从Expression <Func <TModel,TProperty >>获取属性,作为字符串

Question

我使用了一些强类型表达式,这些表达式被序列化以允许我的UI代码具有强类型排序和搜索表达式.这些是类型的Expression<Func<TModel,TProperty>>并且如此使用:SortOption.Field = (p => p.FirstName);.对于这个简单的案例,我已经完美地完成了这项工作.

我用来解析"FirstName"属性的代码实际上重用了我们使用的第三方产品中的一些现有功能,并且它工作得很好,直到我们开始使用深层嵌套的属性(SortOption.Field = (p => p.Address.State.Abbreviation);).此代码在支持深层嵌套属性的需求方面有一些非常不同的假设.

至于这段代码的作用,我并不是真的理解它而不是改变代码,我想我应该从头开始编写这个功能.但是,我不知道这样做的好方法.我怀疑我们可以做一些比做ToString()和执行字符串解析更好的事情.那么有什么好办法来处理琐碎和深层嵌套的案例呢？

要求:

• 鉴于表达式p => p.FirstName我需要一串"FirstName".
• 鉴于表达式p => p.Address.State.Abbreviation我需要一串"Address.State.Abbreviation"

---

虽然对我的问题的答案并不重要,但我怀疑我的序列化/反序列化代码对将来发现这个问题的其他人有用,所以它在下面.同样,这段代码对这个问题并不重要 - 我只是觉得它可能对某些人有所帮助.请注意,DynamicExpression.ParseLambda它来自动态LINQ的东西,Property.PropertyToString()这是这个问题的关键.

/// <summary>
/// This defines a framework to pass, across serialized tiers, sorting logic to be performed.
/// </summary>
/// <typeparam name="TModel">This is the object type that you are filtering.</typeparam>
/// <typeparam name="TProperty">This is the property on the object that you are filtering.</typeparam>
[Serializable]
public class SortOption<TModel, TProperty> : ISerializable where TModel : class
{
    /// <summary>
    /// Convenience constructor.
    /// </summary>
    /// <param name="property">The property to sort.</param>
    /// <param name="isAscending">Indicates if the sorting should be ascending or descending</param>
    /// <param name="priority">Indicates the sorting priority where 0 is a higher priority than 10.</param>
    public SortOption(Expression<Func<TModel, TProperty>> property, bool isAscending = true, int priority = 0)
    {
        Property = property;
        IsAscending = isAscending;
        Priority = priority;
    }

    /// <summary>
    /// Default Constructor.
    /// </summary>
    public SortOption()
        : this(null)
    {
    }

    /// <summary>
    /// This is the field on the object to filter.
    /// </summary>
    public Expression<Func<TModel, TProperty>> Property { get; set; }

    /// <summary>
    /// This indicates if the sorting should be ascending or descending.
    /// </summary>
    public bool IsAscending { get; set; }

    /// <summary>
    /// This indicates the sorting priority where 0 is a higher priority than 10.
    /// </summary>
    public int Priority { get; set; }

    #region Implementation of ISerializable

    /// <summary>
    /// This is the constructor called when deserializing a SortOption.
    /// </summary>
    protected SortOption(SerializationInfo info, StreamingContext context)
    {
        IsAscending = info.GetBoolean("IsAscending");
        Priority = info.GetInt32("Priority");

        // We just persisted this by the PropertyName. So let's rebuild the Lambda Expression from that.
        Property = DynamicExpression.ParseLambda<TModel, TProperty>(info.GetString("Property"), default(TModel), default(TProperty));
    }

    /// <summary>
    /// Populates a <see cref="T:System.Runtime.Serialization.SerializationInfo"/> with the data needed to serialize the target object.
    /// </summary>
    /// <param name="info">The <see cref="T:System.Runtime.Serialization.SerializationInfo"/> to populate with data. </param>
    /// <param name="context">The destination (see <see cref="T:System.Runtime.Serialization.StreamingContext"/>) for this serialization. </param>
    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        // Just stick the property name in there. We'll rebuild the expression based on that on the other end.
        info.AddValue("Property", Property.PropertyToString());
        info.AddValue("IsAscending", IsAscending);
        info.AddValue("Priority", Priority);
    }

    #endregion
}

Answer 1

这是诀窍:这种形式的任何表达......

obj => obj.A.B.C // etc.

......实际上只是一堆嵌套MemberExpression对象.

