[英]Dynamic expression tree with method 'Select'
我正在嘗試使用表達式樹->構建以下lambda表達式
info => info.event_objects.Select(x => x.object_info.contact_info)
我做了很多研究,並在StackOverflow上找到了一些答案。 這幫助我建立了
info =>
info.event_objects.Any(x => x.object_info.contact_info.someBool == true)
如您所見,方法“ Any”很容易獲得。
var anyMethod = typeof(Enumerable).GetMethods().Single(m => m.Name == "Any"
&& m.GetParameters().Length == 2);
anyMethod = anyMethod.MakeGenericMethod(childType);
主要問題在於方法“選擇”。 如果嘗試將名稱“ Any”更改為“ Select”,則會出現以下異常:
var selectMethod = typeof(Enumerable).GetMethods().Single(m => m.Name ==
"Select" && m.GetParameters().Length == 2);
selectMethod = selectMethod.MakeGenericMethod(childType);
附加信息:序列包含多個匹配元素
我嘗試過的另一種方式:
MethodInfo selectMethod = null;
foreach (MethodInfo m in typeof(Enumerable).GetMethods().Where(m => m.Name
== "Select"))
foreach (ParameterInfo p in m.GetParameters().Where(p =>
p.Name.Equals("selector")))
if (p.ParameterType.GetGenericArguments().Count() == 2)
selectMethod = (MethodInfo)p.Member;
似乎可行,但隨后出現異常:
navigationPropertyPredicate = Expression.Call(selectMethod, parameter,
navigationPropertyPredicate);
Additional information: Method
System.Collections.Generic.IEnumerable`1[TResult] Select[TSource,TResult]
(System.Collections.Generic.IEnumerable`1[TSource],
System.Func`2[TSource,TResult]) is a generic method definition>
之后,我嘗試使用:
selectMethod = selectMethod.MakeGenericMethod(typeof(event_objects),
typeof(contact_info));
實際上,它沒有幫助。
這是我的完整代碼
public static Expression GetNavigationPropertyExpression(Expression parameter, params string[] properties)
{
Expression resultExpression = null;
Expression childParameter, navigationPropertyPredicate;
Type childType = null;
if (properties.Count() > 1)
{
//build path
parameter = Expression.Property(parameter, properties[0]);
var isCollection = typeof(IEnumerable).IsAssignableFrom(parameter.Type);
//if it´s a collection we later need to use the predicate in the methodexpressioncall
if (isCollection)
{
childType = parameter.Type.GetGenericArguments()[0];
childParameter = Expression.Parameter(childType, "x");
}
else
{
childParameter = parameter;
}
//skip current property and get navigation property expression recursivly
var innerProperties = properties.Skip(1).ToArray();
navigationPropertyPredicate = GetNavigationPropertyExpression(childParameter, innerProperties);
if (isCollection)
{
//var selectMethod = typeof(Enumerable).GetMethods().Single(m => m.Name == "Select" && m.GetParameters().Length == 2);
//selectMethod = selectMethod.MakeGenericMethod(childType);
MethodInfo selectMethod = null;
foreach (MethodInfo m in typeof(Enumerable).GetMethods().Where(m => m.Name == "Select"))
foreach (ParameterInfo p in m.GetParameters().Where(p => p.Name.Equals("selector")))
if (p.ParameterType.GetGenericArguments().Count() == 2)
selectMethod = (MethodInfo)p.Member;
navigationPropertyPredicate = Expression.Call(selectMethod, parameter, navigationPropertyPredicate);
resultExpression = MakeLambda(parameter, navigationPropertyPredicate);
}
else
{
resultExpression = navigationPropertyPredicate;
}
}
else
{
var childProperty = parameter.Type.GetProperty(properties[0]);
var left = Expression.Property(parameter, childProperty);
var right = Expression.Constant(true, typeof(bool));
navigationPropertyPredicate = Expression.Lambda(left);
resultExpression = MakeLambda(parameter, navigationPropertyPredicate);
}
return resultExpression;
}
private static Expression MakeLambda(Expression parameter, Expression predicate)
{
var resultParameterVisitor = new ParameterVisitor();
resultParameterVisitor.Visit(parameter);
var resultParameter = resultParameterVisitor.Parameter;
return Expression.Lambda(predicate, (ParameterExpression)resultParameter);
}
private class ParameterVisitor : ExpressionVisitor
{
public Expression Parameter
{
get;
private set;
}
protected override Expression VisitParameter(ParameterExpression node)
{
Parameter = node;
return node;
}
}
[TestMethod]
public void TestDynamicExpression()
{
var parameter = Expression.Parameter(typeof(event_info), "x");
var expression = GetNavigationPropertyExpression(parameter, "event_objects", "object_info", "contact_info");
}
編輯:不幸的是,我已經嘗試過從這個問題的答案,但似乎不起作用
“其他信息:序列包含多個匹配元素”
與“ Any()”不同,對於“ Select()”,有兩個參數兩個的重載:
Select<TS, TR>(IE<TS> source, Func<TS, TR> selector)
Select<TS, TR>(IE<TS> source, Func<TS, int, TR> selector)
(采用“(item,index)=>”選擇器lambda) 由於您的代碼無論如何已經依賴於“深奧的知識”,因此只需選擇其中的第一個即可:
var selectMethod = typeof(Enumerable).GetMethods()
.First(m => m.Name == nameof(Enumerable.Select)
&& m.GetParameters().Length == 2);
通過使用兩個接受string methodName
和Type[] typeArguments
Expression.Call
方法重載(一個用於static
方法,一個用於instance
方法),可以避免通過反射找到正確的泛型方法重載(這已經很復雜並且容易出錯,正如您已經注意到的那樣)。 Type[] typeArguments
。
另外,由於缺乏清晰的表達和lambda表達構建的分離,當前的實現過於復雜並包含其他問題。
這是一個正確的工作實現:
public static LambdaExpression GetNavigationPropertySelector(Type type, params string[] properties)
{
return GetNavigationPropertySelector(type, properties, 0);
}
private static LambdaExpression GetNavigationPropertySelector(Type type, string[] properties, int depth)
{
var parameter = Expression.Parameter(type, depth == 0 ? "x" : "x" + depth);
var body = GetNavigationPropertyExpression(parameter, properties, depth);
return Expression.Lambda(body, parameter);
}
private static Expression GetNavigationPropertyExpression(Expression source, string[] properties, int depth)
{
if (depth >= properties.Length)
return source;
var property = Expression.Property(source, properties[depth]);
if (typeof(IEnumerable).IsAssignableFrom(property.Type))
{
var elementType = property.Type.GetGenericArguments()[0];
var elementSelector = GetNavigationPropertySelector(elementType, properties, depth + 1);
return Expression.Call(
typeof(Enumerable), "Select", new Type[] { elementType, elementSelector.Body.Type },
property, elementSelector);
}
else
{
return GetNavigationPropertyExpression(property, properties, depth + 1);
}
}
第一種是公共方法。 它在內部使用接下來的兩個私有方法來遞歸地構建所需的lambda。 如您所見,我區分了構建lambda表達式和僅用作lambda主體的表達式。
測試:
var selector = GetNavigationPropertySelector(typeof(event_info),
"event_objects", "object_info", "contact_info");
結果:
x => x.event_objects.Select(x1 => x1.object_info.contact_info)
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