对于EF对数据库的缓存，EF本身也有，但是不能灵活的控制，而且实体对象释放了缓存就没有了，总不能使用同一个实体对象(实体对象不支持多线程)，基本上就是用完就释放，而EF的一个扩展框架也提供了缓存操作（源码：https://github.com/loresoft/EntityFramework.Extended），大体的操作：
//默认配置的缓存 var tasks = db.Tasks .Where(t => t.CompleteDate == null) .FromCache(); //设置缓存时间300s var tasks = db.Tasks .Where(t => t.AssignedId == myUserId && t.CompleteDate == null) .FromCache(CachePolicy.WithDurationExpiration(TimeSpan.FromSeconds(300)));
看了下这个扩展框架源码是如何对linq表达式进行解析的，主要是继承ExpressionVisitor(采用访问者模式)进行解析，其中有段代码是最重要的了：
private Expression Evaluate(Expression e)
{
if (e.NodeType == ExpressionType.Constant)
{
return e;
}
LambdaExpression lambda = Expression.Lambda(e);
Delegate fn = lambda.Compile();
return Expression.Constant(fn.DynamicInvoke(null), e.Type);
}
上面代码中调用DynamicInvoke会对Where中的变量进行解析为实际值，例如上面的Where中的t.AssignedId会解析为实际的值；用着还是挺好的，但是做了次性能分析，发现DynamicInvoke的调用非常费性能(看到了dynamic字头的也知道会费性能的了)，那么要么硬着头皮去用，要么就重新对Expression进行解析了，下面是对上面代码改进：
private Expression Evaluate(Expression e)
{
if (e.NodeType == ExpressionType.Constant)
{
return e;
}
if (e is MemberExpression)
{
var expmember = ExpressionVisitorHelper.GetMemberExpression(e);
if (expmember != null)
{
return expmember;
}
}
return ExpressionVisitorHelper.GetMethodCallExpression(e);
}
//ExpressionVisitorHelper类：
internal static class ExpressionVisitorHelper
{
public static Expression GetMethodCallExpression(Expression e)
{
object val = null;
if (e is MethodCallExpression)
{
val = DoToGetMethodCallVal(e as MethodCallExpression);
}
else if (e is UnaryExpression)
{
val = GetUnaryExpressionVal(e as UnaryExpression);
}
if (val == null || val.GetType() != e.Type)
{
LambdaExpression lambda = Expression.Lambda(e);
return Expression.Constant(lambda.Compile().DynamicInvoke(null), e.Type);
}
else
{
return Expression.Constant(val, e.Type);
}
}
public static Expression GetMemberExpression(Expression exp)
{
var val = GetMemberExpVal(exp);
if (val != null)
{
return Expression.Constant(val, exp.Type);
}
return null;
}
static object GetUnaryExpressionVal(UnaryExpression exp)
{
var val = GetMemberExpVal(exp.Operand);
if (val == null)
{
val = GetConstantExpVal(exp.Operand, null);
}
return val;
}
static object DoToGetMethodCallVal(MethodCallExpression expMethod)
{
//可能出现的问题expMethod.Object又是一个MethodCallExpression，例如：l.Serverid.StartsWith(model.Name.TrimEnd().TrimStart().ToLower())
object val = null;
if (expMethod.Object is MethodCallExpression)
{
Expression tempExp = expMethod.Object;
MethodCallExpression tempExpMethod;
var listMemberExp = new List<MethodCallExpression>();
while (tempExp is MethodCallExpression)
{
tempExpMethod = tempExp as MethodCallExpression;
listMemberExp.Add(tempExpMethod);
tempExp = tempExpMethod.Object;
}
//进行第一个Method的调用(TrimEnd):
tempExpMethod = listMemberExp[listMemberExp.Count - 1];
val = GetMemberExpVal(tempExpMethod.Object);
val = GetMethodCallVal(tempExpMethod, val);
//循环调用其余的Method(TrimStart)
for (int i = listMemberExp.Count - 2; i >= 0; i--)
{
val = GetMethodCallVal(listMemberExp[i], val);
}
//进行最后一个Method的调用(ToLower)
val = GetMethodCallVal(expMethod, val);
}
else
{