首先你有:

MemberExpression: obj.A.B.C
Expression:       obj.A.B   // MemberExpression
Member:           C

评估Expression上面的MemberExpression为您提供:

MemberExpression: obj.A.B
Expression:       obj.A     // MemberExpression
Member:           B

最后,以上这(在"顶部"),你必须:

MemberExpression: obj.A
Expression:       obj       // note: not a MemberExpression
Member:           A

所以似乎很清楚,解决这个问题的方法是检查Expression一个MemberExpressionup 的属性,直到它不再是一个a MemberExpression.

更新:似乎你的问题有一个额外的旋转.可能你有一些看起来像Func<T, int>......的lambda

p => p.Age

......但实际上是一个Func<T, object>; 在这种情况下,编译器会将上面的表达式转换为:

p => Convert(p.Age)

实际上,调整此问题并不像看起来那么难.看看我的更新代码,找到一种方法来处理它.请注意,通过抽象代码来获取MemberExpression它自己的方法(TryFindMemberExpression),这种方法使GetFullPropertyName方法相当干净,并允许您将来添加额外的检查 - 如果您可能发现自己面临一个新的方案,你没有最初的原因 - 不必涉及过多的代码.

为了说明:这段代码对我有用.

// code adjusted to prevent horizontal overflow
static string GetFullPropertyName<T, TProperty>
(Expression<Func<T, TProperty>> exp)
{
    MemberExpression memberExp;
    if (!TryFindMemberExpression(exp.Body, out memberExp))
        return string.Empty;

    var memberNames = new Stack<string>();
    do
    {
        memberNames.Push(memberExp.Member.Name);
    }
    while (TryFindMemberExpression(memberExp.Expression, out memberExp));

    return string.Join(".", memberNames.ToArray());
}

// code adjusted to prevent horizontal overflow
private static bool TryFindMemberExpression
(Expression exp, out MemberExpression memberExp)
{
    memberExp = exp as MemberExpression;
    if (memberExp != null)
    {
        // heyo! that was easy enough
        return true;
    }

    // if the compiler created an automatic conversion,
    // it'll look something like...
    // obj => Convert(obj.Property) [e.g., int -> object]
    // OR:
    // obj => ConvertChecked(obj.Property) [e.g., int -> long]
    // ...which are the cases checked in IsConversion
    if (IsConversion(exp) && exp is UnaryExpression)
    {
        memberExp = ((UnaryExpression)exp).Operand as MemberExpression;
        if (memberExp != null)
        {
            return true;
        }
    }

    return false;
}

private static bool IsConversion(Expression exp)
{
    return (
        exp.NodeType == ExpressionType.Convert ||
        exp.NodeType == ExpressionType.ConvertChecked
    );
}

用法:

Expression<Func<Person, string>> simpleExp = p => p.FirstName;
Expression<Func<Person, string>> complexExp = p => p.Address.State.Abbreviation;
Expression<Func<Person, object>> ageExp = p => p.Age;

Console.WriteLine(GetFullPropertyName(simpleExp));
Console.WriteLine(GetFullPropertyName(complexExp));
Console.WriteLine(GetFullPropertyName(ageExp));

输出:

FirstName
Address.State.Abbreviation
Age

Answer 2

这是一个允许您获取字符串表示的方法,即使您有嵌套属性:

public static string GetPropertySymbol<T,TResult>(Expression<Func<T,TResult>> expression)
{
    return String.Join(".",
        GetMembersOnPath(expression.Body as MemberExpression)
            .Select(m => m.Member.Name)
            .Reverse());  
}

private static IEnumerable<MemberExpression> GetMembersOnPath(MemberExpression expression)
{
    while(expression != null)
    {
        yield return expression;
        expression = expression.Expression as MemberExpression;
    }
}

如果你仍然在.NET 3.5上,你需要ToArray()在调用之后粘贴Reverse(),因为在.NET 4中首先添加了String.Join一个重载IEnumerable.

Answer 3

对于来自p => p.FirstName的"FirstName"

Expression<Func<TModel, TProperty>> expression; //your given expression
string fieldName = ((MemberExpression)expression.Body).Member.Name; //watch out for runtime casting errors

我建议您查看ASP.NET MVC 2代码(来自aspnet.codeplex.com),因为它有类似的Html助手API ... Html.TextBoxFor(p => p.FirstName)等

Answer 4

另一种简单的方法是使用System.Web.Mvc.ExpressionHelper.GetExpressionText方法.在我的下一次打击中,我会写更详细的内容.看看http://carrarini.blogspot.com/.