简单理解ORM，实体类生成查询SQL语句

       目前有很多开源的ORM项目，大多情况下也不需要我们重复去造轮子，我们只需要了解轮子怎么造的，怎么用就可以，下面简单说一下怎么通过实体生成一个SQL语句；

        先建立2个Attribute类，TableAttribute、ColumnAttribute   ， 且希望TableAttribute只想标记在实体类上，所以限制 [AttributeUsage(AttributeTargets.Class)]，而希望ColumnAttribute只标记在属性上 [AttributeUsage(AttributeTargets.Property)]

    [AttributeUsage(AttributeTargets.Class)]
    public class TableAttribute : Attribute
    {
        private string _TableName = "";
        public TableAttribute(string TableName)
        {
            this._TableName = TableName;
        }
        public string GetTableName()
        {
            return this._TableName;
        }
    }

   [AttributeUsage(AttributeTargets.Property)]
    public class ColumnAttribute:Attribute
    {
        private string _ColumnName = "";  
        public ColumnAttribute(string ColumnName)
        {
            this._ColumnName = ColumnName;
        }
        public string GetColumnName()
        {
            return this._ColumnName;
        }
    }

     再做一个静态扩展类，增加2个扩展方法 一个针对类型的、一个针对属性的扩展方法

public static class AttributeExtend
    {
        public static string GetMappingName<T>(this T t) where T : BaseModel
        {
            if (t.GetType().IsDefined(typeof(TableAttribute), true))
            {
                TableAttribute attribute = (TableAttribute)t.GetType().GetCustomAttributes(typeof(TableAttribute), true)[0];
                return attribute.GetTableName();
            }
            else
            {
                return t.GetType().Name;
            }
        }

        public static string GetMappingName(this PropertyInfo prop)
        {
            if (prop.IsDefined(typeof(ColumnAttribute), true))
            {
                ColumnAttribute attribute = (ColumnAttribute)prop.GetCustomAttribute(typeof(ColumnAttribute), true);
                return attribute.GetColumnName();
            }
            else
            {
                return prop.Name;
            }
        } 

        public static string GetMappingName(this Type type)
        {
            if (type.IsDefined(typeof(TableAttribute), true))
            {
                TableAttribute attribute = (TableAttribute)type.GetCustomAttribute(typeof(TableAttribute), true);
                return attribute.GetTableName();
            }
            else
            {
                return type.Name;
            }
        }
    }

         获取sql语句方法，目前只简单写了查询所有的，及根据ID查询，如果想丰富查询操作需要用到表达式目录树

  public class OrmSql
    {    
        public string GetAllSelectSQL<T>() where T :BaseModel
        {
           Type type = typeof(T);
           var props = type.GetProperties();   
           string columnString = string.Join(",", props.Select(m => $"[{m.GetMappingName()}]"));
           string SelectSQL = $"select  {columnString} from {type.GetMappingName()}"; 
           return SelectSQL;
        }
        public string GetSelectSQLByID<T>(T t)  where T :BaseModel
        {
            Type type = typeof(T);
            var props = type.GetProperties();
            string columnString = string.Join(",", props.Select(m => $"[{m.GetMappingName()}]"));
            string SelectSQL = $"select  {columnString}  from {type.GetMappingName()} where id= '{t.Id}'";
            return SelectSQL;
        }

    }

         调用方法

 public class Program
    {
        public static void Main(string[] args)
        {
            OrmSql orm = new OrmSql();
            Console.WriteLine(orm.GetAllSelectSQL<Donator>() );
            Console.WriteLine(orm.GetSelectSQLByID<Donator>(new Donator() {  Id=1}) );
        }
    }

     运行截图：