目前我們有一個(gè)包可以從字符串中的字段動(dòng)態(tài)生成 linq 選擇。它適用于平面屬性，但它不是設(shè)計(jì)用于嵌套字段，如 someObj.NestedObj.SomeField。我們當(dāng)前的代碼在服務(wù)方法中的工作方式如下：_context.Shipments .Where(s => s.Id == request.Id) // it does not matter just an example .Select(request.Fields) .ToPage(request); // ToPage extension comes from a nuget package請(qǐng)求對(duì)象的參數(shù)“字段”只是一個(gè)以逗號(hào)分隔的字符串，包括 Shipment 對(duì)象的屬性。我對(duì) Shipment 進(jìn)行了一些重構(gòu)，我將一些字段分組到一個(gè)名為 Address 的新類中，并將其添加到 Shipment 中，如下所示：// before refactoringclass Shipment { // other fields... public string SenderAddress; public string SenderCityName; public string SenderCityId; public string RecipientAddress; public string CityName; public string CityId;}// after refactoringclass Shipment { // other fields... public Address Sender; public Address Recipient;}class Address { public string AddressText; public string CityName; public string CityId;}為了當(dāng)前的數(shù)據(jù)庫(kù)映射，我添加了相應(yīng)的映射：public class ShipmentMap : DataEntityTypeConfiguration<Shipment> { public ShipmentMap() { ToTable("Shipments"); // other property mappings Property(s => s.Recipient.AddressText).HasMaxLength(1100).HasColumnName("RecipientAddress"); Property(s => s.Recipient.CityName).HasMaxLength(100).HasColumnName("CityName"); Property(s => s.Recipient.CityId).IsOptional().HasColumnName("CityId"); Property(s => s.Sender.AddressText).HasMaxLength(1100).HasColumnName("SenderAddress"); Property(s => s.Sender.CityName).HasMaxLength(100).HasColumnName("SenderCityName"); Property(s => s.Sender.CityId).IsOptional().HasColumnName("SenderCityId"); } }DataEntityTypeConfiguration 來(lái)自 nuget 包： public abstract class DataEntityTypeConfiguration<T> : EntityTypeConfiguration<T> where T : class { protected virtual void PostInitialize(); }所以，我的問(wèn)題是 select(fields) 在 fields = "Recipient.CityId" 時(shí)不起作用。如何動(dòng)態(tài)生成用于選擇嵌套字段的 linq？
3 回答
拉風(fēng)的咖菲貓
TA貢獻(xiàn)1995條經(jīng)驗(yàn) 獲得超2個(gè)贊
很高興您找到了解決您的特定問(wèn)題的方法。
這是一個(gè)更通用的解決方案，一旦原始屬性名稱和類型匹配（例如Entity->Dto等），以及多級(jí)嵌套，它就會(huì)處理不同的源和目標(biāo)類型：
public static Expression<Func<TSource, TTarget>> BuildSelector<TSource, TTarget>(string members) =>
BuildSelector<TSource, TTarget>(members.Split(',').Select(m => m.Trim()));
public static Expression<Func<TSource, TTarget>> BuildSelector<TSource, TTarget>(IEnumerable<string> members)
{
var parameter = Expression.Parameter(typeof(TSource), "e");
var body = NewObject(typeof(TTarget), parameter, members.Select(m => m.Split('.')));
return Expression.Lambda<Func<TSource, TTarget>>(body, parameter);
}
static Expression NewObject(Type targetType, Expression source, IEnumerable<string[]> memberPaths, int depth = 0)
{
var bindings = new List<MemberBinding>();
var target = Expression.Constant(null, targetType);
foreach (var memberGroup in memberPaths.GroupBy(path => path[depth]))
{
var memberName = memberGroup.Key;
var targetMember = Expression.PropertyOrField(target, memberName);
var sourceMember = Expression.PropertyOrField(source, memberName);
var childMembers = memberGroup.Where(path => depth + 1 < path.Length);
var targetValue = !childMembers.Any() ? sourceMember :
NewObject(targetMember.Type, sourceMember, childMembers, depth + 1);
bindings.Add(Expression.Bind(targetMember.Member, targetValue));
}
return Expression.MemberInit(Expression.New(targetType), bindings);
}
前兩種方法只是公開暴露的高級(jí)助手。實(shí)際工作是通過(guò)私有遞歸NewObject方法完成的。它對(duì)當(dāng)前級(jí)別的屬性進(jìn)行分組，并且對(duì)于每個(gè)分組，要么創(chuàng)建簡(jiǎn)單的分配，就像PropertyN = source.Property1.Property2...PropertyN它是最后一個(gè)級(jí)別一樣，要么遞歸地PropertyN = new TypeN { … }否則。
與您的示例中的表達(dá)式匹配的示例用法：
var test = BuildSelector<Shipment, Shipment>(
"Recipient.CityName, Sender.CityId, Sender.CityName, ParcelUniqueId");
只需Compile在需要時(shí)調(diào)用Func。
素胚勾勒不出你
TA貢獻(xiàn)1827條經(jīng)驗(yàn) 獲得超9個(gè)贊
最后我找到了解決方案。它為兩級(jí)嵌套屬性（如 Shipment.Sender.CityName）生成正確的 lambda。所以任何需要同樣?xùn)|西的人都可以使用它。
我希望它有幫助。
/* this comes from request
* request.Fields = "Sender.CityId,Sender.CityName,Recipient.CityName,parcelUniqueId"
*/
// in the service method
var shipmentList = _context.Shipments.
