【发布时间】：2011-11-26 02:37:59
【问题描述】：
我正在尝试为我的 EF 存储库实现一种缓存方案，类似于博客 here 中的缓存方案。正如作者和评论者所报告的那样，限制是密钥生成方法不能生成随给定查询参数而变化的缓存密钥。这里是缓存键的生成方法：
private static string GetKey<T>(IQueryable<T> query)
{
string key = string.Concat(query.ToString(), "\n\r",
typeof(T).AssemblyQualifiedName);
return key;
}
因此以下查询将产生相同的缓存键：
var isActive = true;
var query = context.Products
.OrderBy(one => one.ProductNumber)
.Where(one => one.IsActive == isActive).AsCacheable();
和
var isActive = false;
var query = context.Products
.OrderBy(one => one.ProductNumber)
.Where(one => one.IsActive == isActive).AsCacheable();
请注意，唯一的区别是第一个查询中的 isActive = true 和第二个查询中的 isActive = false。
任何关于有效生成因IQueryable 参数而异的缓存键的建议/见解将不胜感激。
感谢 Sergey Barskiy 分享 EF CodeFirst 缓存方案。
更新
我自己采取了遍历 IQueryable 的表达式树的方法，目的是解析查询中使用的参数的值。根据maxlego's 的建议，我扩展了System.Linq.Expressions.ExpressionVisitor 类以访问我们感兴趣的表达式节点——在本例中为MemberExpression。更新后的 GetKey 方法如下所示：
public static string GetKey<T>(IQueryable<T> query)
{
var keyBuilder = new StringBuilder(query.ToString());
var queryParamVisitor = new QueryParameterVisitor(keyBuilder);
queryParamVisitor.GetQueryParameters(query.Expression);
keyBuilder.Append("\n\r");
keyBuilder.Append(typeof (T).AssemblyQualifiedName);
return keyBuilder.ToString();
}
而QueryParameterVisitor 类的灵感来自Bryan Watts 和Marc Gravell 对此question 的回答，看起来像这样：
/// <summary>
/// <see cref="ExpressionVisitor"/> subclass which encapsulates logic to
/// traverse an expression tree and resolve all the query parameter values
/// </summary>
internal class QueryParameterVisitor : ExpressionVisitor
{
public QueryParameterVisitor(StringBuilder sb)
{
QueryParamBuilder = sb;
Visited = new Dictionary<int, bool>();
}
protected StringBuilder QueryParamBuilder { get; set; }
protected Dictionary<int, bool> Visited { get; set; }
public StringBuilder GetQueryParameters(Expression expression)
{
Visit(expression);
return QueryParamBuilder;
}
private static object GetMemberValue(MemberExpression memberExpression, Dictionary<int, bool> visited)
{
object value;
if (!TryGetMemberValue(memberExpression, out value, visited))
{
UnaryExpression objectMember = Expression.Convert(memberExpression, typeof (object));
Expression<Func<object>> getterLambda = Expression.Lambda<Func<object>>(objectMember);
Func<object> getter = null;
try
{
getter = getterLambda.Compile();
}
catch (InvalidOperationException)
{
}
if (getter != null) value = getter();
}
return value;
}
private static bool TryGetMemberValue(Expression expression, out object value, Dictionary<int, bool> visited)
{
if (expression == null)
{
// used for static fields, etc
value = null;
return true;
}
// Mark this node as visited (processed)
int expressionHash = expression.GetHashCode();
if (!visited.ContainsKey(expressionHash))
{
visited.Add(expressionHash, true);
}
// Get Member Value, recurse if necessary
switch (expression.NodeType)
{
case ExpressionType.Constant:
value = ((ConstantExpression) expression).Value;
return true;
case ExpressionType.MemberAccess:
var me = (MemberExpression) expression;
object target;
if (TryGetMemberValue(me.Expression, out target, visited))
{
// instance target
switch (me.Member.MemberType)
{
case MemberTypes.Field:
value = ((FieldInfo) me.Member).GetValue(target);
return true;
case MemberTypes.Property:
value = ((PropertyInfo) me.Member).GetValue(target, null);
return true;
}
}
break;
}
// Could not retrieve value
value = null;
return false;
}
protected override Expression VisitMember(MemberExpression node)
{
// Only process nodes that haven't been processed before, this could happen because our traversal
// is depth-first and will "visit" the nodes in the subtree before this method (VisitMember) does
if (!Visited.ContainsKey(node.GetHashCode()))
{
object value = GetMemberValue(node, Visited);
if (value != null)
{
QueryParamBuilder.Append("\n\r");
QueryParamBuilder.Append(value.ToString());
}
}
return base.VisitMember(node);
}
}
我仍在对缓存密钥生成进行一些性能分析，并希望它不会太昂贵（一旦我得到结果，我会用结果更新问题）。我将保留这个问题，以防有人对如何优化此过程有建议，或者对生成缓存键的更有效方法提出建议，这些方法会随查询参数而变化。虽然这种方法产生了所需的输出，但它绝不是最优的。
【问题讨论】：
标签： entity-framework caching repository ef-code-first iqueryable