C# 具有Lambda的属性构造函数
可以执行以下操作:C# 具有Lambda的属性构造函数,c#,lambda,custom-attributes,C#,Lambda,Custom Attributes,可以执行以下操作: public static void SomeMethod<TFunc>(Expression<TFunc> expr) { //LambdaExpression happily excepts any Expession<TFunc> LambdaExpression lamb = expr; } 或 现有的解决方法是将数字ID传递给构造函数(满足“参数必须是常量”的要求),构造函数使用该ID在先前添加了表达式的字典中进
public static void SomeMethod<TFunc>(Expression<TFunc> expr)
{
//LambdaExpression happily excepts any Expession<TFunc>
LambdaExpression lamb = expr;
}
或
现有的解决方法是将数字ID传递给构造函数(满足“参数必须是常量”的要求),构造函数使用该ID在先前添加了表达式的字典中进行查找。我希望能够改进/简化这一点,但我感觉由于属性构造函数的限制,它并没有变得更好。这是不可能的,因为可以传递到属性中的内容需要符合CLR的二进制DLL格式,并且无法对任意对象初始化进行编码。例如,对于相同的情况,不能传递可为null的值。限制非常严格。这个怎么样:
class ExpandableQueryAttribute : Attribute
{
private LambdaExpression someLambda;
//ctor
public ExpandableQueryAttribute(Type hostingType, string filterMethod)
{
someLambda = (LambdaExpression)hostingType.GetField(filterMethod).GetValue(null);
// could also use a static method
}
}
这应该允许您将lambda分配给一个字段,然后在运行时将其吸进,尽管通常我更喜欢在编译时使用PostSharp之类的工具来完成这项工作
简单用法示例
public class LambdaExpressionAttribute : Attribute
{
public LambdaExpression MyLambda { get; private set; }
//ctor
public LambdaExpressionAttribute(Type hostingType, string filterMethod)
{
MyLambda = (LambdaExpression)hostingType.GetField(filterMethod).GetValue(null);
}
}
public class User
{
public bool IsAdministrator { get; set; }
}
public static class securityExpresions
{
public static readonly LambdaExpression IsAdministrator = (Expression<Predicate<User>>)(x => x.IsAdministrator);
public static readonly LambdaExpression IsValid = (Expression<Predicate<User>>)(x => x != null);
public static void CheckAccess(User user)
{
// only for this POC... never do this in shipping code
System.Diagnostics.StackTrace stackTrace = new System.Diagnostics.StackTrace();
var method = stackTrace.GetFrame(1).GetMethod();
var filters = method.GetCustomAttributes(typeof(LambdaExpressionAttribute), true).OfType<LambdaExpressionAttribute>();
foreach (var filter in filters)
{
if ((bool)filter.MyLambda.Compile().DynamicInvoke(user) == false)
{
throw new UnauthorizedAccessException("user does not have access to: " + method.Name);
}
}
}
}
public static class TheClass
{
[LambdaExpression(typeof(securityExpresions), "IsValid")]
public static void ReadSomething(User user, object theThing)
{
securityExpresions.CheckAccess(user);
Console.WriteLine("read something");
}
[LambdaExpression(typeof(securityExpresions), "IsAdministrator")]
public static void WriteSomething(User user, object theThing)
{
securityExpresions.CheckAccess(user);
Console.WriteLine("wrote something");
}
}
static void Main(string[] args)
{
User u = new User();
try
{
TheClass.ReadSomething(u, new object());
TheClass.WriteSomething(u, new object());
}
catch(Exception e)
{
Console.WriteLine(e);
}
}
公共类LambdaExpressionAttribute:属性
{
公共LambdaExpression MyLambda{get;private set;}
//执行器
公共LambdaExpressionAttribute(类型hostingType,字符串筛选器方法)
{
MyLambda=(LambdaExpression)hostingType.GetField(filterMethod.GetValue(null);
}
}
公共类用户
{
公共布尔IsAdministrator{get;set;}
}
公共静态类securityExpresions
{
公共静态只读LambdaExpression IsAdministrator=(表达式)(x=>x.IsAdministrator);
公共静态只读LambdaExpression IsValid=(表达式)(x=>x!=null);
公共静态无效检查访问(用户)
{
//仅适用于此POC…请勿在装运代码中执行此操作
System.Diagnostics.StackTrace StackTrace=新的System.Diagnostics.StackTrace();
var method=stackTrace.GetFrame(1.GetMethod();
var filters=method.GetCustomAttributes(typeof(LambdaExpressionAttribute),true);
foreach(过滤器中的var过滤器)
{
if((bool)filter.MyLambda.Compile().DynamicInvoke(用户)==false)
{
抛出新的UnauthorizedAccessException(“用户无权访问:“+method.Name”);
}
}
}
}
公共静态类
{
