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誰能這種簡單的LINQ到SQL轉換爲表達式樹:轉換LINQ到SQL表達式表達式樹
List<Region> lst = (from r in dc.Regions
where r.RegionID > 2 && r.RegionDescription.Contains("ern")
select r).ToList();
誰能這種簡單的LINQ到SQL轉換爲表達式樹:轉換LINQ到SQL表達式表達式樹
List<Region> lst = (from r in dc.Regions
where r.RegionID > 2 && r.RegionDescription.Contains("ern")
select r).ToList();
這應做到:
var query = dc.Regions.AsQueryable();
ParameterExpression pe = Expression.Parameter(typeof(Region), "region");
Expression id = Expression.PropertyOrField(pe, "RegionID");
Expression two = Expression.Constant(2);
Expression e1 = Expression.GreaterThan(id, two);
Expression description = Expression.PropertyOrField(pe, "RegionDescription");
MethodInfo method = typeof(string).GetMethod("Contains", new[] {typeof(string)});
Expression ern = Expression.Constant("ern",typeof(string));
Expression e2 = Expression.Call(description, method, ern);
Expression e3 = Expression.And(e1, e2);
MethodCallExpression whereCallExpression = Expression.Call(
typeof(Queryable),
"Where",
new Type[] { query.ElementType },
query.Expression,
Expression.Lambda<Func<Region, bool>>(e3, new ParameterExpression[] { pe }));
var results = query.Provider.CreateQuery<Region>(whereCallExpression);
List<Region> lst = results.ToList();
而且只選擇RegionID
從結果集執行以下操作:
MethodCallExpression selectExpression = Expression.Call(
typeof(Queryable),
"Select",
new[]{ typeof(Region), typeof(int)},
whereCallExpression,
Expression.Lambda<Func<Region, int>>(id, pe));
var regionIDsQuery = query.Provider.CreateQuery<int>(selectExpression);
List<int> regionIDs = regionIDsQuery.ToList();
非常感謝。可以介紹一些關於Expression Tree的文章嗎?你是表達樹的主人嗎?謝謝 – Arian 2011-03-08 04:51:48
請編輯你的文章,並用「ern」替換「widget」。你可以告訴我,如果我只想選擇「RegionID」,我該怎麼做。謝謝 – Arian 2011-03-08 05:26:54
@Nima很高興我能幫忙。謝謝你這麼說,但我不是'Expression Trees'的主人。它們只是一個有趣的結構,我一般都會更好地理解'Linq'。我建議閱讀課程的文檔,但也要編寫一些標準的'Linq'查詢,然後檢查它們的'Expression'屬性。一點一點地這將幫助你瞭解潛在的機制。我會更新我的答案以包含「選擇」呼叫。 – Sorax 2011-03-08 15:21:52