从lambda表达式中检索属性名称-Java 学习之路

452

通过lambda表达式传入时，是否有更好的方法来获取属性名称？这是我现在拥有的 .

例如 .

GetSortingInfo<User>(u => u.UserId);

只有当属性是字符串时，它才能将其作为元素表达式进行处理 . 因为并非所有属性都是字符串我必须使用对象，但它会为那些返回一个单一表达式 .

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;
}

19 回答

3
我在ObjectStateEntry上创建了一个扩展方法，以便能够以类型安全的方式标记属性（Entity Framework POCO类）的属性，因为默认方法只接受字符串 . 这是我从酒店获取名称的方式：
```
public static void SetModifiedProperty<T>(this System.Data.Objects.ObjectStateEntry state, Expression<Func<T>> action)
{
    var body = (MemberExpression)action.Body;
    string propertyName = body.Member.Name;

    state.SetModifiedProperty(propertyName);
}
```
回复于 2024-04-27T13:53:41+08:00

我最近做了一个非常类似的事情来制作一个类型安全的OnPropertyChanged方法 .

这是一个方法，它将返回表达式的PropertyInfo对象 . 如果表达式不是属性，则抛出异常 .

public PropertyInfo GetPropertyInfo<TSource, TProperty>(
    TSource source,
    Expression<Func<TSource, TProperty>> propertyLambda)
{
    Type type = typeof(TSource);

    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));

    return propInfo;
}

使用 source 参数，以便编译器可以对方法调用进行类型推断 . 您可以执行以下操作

var propertyInfo = GetPropertyInfo(someUserObject, u => u.UserID);

回复于 2024-04-27T13:53:41+08:00

我正在为C＃6前项目使用扩展方法，对于那些以C＃6为目标的项目使用nameof() .

public static class MiscExtentions
{
    public static string NameOf<TModel, TProperty>(this object @object, Expression<Func<TModel, TProperty>> propertyExpression)
    {
        var expression = propertyExpression.Body as MemberExpression;
        if (expression == null)
        {
            throw new ArgumentException("Expression is not a property.");
        }

        return expression.Member.Name;
    }
}

我称之为：

public class MyClass 
{
    public int Property1 { get; set; }
    public string Property2 { get; set; }
    public int[] Property3 { get; set; }
    public Subclass Property4 { get; set; }
    public Subclass[] Property5 { get; set; }
}

public class Subclass
{
    public int PropertyA { get; set; }
    public string PropertyB { get; set; }
}

// result is Property1
this.NameOf((MyClass o) => o.Property1);
// result is Property2
this.NameOf((MyClass o) => o.Property2);
// result is Property3
this.NameOf((MyClass o) => o.Property3);
// result is Property4
this.NameOf((MyClass o) => o.Property4);
// result is PropertyB
this.NameOf((MyClass o) => o.Property4.PropertyB);
// result is Property5
this.NameOf((MyClass o) => o.Property5);

它适用于字段和属性 .

回复于 2024-04-27T13:53:41+08:00

我完成了类似于下面方法的 INotifyPropertyChanged 实现 . 这里的属性存储在下面显示的基类的字典中 . 当然并不总是希望使用继承，但对于视图模型，我认为它是可接受的，并在视图模型类中提供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); }
    }
}

更复杂的基类如下所示 . 它处理从lambda表达式到属性名称的转换 . 请注意，属性实际上是伪属性，因为只使用了名称 . 但它对视图模型和视图模型上的属性的引用看起来是透明的 .

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;
}

回复于 2024-04-27T13:53:41+08:00

13

现在在C＃6你可以像这样使用nameof nameof(User.UserId)

这有很多好处，其中包括compile time，而不是运行时 .

https://msdn.microsoft.com/en-us/magazine/dn802602.aspx

回复于 2024-04-27T13:53:41+08:00

这是另一个答案：

public static string GetPropertyName<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
                                                                      Expression<Func<TModel, TProperty>> expression)
    {
        var metaData = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);

        return metaData.PropertyName;
    }

回复于 2024-04-27T13:53:41+08:00

public string GetName<TSource, TField>(Expression<Func<TSource, TField>> Field)
{
    return (Field.Body as MemberExpression ?? ((UnaryExpression)Field.Body).Operand as MemberExpression).Member.Name;
}

这会处理成员和一元表达式 . 不同之处在于，如果表达式表示值类型，则将获得 UnaryExpression ，而如果表达式表示引用类型，则将获得 MemberExpression . 可以将所有内容强制转换为对象，但必须将值类型装箱 . 这就是UnaryExpression存在的原因 . Reference.

