有沒有辦法用對像初始化器和表達式樹創建對象的實例?我的意思是創建一個表達式樹來構建這個lambda:
// my class
public class MyObject {
public bool DisplayValue { get; set; }
}
// my lambda:
var lambda = (Func<bool, MyObject>)
(displayValue => new MyObject { DisplayValue = displayValue });
如何使用表達式樹創建此lambda?
更新:
我試著自己編寫以下代碼:
public static Func<bool, dynamic> Creator;
static void BuildLambda() {
var expectedType = typeof(MyObject);
var displayValueParam = Expression.Parameter(typeof(bool), "displayValue");
var ctor = Expression.New(expectedType);
var local = Expression.Parameter(expectedType, "obj");
var displayValueProperty = Expression.Property(ctor, "DisplayValue");
var returnTarget = Expression.Label(expectedType);
var returnExpression = Expression.Return(returnTarget,local, expectedType);
var returnLabel = Expression.Label(returnTarget, Expression.Default(expectedType));
var block = Expression.Block(
new[] { local },
Expression.Assign(local, ctor),
Expression.Assign(displayValueProperty, displayValueParam),
Expression.Return(Expression.Label(expectedType), local, expectedType),
returnExpression,
returnLabel
);
Creator =
Expression.Lambda<Func<bool, dynamic>>(block, displayValueParam)
.Compile();
}
但它會引發以下錯誤:
無法跳轉到未定義的標籤''。
大家能幫幫我嗎?
要在Expression中表示對像初始值設定項,您應該使用Expression.MemberInit()
:
Expression<Func<bool, MyObject>> BuildLambda() {
var createdType = typeof(MyObject);
var displayValueParam = Expression.Parameter(typeof(bool), "displayValue");
var ctor = Expression.New(createdType);
var displayValueProperty = createdType.GetProperty("DisplayValue");
var displayValueAssignment = Expression.Bind(
displayValueProperty, displayValueParam);
var memberInit = Expression.MemberInit(ctor, displayValueAssignment);
return
Expression.Lambda<Func<bool, MyObject>>(memberInit, displayValueParam);
}
要驗證這實際上是否符合您的要求,您可以在創建的表達式上調用ToString()
。在這種情況下,輸出符合預期:
displayValue => new MyObject() {DisplayValue = displayValue}
最後我找到了答案:
public static Func<bool, dynamic> Creator;
static void BuildLambda() {
var expectedType = typeof(MyObject);
var displayValueParam = Expression.Parameter(typeof(bool), "displayValue");
var ctor = Expression.New(expectedType);
var local = Expression.Parameter(expectedType, "obj");
var displayValueProperty = Expression.Property(local, "DisplayValue");
var returnTarget = Expression.Label(expectedType);
var returnExpression = Expression.Return(returnTarget,local, expectedType);
var returnLabel = Expression.Label(returnTarget, Expression.Default(expectedType));
var block = Expression.Block(
new[] { local },
Expression.Assign(local, ctor),
Expression.Assign(displayValueProperty, displayValueParam),
/* I forgot to remove this line:
* Expression.Return(Expression.Label(expectedType), local, expectedType),
* and now it works.
* */
returnExpression,
returnLabel
);
Creator =
Expression.Lambda<Func<bool, dynamic>>(block, displayValueParam)
.Compile();
}
更新:
雖然它工作正常,但@svick在他的回答中提供了一個更好更短的方式,我正在尋找: MemberInit
。請參閱@ svick的回答。