.OrderByDescending(s => s.Id)
.Skip((request.Page -1) * request.PageSize)
.Take(request.PageSize)
.Select(new SelectLambdaBuilder<Shipment>().CreateNewStatement(request.Fields))
.ToList();
public class SelectLambdaBuilder<T>
{
// as a performence consideration I cached already computed type-properties
private static Dictionary<Type, PropertyInfo[]> _typePropertyInfoMappings = new Dictionary<Type, PropertyInfo[]>();
private readonly Type _typeOfBaseClass = typeof(T);
private Dictionary<string, List<string>> GetFieldMapping(string fields)
{
var selectedFieldsMap = new Dictionary<string, List<string>>();
foreach (var s in fields.Split(','))
{
var nestedFields = s.Split('.').Select(f => f.Trim()).ToArray();
var nestedValue = nestedFields.Length > 1 ? nestedFields[1] : null;
if (selectedFieldsMap.Keys.Any(key => key == nestedFields[0]))
{
selectedFieldsMap[nestedFields[0]].Add(nestedValue);
}
else
{
selectedFieldsMap.Add(nestedFields[0], new List<string> { nestedValue });
}
}
return selectedFieldsMap;
}
public Func<T, T> CreateNewStatement(string fields)
{
ParameterExpression xParameter = Expression.Parameter(_typeOfBaseClass, "s");
NewExpression xNew = Expression.New(_typeOfBaseClass);
var selectFields = GetFieldMapping(fields);
var shpNestedPropertyBindings = new List<MemberAssignment>();
foreach (var keyValuePair in selectFields)
{
PropertyInfo[] propertyInfos;
if (!_typePropertyInfoMappings.TryGetValue(_typeOfBaseClass, out propertyInfos))
{
var properties = _typeOfBaseClass.GetProperties();
propertyInfos = properties;
_typePropertyInfoMappings.Add(_typeOfBaseClass, properties);
}
var propertyType = propertyInfos
.FirstOrDefault(p => p.Name.ToLowerInvariant().Equals(keyValuePair.Key.ToLowerInvariant()))
.PropertyType;
if (propertyType.IsClass)
{
PropertyInfo objClassPropInfo = _typeOfBaseClass.GetProperty(keyValuePair.Key);
MemberExpression objNestedMemberExpression = Expression.Property(xParameter, objClassPropInfo);
NewExpression innerObjNew = Expression.New(propertyType);
var nestedBindings = keyValuePair.Value.Select(v =>
{
PropertyInfo nestedObjPropInfo = propertyType.GetProperty(v);
MemberExpression nestedOrigin2 = Expression.Property(objNestedMemberExpression, nestedObjPropInfo);
var binding2 = Expression.Bind(nestedObjPropInfo, nestedOrigin2);
return binding2;
});
MemberInitExpression nestedInit = Expression.MemberInit(innerObjNew, nestedBindings);
shpNestedPropertyBindings.Add(Expression.Bind(objClassPropInfo, nestedInit));
}
else
{
Expression mbr = xParameter;
mbr = Expression.PropertyOrField(mbr, keyValuePair.Key);
PropertyInfo mi = _typeOfBaseClass.GetProperty( ((MemberExpression)mbr).Member.Name );
var xOriginal = Expression.Property(xParameter, mi);
shpNestedPropertyBindings.Add(Expression.Bind(mi, xOriginal));
}
}
var xInit = Expression.MemberInit(xNew, shpNestedPropertyBindings);
var lambda = Expression.Lambda<Func<T,T>>( xInit, xParameter );
return lambda.Compile();
}
它編譯 lambda 如下：
s => new Shipment {
Recipient = new Address {
CityName = s.Recipient.CityName
},
Sender = new Address {
CityId = s.Sender.CityId,
CityName = s.Sender.CityName
},
ParcelUniqueId = s.ParcelUniqueId
}
我分享了 debug 的一些截圖：
汪汪一只貓
TA貢獻(xiàn)1898條經(jīng)驗(yàn) 獲得超8個(gè)贊
最后我找到了解決方案。它為兩級(jí)嵌套屬性（如 Shipment.Sender.CityName）生成正確的 lambda。所以任何需要同樣?xùn)|西的人都可以使用它。
我希望它有幫助。
/* this comes from request
* request.Fields = "Sender.CityId,Sender.CityName,Recipient.CityName,parcelUniqueId"
*/
// in the service method
var shipmentList = _context.Shipments.
