[英]Using ExpressionVisitor to modify expression for automatic translations
我正在嘗試在我的Entity Framework模型中添加對多語言分類字符串的支持。 這就是我所擁有的:
實體:
public partial class ServiceState : ITranslatableEntity<ServiceStateTranslation>
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<ServiceStateTranslation> Translations { get; set; }
}
ITranslatableEntity界面:
public interface ITranslatableEntity<T>
{
ICollection<T> Translations { get; set; }
}
然后是包含翻譯的實體:
public partial class ServiceStateTranslation
{
public int Id { get; set; }
[Index("IX_ClassificationTranslation", 1, IsUnique = true)]
public int MainEntityId { get; set; }
public ServiceState MainEntity { get; set; }
[Index("IX_ClassificationTranslation", 2, IsUnique = true)]
public string LanguageCode { get; set; }
public string Name { get; set; }
}
包含本地化字符串的屬性的名稱在主實體和轉換實體(本例中為Name
)中始終相同。
使用這樣的模型我可以做這樣的事情:
var result = query.Select(x => new
{
Name = x.Name,
StateName =
currentLanguageCode == DEFAULTLANGUAGECODE
? x.ServiceState.Name
: x.ServiceState.Translations.Where(i => i.LanguageCode == currentLanguageCode)
.Select(i => i.Name)
.FirstOrDefault() ?? x.ServiceState.Name
}).ToList();
問題是我不喜歡為包含任何可轉換實體的每個查詢編寫這種代碼,所以我正在考慮使用QueryInterceptor
和ExpressionVisitor
,它會做一些魔術並允許我用這樣的代碼替換查詢:
var result = query.Select(x => new
{
Name = x.Name,
StateName = x.ServiceState.Name
}).ToLocalizedList(currentLanguageCode, DEFAULTLANGUAGECODE);
我想可以創建一個ExpressionVisitor,它將:
ITranslatableEntity<>
接口的導航屬性的Select
塊內的表達式 如果當前語言不是默認語言,請將表達式x.ServiceState.Name
更改為
x.ServiceState.Translations.Where(i => i.LanguageCode == currentLanguageCode) .Select(i => i.Name) .FirstOrDefault() ?? x.ServiceState.Name
但我對表達訪客和樹木並不熟悉,所以我在這里有點失落。 有人能讓我走上正軌嗎?
好的,看起來我已經想出了一個有效的解決方案。
public class ClassificationTranslationVisitor : ExpressionVisitor
{
private string langCode = "en";
private string defaultLangCode = "en";
private string memberName = null;
private Expression originalNode = null;
public ClassificationTranslationVisitor(string langCode, string defaultLanguageCode)
{
this.langCode = langCode;
this.defaultLangCode = defaultLanguageCode;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (langCode == defaultLangCode)
{
return base.VisitParameter(node);
}
if (!node.Type.GetCustomAttributes(typeof(TranslatableAttribute), false).Any() && originalNode == null)
{
return base.VisitParameter(node);
}
if (IsGenericInterface(node.Type, typeof(ITranslatableEntity<>)))
{
return AddTranslation(node);
}
return base.VisitParameter(node);
}
protected override Expression VisitMember(MemberExpression node)
{
if (node == null || node.Member == null || node.Member.DeclaringType == null)
{
return base.VisitMember(node);
}
if (langCode == defaultLangCode)
{
return base.VisitMember(node);
}
if (!node.Member.GetCustomAttributes(typeof(TranslatableAttribute), false).Any() && originalNode == null)
{
return base.VisitMember(node);
}
if (IsGenericInterface(node.Member.DeclaringType, typeof(ITranslatableEntity<>)))
{
memberName = node.Member.Name;
originalNode = node;
return Visit(node.Expression);
}
if (IsGenericInterface(node.Type, typeof(ITranslatableEntity<>)))
{
return AddTranslation(node);
}
return base.VisitMember(node);
}
private Expression AddTranslation(Expression node)
{
var expression = Expression.Property(node, "Translations");
var resultWhere = CreateWhereExpression(expression);
var resultSelect = CreateSelectExpression(resultWhere);
var resultIsNull = Expression.Equal(resultSelect, Expression.Constant(null));
var testResult = Expression.Condition(resultIsNull, originalNode, resultSelect);
memberName = null;
originalNode = null;
return testResult;
}
private Expression CreateWhereExpression(Expression ex)
{
var type = ex.Type.GetGenericArguments().First();
var test = CreateExpression(t => t.LanguageCode == langCode, type);
if (test == null)
return null;
return Expression.Call(typeof(Enumerable), "Where", new[] { type }, ex, test);
}
private Expression CreateSelectExpression(Expression ex)
{
var type = ex.Type.GetGenericArguments().First();
ParameterExpression itemParam = Expression.Parameter(type, "lang");
Expression selector = Expression.Property(itemParam, memberName);
var columnLambda = Expression.Lambda(selector, itemParam);
var result = Expression.Call(typeof(Enumerable), "Select", new[] { type, typeof(string) }, ex, columnLambda);
var stringResult = Expression.Call(typeof(Enumerable), "FirstOrDefault", new[] { typeof(string) }, result);
return stringResult;
}
/// <summary>
/// Adapt a QueryConditional to the member we're currently visiting.
/// </summary>
/// <param name="condition">The condition to adapt</param>
/// <param name="type">The type of the current member (=Navigation property)</param>
/// <returns>The adapted QueryConditional</returns>
private LambdaExpression CreateExpression(Expression<Func<ITranslation, bool>> condition, Type type)
{
var lambda = (LambdaExpression)condition;
var conditionType = condition.GetType().GetGenericArguments().First().GetGenericArguments().First();
// Only continue when the condition is applicable to the Type of the member
if (conditionType == null)
return null;
if (!conditionType.IsAssignableFrom(type))
return null;
var newParams = new[] { Expression.Parameter(type, "bo") };
var paramMap = lambda.Parameters.Select((original, i) => new { original, replacement = newParams[i] }).ToDictionary(p => p.original, p => p.replacement);
var fixedBody = ParameterRebinder.ReplaceParameters(paramMap, lambda.Body);
lambda = Expression.Lambda(fixedBody, newParams);
return lambda;
}
private bool IsGenericInterface(Type type, Type interfaceType)
{
return type.GetInterfaces().Any(x =>
x.IsGenericType &&
x.GetGenericTypeDefinition() == interfaceType);
}
}
public class ParameterRebinder : ExpressionVisitor
{
private readonly Dictionary<ParameterExpression, ParameterExpression> map;
public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
{
this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
}
public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
{
return new ParameterRebinder(map).Visit(exp);
}
protected override Expression VisitParameter(ParameterExpression node)
{
ParameterExpression replacement;
if (map.TryGetValue(node, out replacement))
node = replacement;
return base.VisitParameter(node);
}
}
我還添加了TranslatableAttribute
,這是任何即將翻譯的屬性所必需的。
代碼肯定缺少一些檢查,但它已經適用於我的環境。 我也沒有檢查被替換的表達式是否在Select
塊中,但看起來它與TranslatableAttribute
不一樣。
我已經使用了ParameterRebinder
和其他一些來自這個答案的ExpressionVisitor軟刪除代碼
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