如何使用Linq.Expressions查詢集合

Question

我構建了一個自定義的IQueryable提供程序。該提供者將查詢例如

c.PurchaseDate == new DateTime(2011, 11, 29) && c.Name == "Elizabeth Brown" 

從底層代碼到System.Linq.Expressions.Expression

現在我需要使用LINQ查詢運行它們對這個集合

IQueryable<Customer> customers = _customers.AsQueryable(); 

誰能告訴我如何用Expression查詢集合？

感謝

Answer 1

//Query = c.PurchaseDate == new DateTime(2011, 11, 29) && c.Name 
    // == "Elizabeth Brown") 
IQueryable<Customer> customers = _customers.AsQueryable<Customer>(); 

//Predicate parameter 
ParameterExpression parameter = Expression.Parameter(typeof(Customer), 
                  "customer"); 

//Create left expression 
Expression left = Expression.Property(parameter, typeof(Customer) 
            .GetProperty("PurchaseDate")); 
Expression right = Expression.Constant(new DateTime(2011, 11, 29)); 
Expression leftExp = Expression.Equal(left, right); 

//Create right expression tree 
left = Expression.Property(parameter, typeof(Customer).GetProperty("Name")); 
right = Expression.Constant("Elizabeth Brown", typeof(string)); 
Expression rightExp = Expression.Equal(left, right); 

//Combine the expressions into expression tree 
Expression expressionTree = Expression.AndAlso(leftExp, rightExp); 

//Create an expression tree that represents the expression 
MethodCallExpression methodCall = Expression.Call(
    typeof(Queryable), 
    "Where", 
    new Type[] { customers.ElementType }, 
    customers.Expression, 
    Expression 
       .Lambda<Func<Customer, bool>> 
       (expressionTree, new ParameterExpression[] { parameter })); 

// Create an executable query from the expression tree. 
IQueryable<Customer> results = 
       customers.Provider.CreateQuery<Customer>(methodCall); 

// Enumerate the results 
foreach (Customer customer in results) 
{ 
    Console.WriteLine("{0} {1}", customer.Name, customer.PurchaseDate); 
} 

Console.ReadLine(); 

我完成了這樣的任務。 IQueryable是真的很奇妙的東西。請享用！