当通过lambda表达式传入时,是否有更好的方法来获得属性名?
这是我目前拥有的。
eg.
GetSortingInfo<User>(u => u.UserId);
它只在属性为字符串时才将其转换为成员表达式。因为不是所有的属性都是字符串,我必须使用object,但它会为那些返回一个unaryexpression。
public static RouteValueDictionary GetInfo<T>(this HtmlHelper html,
Expression<Func<T, object>> action) where T : class
{
var expression = GetMemberInfo(action);
string name = expression.Member.Name;
return GetInfo(html, name);
}
private static MemberExpression GetMemberInfo(Expression method)
{
LambdaExpression lambda = method as LambdaExpression;
if (lambda == null)
throw new ArgumentNullException("method");
MemberExpression memberExpr = null;
if (lambda.Body.NodeType == ExpressionType.Convert)
{
memberExpr =
((UnaryExpression)lambda.Body).Operand as MemberExpression;
}
else if (lambda.Body.NodeType == ExpressionType.MemberAccess)
{
memberExpr = lambda.Body as MemberExpression;
}
if (memberExpr == null)
throw new ArgumentException("method");
return memberExpr;
}
public string GetName<TSource, TField>(Expression<Func<TSource, TField>> Field)
{
return (Field.Body as MemberExpression ?? ((UnaryExpression)Field.Body).Operand as MemberExpression).Member.Name;
}
这个函数处理成员表达式和一元表达式。区别在于,如果你的表达式表示值类型,你将得到一个UnaryExpression,而如果你的表达式表示引用类型,你将得到一个MemberExpression。所有内容都可以转换为对象,但值类型必须被装箱。这就是UnaryExpression存在的原因。参考。
出于可读性考虑(@Jowen),这里有一个扩展的等效内容:
public string GetName<TSource, TField>(Expression<Func<TSource, TField>> Field)
{
if (object.Equals(Field, null))
{
throw new NullReferenceException("Field is required");
}
MemberExpression expr = null;
if (Field.Body is MemberExpression)
{
expr = (MemberExpression)Field.Body;
}
else if (Field.Body is UnaryExpression)
{
expr = (MemberExpression)((UnaryExpression)Field.Body).Operand;
}
else
{
const string Format = "Expression '{0}' not supported.";
string message = string.Format(Format, Field);
throw new ArgumentException(message, "Field");
}
return expr.Member.Name;
}
我已经完成了类似于下面方法的INotifyPropertyChanged实现。在这里,属性存储在基类中的字典中,如下所示。当然,使用继承并不总是可取的,但是对于视图模型,我认为它是可以接受的,并且在视图模型类中给出了非常干净的属性引用。
public class PhotoDetailsViewModel
: PropertyChangedNotifierBase<PhotoDetailsViewModel>
{
public bool IsLoading
{
get { return GetValue(x => x.IsLoading); }
set { SetPropertyValue(x => x.IsLoading, value); }
}
public string PendingOperation
{
get { return GetValue(x => x.PendingOperation); }
set { SetPropertyValue(x => x.PendingOperation, value); }
}
public PhotoViewModel Photo
{
get { return GetValue(x => x.Photo); }
set { SetPropertyValue(x => x.Photo, value); }
}
}
稍微复杂一些的基类如下所示。它处理从lambda表达式到属性名的转换。注意,这些属性实际上是伪属性,因为只使用了名称。但是它对于视图模型和视图模型上的属性引用来说是透明的。
public class PropertyChangedNotifierBase<T> : INotifyPropertyChanged
{
readonly Dictionary<string, object> _properties = new Dictionary<string, object>();
protected U GetValue<U>(Expression<Func<T, U>> property)
{
var propertyName = GetPropertyName(property);
return GetValue<U>(propertyName);
}
private U GetValue<U>(string propertyName)
{
object value;
if (!_properties.TryGetValue(propertyName, out value))
{
return default(U);
}
return (U)value;
}
protected void SetPropertyValue<U>(Expression<Func<T, U>> property, U value)
{
var propertyName = GetPropertyName(property);
var oldValue = GetValue<U>(propertyName);
if (Object.ReferenceEquals(oldValue, value))
{
return;
}
_properties[propertyName] = value;
RaisePropertyChangedEvent(propertyName);
}
protected void RaisePropertyChangedEvent<U>(Expression<Func<T, U>> property)
{
var name = GetPropertyName(property);
RaisePropertyChangedEvent(name);
}
protected void RaisePropertyChangedEvent(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
private static string GetPropertyName<U>(Expression<Func<T, U>> property)
{
if (property == null)
{
throw new NullReferenceException("property");
}
var lambda = property as LambdaExpression;
var memberAssignment = (MemberExpression) lambda.Body;
return memberAssignment.Member.Name;
}
public event PropertyChangedEventHandler PropertyChanged;
}
我发现一些建议的答案钻到MemberExpression/UnaryExpression不捕获嵌套/子属性。
o =>。Thing2返回Thing1而不是Thing1.Thing2。
如果您试图使用EntityFramework DbSet.Include(…),这种区别就很重要。
我发现只要解析Expression.ToString()就可以了,而且速度相对较快。我将它与UnaryExpression版本进行了比较,甚至从成员/UnaryExpression中获得ToString,以查看是否更快,但差异可以忽略不计。如果这是个糟糕的主意,请纠正我。
可拓法
/// <summary>
/// Given an expression, extract the listed property name; similar to reflection but with familiar LINQ+lambdas. Technique @via https://stackoverflow.com/a/16647343/1037948
/// </summary>
/// <remarks>Cheats and uses the tostring output -- Should consult performance differences</remarks>
/// <typeparam name="TModel">the model type to extract property names</typeparam>
/// <typeparam name="TValue">the value type of the expected property</typeparam>
/// <param name="propertySelector">expression that just selects a model property to be turned into a string</param>
/// <param name="delimiter">Expression toString delimiter to split from lambda param</param>
/// <param name="endTrim">Sometimes the Expression toString contains a method call, something like "Convert(x)", so we need to strip the closing part from the end</param>
/// <returns>indicated property name</returns>
public static string GetPropertyName<TModel, TValue>(this Expression<Func<TModel, TValue>> propertySelector, char delimiter = '.', char endTrim = ')') {
var asString = propertySelector.ToString(); // gives you: "o => o.Whatever"
var firstDelim = asString.IndexOf(delimiter); // make sure there is a beginning property indicator; the "." in "o.Whatever" -- this may not be necessary?
return firstDelim < 0
? asString
: asString.Substring(firstDelim+1).TrimEnd(endTrim);
}//-- fn GetPropertyNameExtended
(检查分隔符甚至可能是多余的)
演示 (LinqPad)
演示+比较代码—https://gist.github.com/zaus/6992590
使用c# 7模式匹配:
public static string GetMemberName<T>(this Expression<T> expression)
{
switch (expression.Body)
{
case MemberExpression m:
return m.Member.Name;
case UnaryExpression u when u.Operand is MemberExpression m:
return m.Member.Name;
default:
throw new NotImplementedException(expression.GetType().ToString());
}
}
例子:
public static RouteValueDictionary GetInfo<T>(this HtmlHelper html,
Expression<Func<T, object>> action) where T : class
{
var name = action.GetMemberName();
return GetInfo(html, name);
}
[更新]c# 8模式匹配:
public static string GetMemberName<T>(this Expression<T> expression) => expression.Body switch
{
MemberExpression m => m.Member.Name,
UnaryExpression u when u.Operand is MemberExpression m => m.Member.Name,
_ => throw new NotImplementedException(expression.GetType().ToString())
};