DataTabel转换List时GetProperty反射赋值报错：从“System.String”到“System.Nullable[[System.DateTime]的强制转换无效

在写代码时经常会碰见可空类型的数据段，Table转换List和List转Table时会很头疼，现在记录下解决方案。

DataTable转换为List

下方代码适合所有可空类型的转换——TableToList：

public static List<T> TableToEntity<T>(DataTable dt) where T : class, new()
       {
           // 定义集合  
           List<T> ts = new List<T>();

           if (dt != null && dt.Rows.Count > 0)
           {
               // 获得此模型的类型  
               Type type = typeof(T);
               string tempName = "";
               foreach (DataRow dr in dt.Rows)
               {
                   T t = new T();
                   // 获得此模型的公共属性  
                   PropertyInfo[] propertys = t.GetType().GetProperties();
                   foreach (PropertyInfo pi in propertys)
                   {
                       tempName = pi.Name;
                       // 检查DataTable是否包含此列  
                       if (dt.Columns.Contains(tempName))
                       {
                           // 判断此属性是否有Setter  
                           if (!pi.CanWrite)
                               continue;
                           object value = dr[tempName];
                           if (value != DBNull.Value)
                           {
                               //pi.SetValue(t, value, null);  
                               // pi.SetValue(t, Convert.ChangeType(value, pi.PropertyType, CultureInfo.CurrentCulture), null);
                               pi.SetValue(t, ChanageType(value,pi.PropertyType), null);
                           }
                       }
                   }
                   ts.Add(t);
               }
           }

           return ts;
       }
      //转换可空类型 如：DateTime? 
       private static object ChanageType(object value, Type convertsionType)
       {
           //判断convertsionType类型是否为泛型，因为nullable是泛型类,
           if (convertsionType.IsGenericType &&
               //判断convertsionType是否为nullable泛型类
               convertsionType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
           {
               if (value == null || value.ToString().Length == 0)
               {
                   return null;
               }

               //如果convertsionType为nullable类，声明一个NullableConverter类，该类提供从Nullable类到基础基元类型的转换
               NullableConverter nullableConverter = new NullableConverter(convertsionType);
               //将convertsionType转换为nullable对的基础基元类型
               convertsionType = nullableConverter.UnderlyingType;
           }
           return Convert.ChangeType(value, convertsionType);
       }

List转换为DataTable

下方代码为List转Table:

/// <summary>  
      /// 转化一个DataTable  
      /// </summary>  
      /// <typeparam name="T"></typeparam>  
      /// <param name="list"></param>  
      /// <returns></returns>  
      public static DataTable ToDataTable<T>(IEnumerable<T> list)
      {
          //创建属性的集合  
          List<PropertyInfo> pList = new List<PropertyInfo>();
          //获得反射的入口  
          Type type = typeof(T);
          DataTable dt = new DataTable();
          //把所有的public属性加入到集合 并添加DataTable的列  
          Array.ForEach<PropertyInfo>(type.GetProperties(), p => { pList.Add(p); dt.Columns.Add(p.Name, p.PropertyType); });
          foreach (var item in list)
          {
              //创建一个DataRow实例  
              DataRow row = dt.NewRow();
              //给row 赋值  
              pList.ForEach(p => row[p.Name] = p.GetValue(item, null));
              //加入到DataTable  
              dt.Rows.Add(row);
          }
          return dt;
      }

但是如果实体类有可空类型的又会报错：System.NotSupportedException: DataSet 不支持 System.Nullable<>

解决方法：

public static DataTable ToDataTable<T>(IEnumerable<T> list)
       {
           try
           {
               DataTable dtReturn = new DataTable();
               // column names
               PropertyInfo[] oProps = null;
               // Could add a check to verify that there is an element 0
               foreach (T rec in list)
               {
                   // Use reflection to get property names, to create table, Only first time, others will follow
                   if (oProps == null)
                   {
                       oProps = ((Type)rec.GetType()).GetProperties();
                       foreach (PropertyInfo pi in oProps)
                       {
                           Type colType = pi.PropertyType; if ((colType.IsGenericType) && (colType.GetGenericTypeDefinition() == typeof(Nullable<>)))
                           {
                               colType = colType.GetGenericArguments()[0];
                           }
                           dtReturn.Columns.Add(new DataColumn(pi.Name, colType));
                       }
                   }
                   DataRow dr = dtReturn.NewRow(); foreach (PropertyInfo pi in oProps)
                   {
                       dr[pi.Name] = pi.GetValue(rec, null) == null ? DBNull.Value : pi.GetValue(rec, null);
                   }
                   dtReturn.Rows.Add(dr);
               }
               return dtReturn;
           }
           catch(Exception es)
           {
               throw es;
           }
         
       }