簡單理解ORM，實體類生成查詢SQL語句

       目前有不少開源的ORM項目，大多狀況下也不須要咱們重複去造輪子，咱們只須要了解輪子怎麼造的，怎麼用就能夠，下面簡單說一下怎麼經過實體生成一個SQL語句；

        先創建2個Attribute類，TableAttribute、ColumnAttribute   ， 且但願TableAttribute只想標記在實體類上，因此限制 [AttributeUsage(AttributeTargets.Class)]，而但願ColumnAttribute只標記在屬性上 [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; } }

     再作一個靜態擴展類，增長2個擴展方法 一個針對類型的、一個針對屬性的擴展方法

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; } } }

         獲取sql語句方法，目前只簡單寫了查詢全部的，及根據ID查詢，若是想豐富查詢操做須要用到表達式目錄樹

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; } }

         調用方法

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}) ); } }

     運行截圖：

     

 

  

 

原文出處：https://www.cnblogs.com/lxshwyan/p/10955323.html