[LambdaExpression(typeof(securityExpresions),“IsValid”)]
公共静态void ReadSomething(用户、对象)
{
securityExpresions.CheckAccess(用户);
Console.WriteLine(“读东西”);
}
[LambdaExpression(typeof(securityexpressions),“IsAdministrator”)]
公共静态无效写入方法(用户、对象和内容)
{
securityExpresions.CheckAccess(用户);
Console.WriteLine(“写了什么”);
}
}
静态void Main(字符串[]参数)
{
用户u=新用户();
尝试
{
ReadSomething(u,newobject());
writeMething类(u,newobject());
}
捕获(例外e)
{
控制台写入线(e);
}
}
使用动态linq:
您可以使属性的构造函数采用一个字符串,该字符串的计算结果为表达式。虽然您不能为属性使用复杂的构造函数,但在某些情况下,我们需要为该属性创建一个公共属性,并在运行时更新它 self指向类的对象,该对象的属性中包含一些属性。LocalDisplayNameAttribute是一个自定义属性 以下代码将在运行时设置自定义属性类的ResourceKey属性。然后,您可以覆盖DisplayName以输出您想要的任何文本
static public void UpdateAttributes(object self)
{
foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(self))
{
LocalDisplayNameAttribute attr =
prop.Attributes[typeof(LocalDisplayNameAttribute)]
as LocalDisplayNameAttribute;
if (attr == null)
{
continue;
}
attr.ResourceKey = prop.Name;
}
}
跟随你的感觉。。。属性参数的限制是相当清楚的。这个问题也在这个链接中被问到。答案是目前还不可能@Jamey Yep,这是我列出的最后一个选择,我知道这是一个限制。我希望通过将表达式声明为变量来解决这个问题,但后来“必须是常量”的要求让我明白了。不过,这里的解决方法很有趣,我将尝试它的一个变体。@chrismcall的可能副本添加了一个简单的示例OK我现在明白了,在其他地方声明lambda并通过其名称引用它。。。聪明!
//lambdas aren't allowed inline for an attribute, as far as I know
[ExpandableQueryAttribute(q => q.FirstOrDefault())]
// ...
class ExpandableQueryAttribute : Attribute
{
private LambdaExpression someLambda;
//ctor
public ExpandableQueryAttribute(Type hostingType, string filterMethod)
{
someLambda = (LambdaExpression)hostingType.GetField(filterMethod).GetValue(null);
// could also use a static method
}
}
public class LambdaExpressionAttribute : Attribute
{
public LambdaExpression MyLambda { get; private set; }
//ctor
public LambdaExpressionAttribute(Type hostingType, string filterMethod)
{
MyLambda = (LambdaExpression)hostingType.GetField(filterMethod).GetValue(null);
}
}
public class User
{
public bool IsAdministrator { get; set; }
}
public static class securityExpresions
{
public static readonly LambdaExpression IsAdministrator = (Expression<Predicate<User>>)(x => x.IsAdministrator);
public static readonly LambdaExpression IsValid = (Expression<Predicate<User>>)(x => x != null);
public static void CheckAccess(User user)
{
// only for this POC... never do this in shipping code
System.Diagnostics.StackTrace stackTrace = new System.Diagnostics.StackTrace();
var method = stackTrace.GetFrame(1).GetMethod();
var filters = method.GetCustomAttributes(typeof(LambdaExpressionAttribute), true).OfType<LambdaExpressionAttribute>();
foreach (var filter in filters)
{
if ((bool)filter.MyLambda.Compile().DynamicInvoke(user) == false)
{
throw new UnauthorizedAccessException("user does not have access to: " + method.Name);
}
}
}
}
public static class TheClass
{
[LambdaExpression(typeof(securityExpresions), "IsValid")]
public static void ReadSomething(User user, object theThing)
{
securityExpresions.CheckAccess(user);
Console.WriteLine("read something");
}
[LambdaExpression(typeof(securityExpresions), "IsAdministrator")]
public static void WriteSomething(User user, object theThing)
{
securityExpresions.CheckAccess(user);
Console.WriteLine("wrote something");
}
}
static void Main(string[] args)
{
User u = new User();
try
{
TheClass.ReadSomething(u, new object());
TheClass.WriteSomething(u, new object());
}
catch(Exception e)
{
Console.WriteLine(e);
}
}
static public void UpdateAttributes(object self)
{
foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(self))
{
LocalDisplayNameAttribute attr =
prop.Attributes[typeof(LocalDisplayNameAttribute)]
as LocalDisplayNameAttribute;
if (attr == null)
{
continue;
}
attr.ResourceKey = prop.Name;
}
}