.OrderByDescending(s => s.Id)
.Skip((request.Page -1) * request.PageSize)
.Take(request.PageSize)
.Select(new SelectLambdaBuilder<Shipment>().CreateNewStatement(request.Fields))
.ToList();
public class SelectLambdaBuilder<T>
{
// as a performence consideration I cached already computed type-properties
private static Dictionary<Type, PropertyInfo[]> _typePropertyInfoMappings = new Dictionary<Type, PropertyInfo[]>();
private readonly Type _typeOfBaseClass = typeof(T);
private Dictionary<string, List<string>> GetFieldMapping(string fields)
{
var selectedFieldsMap = new Dictionary<string, List<string>>();
foreach (var s in fields.Split(','))
{
var nestedFields = s.Split('.').Select(f => f.Trim()).ToArray();
var nestedValue = nestedFields.Length > 1 ? nestedFields[1] : null;
if (selectedFieldsMap.Keys.Any(key => key == nestedFields[0]))
{
selectedFieldsMap[nestedFields[0]].Add(nestedValue);
}
else
{
selectedFieldsMap.Add(nestedFields[0], new List<string> { nestedValue });
}
}
return selectedFieldsMap;
}
public Func<T, T> CreateNewStatement(string fields)
{
ParameterExpression xParameter = Expression.Parameter(_typeOfBaseClass, "s");
NewExpression xNew = Expression.New(_typeOfBaseClass);
var selectFields = GetFieldMapping(fields);
var shpNestedPropertyBindings = new List<MemberAssignment>();
foreach (var keyValuePair in selectFields)
{
PropertyInfo[] propertyInfos;
if (!_typePropertyInfoMappings.TryGetValue(_typeOfBaseClass, out propertyInfos))
{
var properties = _typeOfBaseClass.GetProperties();
propertyInfos = properties;
_typePropertyInfoMappings.Add(_typeOfBaseClass, properties);
}
var propertyType = propertyInfos
.FirstOrDefault(p => p.Name.ToLowerInvariant().Equals(keyValuePair.Key.ToLowerInvariant()))
.PropertyType;
if (propertyType.IsClass)
{
PropertyInfo objClassPropInfo = _typeOfBaseClass.GetProperty(keyValuePair.Key);
MemberExpression objNestedMemberExpression = Expression.Property(xParameter, objClassPropInfo);
NewExpression innerObjNew = Expression.New(propertyType);
var nestedBindings = keyValuePair.Value.Select(v =>
{
PropertyInfo nestedObjPropInfo = propertyType.GetProperty(v);
MemberExpression nestedOrigin2 = Expression.Property(objNestedMemberExpression, nestedObjPropInfo);
var binding2 = Expression.Bind(nestedObjPropInfo, nestedOrigin2);
return binding2;
});
MemberInitExpression nestedInit = Expression.MemberInit(innerObjNew, nestedBindings);
shpNestedPropertyBindings.Add(Expression.Bind(objClassPropInfo, nestedInit));
}
else
{
Expression mbr = xParameter;
mbr = Expression.PropertyOrField(mbr, keyValuePair.Key);
PropertyInfo mi = _typeOfBaseClass.GetProperty( ((MemberExpression)mbr).Member.Name );
var xOriginal = Expression.Property(xParameter, mi);
shpNestedPropertyBindings.Add(Expression.Bind(mi, xOriginal));
}
}
var xInit = Expression.MemberInit(xNew, shpNestedPropertyBindings);
var lambda = Expression.Lambda<Func<T,T>>( xInit, xParameter );
return lambda.Compile();
}
它編譯 lambda 如下：
s => new Shipment {
Recipient = new Address {
CityName = s.Recipient.CityName
},
Sender = new Address {
CityId = s.Sender.CityId,
CityName = s.Sender.CityName
},
ParcelUniqueId = s.ParcelUniqueId
}
我分享了 debug 的一些截圖：我相信你的問(wèn)題出在這段代碼中：
string[] nestedProps = o.Split('.');
Expression mbr = xParameter;
foreach (var prop in nestedProps)
mbr = Expression.PropertyOrField(mbr, prop);
// property "Field1"
PropertyInfo mi = typeof( Shipment ).GetProperty( ((MemberExpression)mbr).Member.Name );
該foreach循環(huán)被重復(fù)地將值分配給mbr，然后覆蓋它，這意味著它的最終值將是表達(dá)等效過(guò)去值的nestedProps。假設(shè)輸入字符串是"Recipient.CityId",mbr將是 的表達(dá)式CityId。然后，您嘗試GetProperty在Shipment類型上查找名稱為 的屬性，該屬性CityId當(dāng)然不存在（CityId作為 的屬性Address）。
不過(guò)，我不確定要解決什么問(wèn)題，因?yàn)槲也淮_定您最終想要什么。
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