[英]Replace parameter to point to nested parameter in lambda expression
由於前一個問題的一些答案,我可以成功地替換lambda表達式中的簡單參數類型,但我無法弄清楚如何將傳入的lambda中的參數替換為嵌套參數。
考慮以下對象:
public class DtoColour {
public DtoColour(string name)
{
Name = name;
}
public string Name { get; set; }
public ICollection<DtoFavouriteColour> FavouriteColours { get; set; }
}
public class DtoPerson
{
public DtoPerson(string firstName, string lastName)
{
FirstName = firstName;
LastName = lastName;
FavouriteColours = new Collection<DtoFavouriteColour>();
}
public string FirstName { get; private set; }
public string LastName { get; private set; }
public ICollection<DtoFavouriteColour> FavouriteColours { get; set; }
}
public class DtoFavouriteColour
{
public DtoColour Colour { get; set; }
public DtoPerson Person { get; set; }
}
public class DomainColour {
public DomainColour(string name)
{
Name = name;
}
public string Name { get; set; }
public ICollection<DomainPerson> People { get; set; }
}
public class DomainPerson {
public DomainPerson(string firstName, string lastName)
{
FirstName = firstName;
LastName = lastName;
Colours = new Collection<DomainColour>();
}
public string FirstName { get; private set; }
public string LastName { get; private set; }
public ICollection<DomainColour> Colours { get; set; }
}
和存儲庫:
public class ColourRepository {
private IList<DtoColour> Colours { get; set; }
public ColourRepository()
{
var favColours = new Collection<DtoFavouriteColour>
{
new DtoFavouriteColour() { Person = new DtoPerson("Peter", "Parker") },
new DtoFavouriteColour() { Person = new DtoPerson("John", "Smith") },
new DtoFavouriteColour() { Person = new DtoPerson("Joe", "Blogs") }
};
Colours = new List<DtoColour>
{
new DtoColour("Red") { FavouriteColours = favColours },
new DtoColour("Blue"),
new DtoColour("Yellow")
};
}
public IEnumerable<DomainColour> GetWhere(Expression<Func<DomainColour, bool>> predicate)
{
var coonvertedPred = MyExpressionVisitor.Convert(predicate);
return Colours.Where(coonvertedPred).Select(c => new DomainColour(c.Name)).ToList();
}
}
最后是一個表達式訪問者,它應該將謂詞轉換為Dto模型的正確謂詞
public class MyExpressionVisitor : ExpressionVisitor
{
private ReadOnlyCollection<ParameterExpression> _parameters;
public static Func<DtoColour, bool> Convert<T>(Expression<T> root)
{
var visitor = new MyExpressionVisitor();
var expression = (Expression<Func<DtoColour, bool>>)visitor.Visit(root);
return expression.Compile();
}
protected override Expression VisitParameter(ParameterExpression node)
{
var param = _parameters?.FirstOrDefault(p => p.Name == node.Name);
if (param != null)
{
return param;
}
if(node.Type == typeof(DomainColour))
{
return Expression.Parameter(typeof(DtoColour), node.Name);
}
if (node.Type == typeof(DomainPerson))
{
return Expression.Parameter(typeof(DtoFavouriteColour), node.Name);
}
return node;
}
protected override Expression VisitLambda<T>(Expression<T> node)
{
_parameters = VisitAndConvert<ParameterExpression>(node.Parameters, "VisitLambda");
return Expression.Lambda(Visit(node.Body), _parameters);
}
protected override Expression VisitMember(MemberExpression node)
{
var exp = Visit(node.Expression);
if (node.Member.DeclaringType == typeof(DomainColour))
{
if (node.Type == typeof(ICollection<DomainPerson>))
{
return Expression.MakeMemberAccess(exp, typeof(DtoColour).GetProperty("FavouriteColours"));
}
return Expression.MakeMemberAccess(exp, typeof(DtoColour).GetProperty(node.Member.Name));
}
if (node.Member.DeclaringType == typeof(DomainPerson))
{
var nested = Expression.MakeMemberAccess(exp, typeof(DtoFavouriteColour).GetProperty("Person"));
return Expression.MakeMemberAccess(nested, typeof(DtoPerson).GetProperty(node.Member.Name));
}
return base.VisitMember(node);
}
}
目前我得到以下例外
[System.ArgumentException:類型'System.Collections.Generic.ICollection
1[ExpressionVisitorTests.DtoFavouriteColour]' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable
11[ExpressionVisitorTests.DtoFavouriteColour]' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable
[ExpressionVisitorTests.DomainPerson]''方法'布爾值任意[DomainPerson](System.Collections.Generic.IEnumerable1[ExpressionVisitorTests.DomainPerson], System.Func
2 [ExpressionVisitorTests.DomainPerson,System.Boolean])']
這是一個不起作用的dotnetfiddle 。
提前感謝您的幫助。