对于可读性（@Jowen），这是一个扩展的等价物：

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;
}

回复于 2024-04-27T13:53:41+08:00

对于 Array .Length来说，有一个优势 . 虽然'Length'作为属性公开，但您不能在以前提出的任何解决方案中使用它 .

using Contract = System.Diagnostics.Contracts.Contract;
using Exprs = System.Linq.Expressions;

static string PropertyNameFromMemberExpr(Exprs.MemberExpression expr)
{
    return expr.Member.Name;
}

static string PropertyNameFromUnaryExpr(Exprs.UnaryExpression expr)
{
    if (expr.NodeType == Exprs.ExpressionType.ArrayLength)
        return "Length";

    var mem_expr = expr.Operand as Exprs.MemberExpression;

    return PropertyNameFromMemberExpr(mem_expr);
}

static string PropertyNameFromLambdaExpr(Exprs.LambdaExpression expr)
{
         if (expr.Body is Exprs.MemberExpression)   return PropertyNameFromMemberExpr(expr.Body as Exprs.MemberExpression);
    else if (expr.Body is Exprs.UnaryExpression)    return PropertyNameFromUnaryExpr(expr.Body as Exprs.UnaryExpression);

    throw new NotSupportedException();
}

public static string PropertyNameFromExpr<TProp>(Exprs.Expression<Func<TProp>> expr)
{
    Contract.Requires<ArgumentNullException>(expr != null);
    Contract.Requires<ArgumentException>(expr.Body is Exprs.MemberExpression || expr.Body is Exprs.UnaryExpression);

    return PropertyNameFromLambdaExpr(expr);
}

public static string PropertyNameFromExpr<T, TProp>(Exprs.Expression<Func<T, TProp>> expr)
{
    Contract.Requires<ArgumentNullException>(expr != null);
    Contract.Requires<ArgumentException>(expr.Body is Exprs.MemberExpression || expr.Body is Exprs.UnaryExpression);

    return PropertyNameFromLambdaExpr(expr);
}

现在示例用法：

int[] someArray = new int[1];
Console.WriteLine(PropertyNameFromExpr( () => someArray.Length ));

如果 PropertyNameFromUnaryExpr 没有检查 ArrayLength ，"someArray"将被打印到控制台（编译器似乎生成直接访问支持Length字段，作为优化，即使在Debug中，因此特殊情况） .

回复于 2024-04-27T13:53:41+08:00

这是另一种方式得到PropertyInfo基于this answer.它消除了对象实例的需要 .

/// <summary>
/// Get metadata of property referenced by expression. Type constrained.
/// </summary>
public static PropertyInfo GetPropertyInfo<TSource, TProperty>(Expression<Func<TSource, TProperty>> propertyLambda)
{
    return GetPropertyInfo((LambdaExpression) propertyLambda);
}

/// <summary>
/// Get metadata of property referenced by expression.
/// </summary>
public static PropertyInfo GetPropertyInfo(LambdaExpression propertyLambda)
{
    // https://stackoverflow.com/questions/671968/retrieving-property-name-from-lambda-expression
    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if(propertyLambda.Parameters.Count() == 0)
        throw new ArgumentException(String.Format(
            "Expression '{0}' does not have any parameters. A property expression needs to have at least 1 parameter.",
            propertyLambda.ToString()));

    var type = propertyLambda.Parameters[0].Type;
    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(String.Format(
            "Expression '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));
    return propInfo;
}

它可以像这样调用：

var propertyInfo = GetPropertyInfo((User u) => u.UserID);

回复于 2024-04-27T13:53:41+08:00

我发现一些深入到 MemberExpression / UnaryExpression 的suggested answers不会捕获嵌套/子属性 .

ex） o => o.Thing1.Thing2 返回 Thing1 而不是 Thing1.Thing2 .

如果您尝试使用EntityFramework DbSet.Include(...) ，这种区别很重要 .

我发现只是解析 Expression.ToString() 似乎工作正常，而且相对较快 . 我将它与 UnaryExpression 版本进行了比较，甚至将 ToString 从 Member/UnaryExpression 中移除，看看它是否更快，但差异可以忽略不计 . 如果这是一个可怕的想法，请纠正我 .

