c# - 从 Expression> 获取字符串形式的属性

Question

我使用一些被序列化的强类型表达式，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 Expression<Func<TModel,TProperty>>并按原样使用:SortOption.Field = (p => p.FirstName); .对于这个简单的案例，我已经让它完美地工作了。

我用来解析“FirstName”属性的代码实际上是在我们使用的第三方产品中重用了一些现有功能，并且效果很好，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要方面有一些非常不同的假设。

至于这段代码的作用，我不是很理解，与其修改那段代码，我想我应该从头开始编写这个功能。但是，我不知道有什么好的方法可以做到这一点。我怀疑我们可以做一些比执行 ToString() 和执行字符串解析更好的事情。那么，什么是处理琐碎和深层嵌套情况的好方法呢？

要求:

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

---

虽然这对我的问题的回答并不重要，但我怀疑我的序列化/反序列化代码可能对将来发现此问题的其他人有用，所以它在下面。同样，这段代码对问题并不重要——我只是认为它可能对某人有所帮助。注意 DynamicExpression.ParseLambda来自 Dynamic LINQ东西和Property.PropertyToString()是关于这个问题的。

/// <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

诀窍是:任何这种形式的表达...

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

...实际上只是一堆嵌套的 MemberExpression对象。

首先你有:

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

正在评估 Expression以上为MemberExpression 给你:

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

最后，那个(在“顶部”)你有:

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

所以很明显解决这个问题的方法是检查 Expression MemberExpression 的属性直到它本身不再是 MemberExpression 为止.

---

更新:您的问题似乎有更多的变化。可能是您有一些 看起来 像 Func<T, int> 的 lambda。 ...

p => p.Age

...但是实际上是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)

针对此问题进行调整实际上并不像看起来那么困难。查看我更新的代码，了解一种处理方法。请注意，通过抽象代码以获得 MemberExpression进入它自己的方法(TryFindMemberExpression)，这种方法保留了GetFullPropertyName方法相当干净，允许您在将来添加额外的检查——如果，也许，您发现自己面临一个您最初没有考虑过的新场景——而不必费力费力代码。

---

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