在進行了一些搜索之后,我遇到了John Skeet的 這個答案 ,這導致我想出了一個有效的解決方案,其中包括在ExpressionVisitor
上添加一個VisitMethodCall
方法的覆蓋,用一個新的MethodInfo
替換正確的集合類型。
protected override Expression VisitMethodCall(MethodCallExpression node)
{
if (node.Method.DeclaringType == typeof(Enumerable) && node.Arguments[0].Type == typeof(ICollection<DomainPerson>))
{
Expression obj = Visit(node.Object);
IEnumerable<Expression> args = Visit(node.Arguments);
if (obj != node.Object || args != node.Arguments)
{
var generic = typeof(Enumerable).GetMethods()
.Where(m => m.Name == node.Method.Name)
.Where(m => m.GetParameters().Length == node.Arguments.Count)
.Single();
var constructed = generic.MakeGenericMethod(typeof(DtoFavouriteColour));
return Expression.Call(obj, constructed, args);
}
}
return node;
}
我還需要確保我對_parameters
集合的引用沒有被訪問node.Body
可能發生的對VisitLambda<T>
的嵌套調用所取代。
protected override Expression VisitLambda<T>(Expression<T> node)
{
var parameters = VisitAndConvert(node.Parameters, "VisitLambda");
// ensure parameters set but dont let original reference
// be overidden by nested calls
_parameters = parameters;
return Expression.Lambda(Visit(node.Body), parameters);
}
請參閱dotnetfiddle了解完全可行的解決方案。
如果有人有更好/更優雅的解決方案,請為我添加一個答案。
你已經解決了具體的問題,所以我不能說我要提出的建議是更好/更優雅,但肯定是更通用(刪除了具體的類型/屬性/假設),因此可以重用於翻譯來自不同模型類型的類似表達式。
這是代碼:
public class ExpressionMap
{
private Dictionary<Type, Type> typeMap = new Dictionary<Type, Type>();
private Dictionary<MemberInfo, Expression> memberMap = new Dictionary<MemberInfo, Expression>();
public ExpressionMap Add<TFrom, TTo>()
{
typeMap.Add(typeof(TFrom), typeof(TTo));
return this;
}
public ExpressionMap Add<TFrom, TFromMember, TTo, TToMember>(Expression<Func<TFrom, TFromMember>> from, Expression<Func<TTo, TToMember>> to)
{
memberMap.Add(((MemberExpression)from.Body).Member, to.Body);
return this;
}
public Expression Map(Expression source) => new MapVisitor { map = this }.Visit(source);
private class MapVisitor : ExpressionVisitor
{
public ExpressionMap map;
private Dictionary<Type, ParameterExpression> parameterMap = new Dictionary<Type, ParameterExpression>();
protected override Expression VisitLambda<T>(Expression<T> node)
{
return Expression.Lambda(Visit(node.Body), node.Parameters.Select(Map));
}
protected override Expression VisitParameter(ParameterExpression node) => Map(node);
protected override Expression VisitMember(MemberExpression node)
{
var expression = Visit(node.Expression);
if (expression == node.Expression)
return node;
Expression mappedMember;
if (map.memberMap.TryGetValue(node.Member, out mappedMember))
return Visit(mappedMember);
return Expression.PropertyOrField(expression, node.Member.Name);
}
protected override Expression VisitMethodCall(MethodCallExpression node)
{
if (node.Object == null && node.Method.IsGenericMethod)
{
// Static generic method
var arguments = Visit(node.Arguments);
var genericArguments = node.Method.GetGenericArguments().Select(Map).ToArray();
var method = node.Method.GetGenericMethodDefinition().MakeGenericMethod(genericArguments);
return Expression.Call(method, arguments);
}
return base.VisitMethodCall(node);
}
private Type Map(Type type)
{
Type mappedType;
return map.typeMap.TryGetValue(type, out mappedType) ? mappedType : type;
}
private ParameterExpression Map(ParameterExpression parameter)
{
var mappedType = Map(parameter.Type);
ParameterExpression mappedParameter;
if (!parameterMap.TryGetValue(mappedType, out mappedParameter))
parameterMap.Add(mappedType, mappedParameter = Expression.Parameter(mappedType, parameter.Name));
return mappedParameter;
}
}
}
以及具體示例的用法:
public IEnumerable<DomainColour> GetWhere(Expression<Func<DomainColour, bool>> predicate)
{
var map = new ExpressionMap()
.Add<DomainColour, DtoColour>()
.Add((DomainColour c) => c.People, (DtoColour c) => c.FavouriteColours.Select(fc => fc.Person))
.Add<DomainPerson, DtoPerson>();
var mappedPredicate = ((Expression<Func<DtoColour, bool>>)map.Map(predicate));
return Colours.Where(mappedPredicate.Compile()).Select(c => new DomainColour(c.Name)).ToList();
}
如您所見,它允許您使用帶有lambda表達式的“fluent”語法定義從一種類型到另一種類型的簡單映射,並且可選地從一種類型的成員到另一種類型的成員/表達式(一旦它們兼容)。 沒有指定映射的成員按名稱映射,如原始代碼中所示。
定義映射后,當然實際處理由自定義ExpressionVisitor
,與您的類似。 區別在於它按類型映射和合並ParameterExpression
,並且還轉換每個靜態泛型方法 ,因此也應該與Queryable
和類似工作。
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