val = GetMemberExpVal(expMethod.Object);
val = GetMethodCallVal(expMethod, val);
}
return val;
}
static object GetMethodCallVal(MethodCallExpression expMethod, object val)
{
if (val != null)
{
switch (expMethod.Method.Name)
{
case "ToString":
case "ToLower":
case "ToUpper":
{
val = expMethod.Method.Invoke(val, null);
}
break;
case "Trim":
case "TrimStart":
case "TrimEnd":
{
object[] objArray = null;
if (expMethod.Arguments != null && expMethod.Arguments.Count > 0)
{
NewArrayExpression arg = null;
ConstantExpression expConst = null;
objArray = new object[expMethod.Arguments.Count];
char[] valArray = null;
for (int i = 0, j = 0; i < expMethod.Arguments.Count; i++)
{
arg = expMethod.Arguments[i] as NewArrayExpression;
if (arg != null && arg.Expressions != null && arg.Expressions.Count > 0)
{
valArray = new char[arg.Expressions.Count];
for (j = 0; j < arg.Expressions.Count; j++)
{
expConst = arg.Expressions[j] as ConstantExpression;
if (expConst != null)
{
valArray[j] = (char)expConst.Value;
}
}
}
objArray[i] = valArray;
}
}
val = expMethod.Method.Invoke(val, objArray);
}
break;
default:
break;
}
}
return val;
}
static object GetMemberExpVal(Expression exp)
{
if (exp is MemberExpression)
{
var node = exp as MemberExpression;
if (node.Expression is MemberExpression)
{
MemberExpression lastMemberExp;
List<MemberExpression> listMemberExp;
var constExp = MemberExpToConstantExp(node.Expression, out lastMemberExp, out listMemberExp);
var constVal = GetConstantExpVal(constExp, lastMemberExp.Member);
if (constVal != null)
{
if (listMemberExp != null && listMemberExp.Count > 0)
{
for (int i = listMemberExp.Count - 1; i >= 0; i--)
{
constVal = GetMemberVal(listMemberExp[i].Member, constVal);
}
return GetMemberVal(node.Member, constVal);
}
else
{
return GetMemberVal(node.Member, constVal);
}
}
}
else if (node.Expression is ConstantExpression)
{
return GetConstantExpVal(node.Expression, node.Member);
}
else if (node.Expression == null)
{
//获取静态的成员数据
return GetConstantExpVal(node.Expression, node.Member);
}
}
return null;
}
static object GetConstantExpVal(Expression exp, MemberInfo member)
{
object val = null;
if (exp is ConstantExpression)
{
val = (exp as ConstantExpression).Value;
if (member != null)
{
val = GetMemberVal(member, val);
}
}
else if (exp == null)
{
//获取静态的成员数据
val = GetMemberVal(member, member.Name);
}
return val;
}
static object GetMemberVal(MemberInfo member, object obj)
{
object val = null;
if (member is PropertyInfo)
{
val = (member as PropertyInfo).GetValue(obj);
}
else if (member is FieldInfo)
{
val = (member as FieldInfo).GetValue(obj);
}
return val;
}
// 获取最后的MemberExpression和ConstantExpression
static Expression MemberExpToConstantExp(Expression exp, out MemberExpression lastMemberExp, out List<MemberExpression> listMemberExp)
{
lastMemberExp = null;
listMemberExp = new List<MemberExpression>();
while (exp is MemberExpression)
{
if (lastMemberExp != null)
{
listMemberExp.Add(lastMemberExp);
}
lastMemberExp = exp as MemberExpression;
exp = lastMemberExp.Expression;
}
return exp;
}
}
时间： 2024-08-18 18:22:39