使用表达式树

Question

更新

多亏了马克的帮助下AlphaPagedList类is now available on CodePlex如果有人有兴趣生成LINQ查询

原始

我试图创建一个表达式树返回以给定字符集开头的元素。

IList<char> chars = new List<char>{'a','b'}; 
IQueryable<Dept>Depts.Where(x=> chars.Contains(x.DeptName[0])); 

我想这是任何IEnumerable的使用，我提供了一个lamdba的财产就如选择：

Depts.Alpha(x=>x.DeptName, chars);

我一直想这一点，但没有运气可言，有什么帮助？

public static IQueryable<T> testing<T>(this IQueryable<T> queryableData, Expression<Func<T,string>> pi, IEnumerable<char> chars) 
{ 
// Compose the expression tree that represents the parameter to the predicate. 

ParameterExpression pe = Expression.Parameter(queryableData.ElementType, "x"); 
ConstantExpression ch = Expression.Constant(chars,typeof(IEnumerable<char>)); 
// ***** Where(x=>chars.Contains(x.pi[0])) ***** 
// pi is a string property 
//Get the string property 

Expression first = Expression.Constant(0); 
//Get the first character of the string 
Expression firstchar = Expression.ArrayIndex(pi.Body, first); 
//Call "Contains" on chars with argument being right 
Expression e = Expression.Call(ch, typeof(IEnumerable<char>).GetMethod("Contains", new Type[] { typeof(char) }),firstchar); 


MethodCallExpression whereCallExpression = Expression.Call(
    typeof(Queryable), 
    "Where", 
    new Type[] { queryableData.ElementType }, 
    queryableData.Expression, 
    Expression.Lambda<Func<T, bool>>(e, new ParameterExpression[] { pe })); 
// ***** End Where ***** 

return (queryableData.Provider.CreateQuery<T>(whereCallExpression)); 
} 

Answer 1

喜欢的东西（重新阅读的问题后可进行编辑） - 但请注意，Expression.Invoke不3.5SP1在EF工作（但它是罚款的LINQ到SQL）：

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Linq.Expressions; 

class Dept 
{ 
    public string DeptName { get; set; } 
} 
public static class Program 
{ 
    static void Main() 
    { 
     IList<char> chars = new List<char>{'a','b'}; 
     Dept[] depts = new[] { new Dept { DeptName = "alpha" }, new Dept { DeptName = "beta" }, new Dept { DeptName = "omega" } }; 
     var count = testing(depts.AsQueryable(), dept => dept.DeptName, chars).Count(); 
    } 

    public static IQueryable<T> testing<T>(this IQueryable<T> queryableData, Expression<Func<T,string>> pi, IEnumerable<char> chars) 
    { 
     var arg = Expression.Parameter(typeof(T), "x"); 
     var prop = Expression.Invoke(pi, arg); 
     Expression body = null; 
     foreach(char c in chars) { 
      Expression thisFilter = Expression.Call(prop, "StartsWith", null, Expression.Constant(c.ToString())); 
      body = body == null ? thisFilter : Expression.OrElse(body, thisFilter); 
     } 
     var lambda = Expression.Lambda<Func<T, bool>>(body ?? Expression.Constant(false), arg); 
     return queryableData.Where(lambda); 
    } 
}