
    /// Represents default type mapping strategy used by Dapper
    /// </summary>
    sealed partial class DefaultTypeMap : SqlMapper.ITypeMap
    {
        /// <summary>
        /// 要映射的实体，所有的字段
        /// </summary>
        private readonly List<FieldInfo> _fields;

        /// <summary>
        /// 要映射的实体，所有的属性
        /// </summary>
        private readonly List<PropertyInfo> _properties;

        /// <summary>
        /// 要映射的实体类型
        /// </summary>
        private readonly Type _type;

        /// <summary>
        /// 构造函数，创建缺省的类型映射策略
        /// Creates default type map
        /// </summary>
        /// <param name="type">Entity type</param>
        public DefaultTypeMap(Type type)
        {
            if (type == null)
                throw new ArgumentNullException("type");

            _fields = GetSettableFields(type);
            _properties = GetSettableProps(type);
            _type = type;
        }

        /// <summary>
        /// 获取属性的setter访问器
        /// </summary>
        /// <param name="propertyInfo"></param>
        /// <param name="type"></param>
        /// <returns></returns>
        internal static MethodInfo GetPropertySetter(PropertyInfo propertyInfo, Type type)
        {
            return propertyInfo.DeclaringType == type ?
                propertyInfo.GetSetMethod(true) :
                propertyInfo.DeclaringType.GetProperty(
                   propertyInfo.Name,
                   BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance,
                   Type.DefaultBinder,
                   propertyInfo.PropertyType,
                   propertyInfo.GetIndexParameters().Select(p => p.ParameterType).ToArray(),
                   null).GetSetMethod(true);
        }

        /// <summary>
        /// 获取某类型所有可写（有set,包括私有）的成员属性
        /// </summary>
        /// <param name="t"></param>
        /// <returns></returns>
        internal static List<PropertyInfo> GetSettableProps(Type t)
        {
            return t
                  .GetProperties(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance)
                  .Where(p => GetPropertySetter(p, t) != null)
                  .ToList();
        }

        /// <summary>
        /// 获取某类型所有的成员字段
        /// </summary>
        /// <param name="t"></param>
        /// <returns></returns>
        internal static List<FieldInfo> GetSettableFields(Type t)
        {
            return t.GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance).ToList();
        }

        /// <summary>
        /// 获取最佳的成员构造器，优先顺序：public的构造器0, 其它构造器1, private的构造器2,然后再按参数匹配
        /// Finds best constructor
        /// </summary>
        /// <param name="names">DataReader column names</param>
        /// <param name="types">DataReader column types</param>
        /// <returns>Matching constructor or default one</returns>
        public ConstructorInfo FindConstructor(string[] names, Type[] types)
        {
            var constructors = _type.GetConstructors(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
            foreach (ConstructorInfo ctor in constructors.OrderBy(c => c.IsPublic ? 0 : (c.IsPrivate ? 2 : 1)).ThenBy(c => c.GetParameters().Length))
            {
                ParameterInfo[] ctorParameters = ctor.GetParameters();
                if (ctorParameters.Length == 0)
                    return ctor;

                if (ctorParameters.Length != types.Length)
                    continue;

                int i = 0;
                for (; i < ctorParameters.Length; i++)
                {
                    if (!String.Equals(ctorParameters[i].Name, names[i], StringComparison.OrdinalIgnoreCase))
                        break;
                    if (types[i] == typeof(byte[]) && ctorParameters[i].ParameterType.FullName == SqlMapper.LinqBinary)
                        continue;
                    var unboxedType = Nullable.GetUnderlyingType(ctorParameters[i].ParameterType) ?? ctorParameters[i].ParameterType;
                    if (unboxedType != types[i]
                        && !(unboxedType.IsEnum && Enum.GetUnderlyingType(unboxedType) == types[i])
                        && !(unboxedType == typeof(char) && types[i] == typeof(string)))
                        break;
                }

                if (i == ctorParameters.Length)
                    return ctor;
            }

            return null;
        }

        /// <summary>
        /// 查找带ExplicitConstructorAttribute特性标识的构造器
        /// Returns the constructor, if any, that has the ExplicitConstructorAttribute on it.
        /// </summary>
        public ConstructorInfo FindExplicitConstructor()
        {
            var constructors = _type.GetConstructors(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
            var withAttr = constructors.Where(c => c.GetCustomAttributes(typeof(ExplicitConstructorAttribute), true).Length > 0).ToList();

            if (withAttr.Count == 1)
            {
                return withAttr[0];
            }

            return null;
        }

        /// <summary>
        /// 根据构造函数和列名
        /// Gets mapping for constructor parameter
        /// </summary>
        /// <param name="constructor">Constructor to resolve</param>
        /// <param name="columnName">DataReader column name</param>
        /// <returns>Mapping implementation</returns>
        public SqlMapper.IMemberMap GetConstructorParameter(ConstructorInfo constructor, string columnName)
        {
            var parameters = constructor.GetParameters();

            //注意构造器参数名与列名是相同的(忽略大小写)
            return new SimpleMemberMap(columnName, parameters.FirstOrDefault(p => string.Equals(p.Name, columnName, StringComparison.OrdinalIgnoreCase)));
        }

        /// <summary>
        /// 为某列找到映射关系，返回IMemberMap
        /// Gets member mapping for column
        /// </summary>
        /// <param name="columnName">DataReader column name</param>
        /// <returns>Mapping implementation</returns>
        public SqlMapper.IMemberMap GetMember(string columnName)
        {
            //优先找与列名相同的属性名（优先找大小写相同的，再找不同的）。
            var property = _properties.FirstOrDefault(p => string.Equals(p.Name, columnName, StringComparison.Ordinal))
               ?? _properties.FirstOrDefault(p => string.Equals(p.Name, columnName, StringComparison.OrdinalIgnoreCase));

            //如果找不到，就尝试去掉列名下划线，再找一次。
            if (property == null && MatchNamesWithUnderscores)
            {
                property = _properties.FirstOrDefault(p => string.Equals(p.Name, columnName.Replace("_", ""), StringComparison.Ordinal))
                    ?? _properties.FirstOrDefault(p => string.Equals(p.Name, columnName.Replace("_", ""), StringComparison.OrdinalIgnoreCase));
            }

            //如果找到就返回一个成员映射对象
            if (property != null)
                return new SimpleMemberMap(columnName, property);

            //如果没有找到匹配的属性，就找DomainObject.Field，按照上面的套路再找一次。
            var field = _fields.FirstOrDefault(p => string.Equals(p.Name, columnName, StringComparison.Ordinal))
               ?? _fields.FirstOrDefault(p => string.Equals(p.Name, columnName, StringComparison.OrdinalIgnoreCase));

            if (field == null && MatchNamesWithUnderscores)
            {
                field = _fields.FirstOrDefault(p => string.Equals(p.Name, columnName.Replace("_", ""), StringComparison.Ordinal))
                    ?? _fields.FirstOrDefault(p => string.Equals(p.Name, columnName.Replace("_", ""), StringComparison.OrdinalIgnoreCase));
            }

            if (field != null)
                return new SimpleMemberMap(columnName, field);

            return null;
        }
        /// <summary>
        /// 按列名匹配属性时，忽略下划线（有些时习惯在数据库中使用下划线）。
        /// Should column names like User_Id be allowed to match properties/fields like UserId ?
        /// </summary>
        public static bool MatchNamesWithUnderscores { get; set; }
    }