扩展方法

/// <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

（检查分隔符可能甚至是过度杀伤）

演示（LinqPad）

演示比较代码 - https://gist.github.com/zaus/6992590

回复于 2024-04-27T13:53:41+08:00

我正在玩同样的事情，并努力工作 . 它没有经过全面测试，但似乎处理了值类型的问题（你遇到的unaryexpression问题）

public static string GetName(Expression<Func<object>> exp)
{
    MemberExpression body = exp.Body as MemberExpression;

    if (body == null) {
       UnaryExpression ubody = (UnaryExpression)exp.Body;
       body = ubody.Operand as MemberExpression;
    }

    return body.Member.Name;
}

回复于 2024-04-27T13:53:41+08:00

167

我更新了@Cameron's answer以对 Convert 类型的lambda表达式进行一些安全检查：

PropertyInfo GetPropertyName<TSource, TProperty>(
Expression<Func<TSource, TProperty>> propertyLambda)
{
  var body = propertyLambda.Body;
  if (!(body is MemberExpression member)
    && !(body is UnaryExpression unary
      && (member = unary.Operand as MemberExpression) != null))
    throw new ArgumentException($"Expression '{propertyLambda}' " +
      "does not refer to a property.");

  if (!(member.Member is PropertyInfo propInfo))
    throw new ArgumentException($"Expression '{propertyLambda}' " +
      "refers to a field, not a property.");

  var type = typeof(TSource);
  if (!propInfo.DeclaringType.GetTypeInfo().IsAssignableFrom(type.GetTypeInfo()))
    throw new ArgumentException($"Expresion '{propertyLambda}' " + 
      "refers to a property that is not from type '{type}'.");

  return propInfo;
}

回复于 2024-04-27T13:53:41+08:00

137

如果你想获得多个字段我会离开这个函数：

/// <summary>
    /// Get properties separated by , (Ex: to invoke 'd => new { d.FirstName, d.LastName }')
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="exp"></param>
    /// <returns></returns>
    public static string GetFields<T>(Expression<Func<T, object>> exp)
    {
        MemberExpression body = exp.Body as MemberExpression;
        var fields = new List<string>();
        if (body == null)
        {
            NewExpression ubody = exp.Body as NewExpression;
            if (ubody != null)
                foreach (var arg in ubody.Arguments)
                {
                    fields.Add((arg as MemberExpression).Member.Name);
                }
        }

        return string.Join(",", fields);
    }

回复于 2024-04-27T13:53:41+08:00

从.NET 4.0开始，您可以使用ExpressionVisitor查找属性：

class ExprVisitor : ExpressionVisitor {
    public bool IsFound { get; private set; }
    public string MemberName { get; private set; }
    public Type MemberType { get; private set; }
    protected override Expression VisitMember(MemberExpression node) {
        if (!IsFound && node.Member.MemberType == MemberTypes.Property) {
            IsFound = true;
            MemberName = node.Member.Name;
            MemberType = node.Type;
        }
        return base.VisitMember(node);
    }
}

以下是您使用此访问者的方式：

var visitor = new ExprVisitor();
visitor.Visit(expr);
if (visitor.IsFound) {
    Console.WriteLine("First property in the expression tree: Name={0}, Type={1}", visitor.MemberName, visitor.MemberType.FullName);
} else {
    Console.WriteLine("No properties found.");
}

回复于 2024-04-27T13:53:41+08:00

5

好吧，没有必要打电话给 .Name.ToString() ，但广泛的是关于它，是的 . 您可能需要的唯一考虑因素是 x.Foo.Bar 是否应返回"Foo"，"Bar"或异常 - 即您是否需要迭代 .

（重新评论）有关灵活排序的更多信息，请参阅here .

回复于 2024-04-27T13:53:41+08:00

314

这是获取struct / class / interface / delegate / array的fields / properties / indexers / methods / extension methods / delegates的字符串名称的一般实现 . 我已经测试了静态/实例和非泛型/通用变体的组合 .

