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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
{
/// <summary>
/// 用户ID
/// </summary>
public int UserId { get; set; }
/// <summary>
/// 表名
/// </summary>
public string TableName { get; set; }
/// <summary>
/// 策略所需字段
/// </summary>
public string PolicyField { get; set; }
/// <summary>
/// 策略所需要值
/// </summary>
public string PolicyValue { get; set; }
/// <summary>
/// 策略操作符
/// </summary>
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<Func<User, bool>> exe = ExpressionExtensions.GenerateExpression<User>(
new string[] { "Age", "UserName" },
new object[] { "", "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
/// <summary>
/// 表达式树的扩展
/// </summary>
public class ExpressionExtensions
{
/// <summary>
/// 构建表达式树
/// 调用：GenerateExpression(new string[]{"username"},new object[]{"zzl"},new string[]{"="});
/// </summary>
/// <typeparam name="T">表类型</typeparam>
/// <param name="keys">字段名</param>
/// <param name="values">字段值</param>
/// <param name="operation">操作符</param>
/// <returns></returns>
public static Expression<Func<T, bool>> GenerateExpression<T>(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 = ; 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 = ; 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<Func<T, bool>>)Expression.Lambda<Func<T, bool>>(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．
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