數組屬性過濾器的動態lambda表達式

Question

根據要求，我想使用C＃創建動態lambda表達式。

比如我想生成像

Employee. Address[1].City 

我怎樣才能做到這一點的動態查詢？請注意，該酒店是一個充滿活力的酒店。

我已經試過這個代碼

var item = Expression.Parameter(typeof(Employee), "item"); 

Expression prop = Expression.Property(item, "Address", new Expression[] { Expression.Constant[1] }); 
prop = Expression.Property(prop, "City"); 

var propValue = Expression.Constant(constraintItem.State); 
var expression = Expression.Equal(prop, propValue); 
var lambda = Expression.Lambda<Func<Line, bool>>(expression, item); 

但沒有奏效。

任何幫助，將不勝感激。

謝謝。

Answer 1

你的「動態查詢」表達（這是不是一個真正的查詢，這是一個簡單的MemberExpression）可以如下產生：

ParameterExpression param = Expression.Parameter(typeof(Employee), "item"); 
MemberExpression address = Expression.Property(param, "Address"); 
BinaryExpression indexedAddress = Expression.ArrayIndex(address, Expression.Constant(1)); 
MemberExpression city = Expression.Property(indexedAddress, "City"); // Assuming "City" is a string. 

// This will give us: item => item.Address[1].City 
Expression<Func<Employee, string>> memberAccessLambda = Expression.Lambda<Func<Employee, string>>(city, param); 

如果你想要一個實際謂詞的一部分使用你的查詢，你只包住MemberExpression與相關比較表達式，即

BinaryExpression eq = Expression.Equal(city, Expression.Constant("New York")); 

// This will give us: item => item.Address[1].City == "New York" 
Expression<Func<Employee, bool>> predicateLambda = Expression.Lambda<Func<Employee, bool>>(eq, param); 

在代碼方面：不知道你爲什麼肌酸g當代表輸入明顯預期爲Employee時，代表類型爲Func<Line, bool>的lambda。參數類型必須始終與委託簽名匹配。

編輯

非數組索引訪問例如：

ParameterExpression param = Expression.Parameter(typeof(Employee), "item"); 
MemberExpression address = Expression.Property(param, "Address"); 

IndexExpression indexedAddress = Expression.MakeIndex(
    address, 
    indexer: typeof(List<string>).GetProperty("Item", returnType: typeof(string), types: new[] { typeof(int) }), 
    arguments: new[] { Expression.Constant(1) } 
); 

// Produces item => item.Address[1]. 
Expression<Func<Employee, string>> lambda = Expression.Lambda<Func<Employee, string>>(indexedAddress, param); 

// Predicate (item => item.Address[1] == "Place"): 
BinaryExpression eq = Expression.Equal(indexedAddress, Expression.Constant("Place")); 
Expression<Func<Employee, bool>> predicateLambda = Expression.Lambda<Func<Employee, bool>>(eq, param);