给定一个具有属性为Dictionary的类
public class Product
{
public Dictionary<string, string> Attributes { get { return attributes; } }
private Dictionary<string, string> attributes = new Dictionary<string, string>();
}
我希望能够根据从数据存储中检索到的格式
的条件匹配产品列表中的产品Brand == Tyco
Color != Blue
我目前的方法是从过滤器构造一个表达式,然后将该表达式作为参数传递给LINQ Where
方法调用,如此
products = products.Where(myConstructedExpression);
其中myConstructedExpression
通常是一个看起来像
p => p.Attributes[attribute] == value
我已经汇总了以下代码用于测试目的,但无论我为他留下表达而尝试过什么,它总是无法调用lambda.Compile()
。
Dictionary<string, ExpressionType> expressionType = new Dictionary<string, ExpressionType>();
expressionType.Add("==", ExpressionType.Equal);
expressionType.Add("!=", ExpressionType.NotEqual);
string filter = "Brand == Tyco";
string[] fields = filter.Split(' ');
string attribute = fields[0];
string op = fields[1];
string value = fields[2];
Product product = new Product();
product.Attributes.Add("Brand", "Tyco");
var parameter = Expression.Parameter(typeof(Product), "p");
var left = /***** THIS IS WHAT I AM FAILING TO CONSTRUCT PROPERLY ********/
var right = Expression.Constant(value);
var operation = Expression.MakeBinary(expressionType[op], left, right);
var lambda = Expression.Lambda<Func<Product, bool>>(operation, parameter);
var result = lambda.Compile()(product);
问题
答案 0 :(得分:4)
所以要获得p => p.Attributes["Brand"] <someoperator> "Tyco"
,你可以这样做。
使用索引类型的“技巧”是使用他们的Item属性(您也可以使用get_item
方法)
var parameter = Expression.Parameter(typeof(Product), "p");
Expression left = Expression.Property(parameter, "Attributes");
left = Expression.Property(left, "Item", new Expression[] { Expression.Constant(attribute) });
修改强>
IDictionary.ContainsKey(<value>)
测试的版本
真的是一步一步,但我认为这首先会使事情变得更加清晰。
//left part of lambda, p
var parameter = Expression.Parameter(typeof(Product), "p");
//right part
//p.Attributes
Expression left = Expression.Property(parameter, "Attributes");
var method = typeof(IDictionary<string, string>).GetMethod("ContainsKey");
//p.Attributes.ContainsKey("Brand");
Expression containsExpression = Expression.Call(left, method, Expression.Constant(attribute));
//p.Attributes.Item["Brand"]
Expression keyExpression= Expression.Property(left, "Item", new Expression[] { Expression.Constant(attribute) });
//"Tyco"
var right = Expression.Constant(value);
//{p => IIF(p.Attributes.ContainsKey("Brand"), (p.Attributes.Item["Brand"] == "Tyco"), False)}
Expression operation = Expression.Condition(
containsExpression,
Expression.MakeBinary(expressionType[op], keyExpression, right),
Expression.Constant(false));
var lambda = Expression.Lambda<Func<Product, bool>>(operation, parameter);