Lind.DDD框架里提出了对数据集的控制，某些权限的用户为某些表添加某些数据集的权限，具体实现是在一张表中存储用户ID,表名，检索字段，检索值和检索操作符，然后用户登陆后，通过自己权限来构建对应表的查询语句，即动态构建表达式树，这种操作一些被写在业务层上，我们可以在业务层需要进行数据集权限控制的地方，添加这种策略，下面具体分析说明一下．

看一下数据集权限表结果

public class User_DataSet_Policies    {        /// 
        /// 用户ID        /// 
        public int UserId { get; set; }        /// 
        /// 表名        /// 
        public string TableName { get; set; }        /// 
        /// 策略所需字段        /// 
        public string PolicyField { get; set; }        /// 
        /// 策略所需要值        /// 
        public string PolicyValue { get; set; }        /// 
        /// 策略操作符        /// 
        public string PolicyOperation { get; set; }    }

看一下，在程序中如何动态构建和使用我们的表达式树

　　　　　　 Expression
     
      
       > exe = ExpressionExtensions.GenerateExpression
       
        (                new string[] { "Age", "UserName" },                new object[] { "12", "zzl" },                new string[] { "=", "=" });            userList.Where(exe.Compile()).ToList().ForEach(i =>            {                Console.WriteLine(i.UserName);            });
       
      
     

下面贡献一下GenerateExpression泛型方法的原码，大家可以把它添加到我们的LinqExtensions模块里

　　 /// 
    /// 表达式树的扩展    /// 
    public class ExpressionExtensions    {        /// 
        /// 构建表达式树        /// 调用：GenerateExpression(new string[]{"username"},new object[]{"zzl"},new string[]{"="});        /// 
        /// 
     
      表类型
             /// 字段名        /// 字段值        /// 操作符        /// 
             public static Expression
     
      
       > GenerateExpression
       
        (string[] keys, object[] values, string[] operation)        {            var TType = typeof(T);            Expression expression_return = Expression.Constant(true);            ParameterExpression expression_param = Expression.Parameter(TType, "p");            Expression expression;            for (int i = 0; i < keys.Length; i++)            {                switch (operation[i])                {                    case "=":                        expression = Expression.Equal(Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                            TType.GetMethod("ToString")),                         Expression.Constant(values[i]));                        expression_return = Expression.And(expression_return, expression);                        break;                    case "%":                        expression = Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                            typeof(string).GetMethod("Contains"),                            Expression.Constant(values[i], typeof(string)));                        expression_return = Expression.And(expression_return, expression);                        break;                    case ">":                        expression = Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                            typeof(double).GetType().GetMethod("GreaterThan"), Expression.Constant(values[i]));                        expression_return = Expression.And(expression_return, expression);                        break;                    case "<":                        expression = Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                            typeof(double).GetType().GetMethod("LessThan"), Expression.Constant(values[i]));                        expression_return = Expression.And(expression_return, expression);                        break;                    case ">=":                        expression = Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                            typeof(double).GetType().GetMethod("GreaterThanOrEqual"), Expression.Constant(values[i]));                        expression_return = Expression.And(expression_return, expression);                        break;                    case "<=":                        expression = Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                            TType.GetProperty(keys[i]).GetType().GetMethod("LessThanOrEqual"), Expression.Constant(values[i]));                        expression_return = Expression.And(expression_return, expression);                        break;                    case "in":                        string[] strarr = values[i].ToString().Split(',');                        Expression or_return = Expression.Constant(false);                        for (int k = 0; k < strarr.Length; k++)                        {                            expression = Expression.Equal(Expression.Call(Expression.Property(expression_param, TType.GetProperty(keys[i])),                                TType.GetMethod("ToString")),                             Expression.Constant(strarr[k]));                            or_return = Expression.Or(or_return, expression);                        }                        expression_return = Expression.And(expression_return, or_return);                        break;                    default:                        throw new ArgumentException("无效的操作符，目前只支持=,%,>,<,>=,<=,in");                }            }            return (Expression
        
         
          >)Expression.Lambda
          
           
            >(expression_return, new ParameterExpression[] { expression_param }); } }
           
          
         
        
       
      
     

对于动态构建表达式的介绍就到这里了，以后在使用过程中如果出现什么问题，请直接回复我．