//involves recursion
public static string GetMemberName(this LambdaExpression memberSelector)
{
    Func<Expression, string> nameSelector = null;  //recursive func
    nameSelector = e => //or move the entire thing to a separate recursive method
    {
        switch (e.NodeType)
        {
            case ExpressionType.Parameter:
                return ((ParameterExpression)e).Name;
            case ExpressionType.MemberAccess:
                return ((MemberExpression)e).Member.Name;
            case ExpressionType.Call:
                return ((MethodCallExpression)e).Method.Name;
            case ExpressionType.Convert:
            case ExpressionType.ConvertChecked:
                return nameSelector(((UnaryExpression)e).Operand);
            case ExpressionType.Invoke:
                return nameSelector(((InvocationExpression)e).Expression);
            case ExpressionType.ArrayLength:
                return "Length";
            default:
                throw new Exception("not a proper member selector");
        }
    };

    return nameSelector(memberSelector.Body);
}

这个东西也可以用简单的 while 循环编写：

//iteration based
public static string GetMemberName(this LambdaExpression memberSelector)
{
    var currentExpression = memberSelector.Body;

    while (true)
    {
        switch (currentExpression.NodeType)
        {
            case ExpressionType.Parameter:
                return ((ParameterExpression)currentExpression).Name;
            case ExpressionType.MemberAccess:
                return ((MemberExpression)currentExpression).Member.Name;
            case ExpressionType.Call:
                return ((MethodCallExpression)currentExpression).Method.Name;
            case ExpressionType.Convert:
            case ExpressionType.ConvertChecked:
                currentExpression = ((UnaryExpression)currentExpression).Operand;
                break;
            case ExpressionType.Invoke:
                currentExpression = ((InvocationExpression)currentExpression).Expression;
                break;
            case ExpressionType.ArrayLength:
                return "Length";
            default:
                throw new Exception("not a proper member selector");
        }
    }
}

我喜欢递归方法，虽然第二个可能更容易阅读 . 人们可以称之为：

someExpr = x => x.Property.ExtensionMethod()[0]; //or
someExpr = x => Static.Method().Field; //or
someExpr = x => VoidMethod(); //or
someExpr = () => localVariable; //or
someExpr = x => x; //or
someExpr = x => (Type)x; //or
someExpr = () => Array[0].Delegate(null); //etc

string name = someExpr.GetMemberName();

打印最后一个成员 .

注意：

如果链接表达式如 A.B.C ，则返回"C" .
这不适用于 const ，数组索引器或 enum （不可能涵盖所有情况） .

回复于 2024-04-27T13:53:41+08:00

这是method proposed by Cameron的更新 . 第一个参数不是必需的 .

public PropertyInfo GetPropertyInfo<TSource, TProperty>(
    Expression<Func<TSource, TProperty>> propertyLambda)
{
    Type type = typeof(TSource);

    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(string.Format(
            "Expresion '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));

    return propInfo;
}

您可以执行以下操作：

var propertyInfo = GetPropertyInfo<SomeType>(u => u.UserID);
var propertyInfo = GetPropertyInfo((SomeType u) => u.UserID);

扩展方法：

public static PropertyInfo GetPropertyInfo<TSource, TProperty>(this TSource source,
    Expression<Func<TSource, TProperty>> propertyLambda) where TSource : class
{
    return GetPropertyInfo(propertyLambda);
}

public static string NameOfProperty<TSource, TProperty>(this TSource source,
    Expression<Func<TSource, TProperty>> propertyLambda) where TSource : class
{
    PropertyInfo prodInfo = GetPropertyInfo(propertyLambda);
    return prodInfo.Name;
}

您可以：

SomeType someInstance = null;
string propName = someInstance.NameOfProperty(i => i.Length);
PropertyInfo propInfo = someInstance.GetPropertyInfo(i => i.Length);

回复于 2024-04-27T13:53:41+08:00

我发现你可以做的另一种方法是强烈输入源和属性，并明确推断lambda的输入 . 不确定这是否是正确的术语，但这是结果 .

public static RouteValueDictionary GetInfo<T,P>(this HtmlHelper html, Expression<Func<T, P>> action) where T : class
{
    var expression = (MemberExpression)action.Body;
    string name = expression.Member.Name;

    return GetInfo(html, name);
}

然后这样称呼它 .

GetInfo((User u) => u.UserId);

并且它有效 .
谢谢大家 .

回复于 2024-04-27T13:53:41+08:00

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：

public static RouteValueDictionary GetInfo<T>(this HtmlHelper html, 
    Expression<Func<T, object>> action) where T : class
{
    var name = action.GetMemberName();
    return GetInfo(html, name);
}

回复于 2024-04-27T13:53:41+08:00

从lambda表达式中检索属性名称

19 回答

扩展方法

演示（LinqPad）

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