c# - 表达式——如何重用业务逻辑？如何将它们结合起来？

Question

注意:这是一篇很长的文章，请滚动到底部查看问题 - 希望这会让我更容易理解我的问题。谢谢!

---

我有“成员(member)”模型，其定义如下:

public class Member
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string ScreenName { get; set; }

    [NotMapped]
    public string RealName
    {
         get { return (FirstName + " " + LastName).TrimEnd(); }
    }

    [NotMapped]
    public string DisplayName
    {
        get
        {
            return string.IsNullOrEmpty(ScreenName) ? RealName : ScreenName;
        }
    }
}

这是现有的项目和模型，我不想更改它。现在，我们收到了通过 DisplayName 启用配置文件检索的请求:

public Member GetMemberByDisplayName(string displayName)
{
     var member = this.memberRepository
                      .FirstOrDefault(m => m.DisplayName == displayName);
     return member;
}

此代码不起作用，因为 DisplayName 未映射到数据库中的字段。好吧，那我就表个态:

public Member GetMemberByDisplayName(string displayName)
{
     Expression<Func<Member, bool>> displayNameSearchExpr = m => (
                string.IsNullOrEmpty(m.ScreenName) 
                    ? (m.Name + " " + m.LastName).TrimEnd() 
                    : m.ScreenName
            ) == displayName;

     var member = this.memberRepository
                      .FirstOrDefault(displayNameSearchExpr);

     return member;
}

这有效。唯一的问题是生成显示名称的业务逻辑被复制/粘贴到两个不同的地方。我想避免这种情况。但我不明白该怎么做。我带来的最好的如下:

  public class Member
    {

        public static Expression<Func<Member, string>> GetDisplayNameExpression()
        {
            return m => (
                            string.IsNullOrEmpty(m.ScreenName)
                                ? (m.Name + " " + m.LastName).TrimEnd()
                                : m.ScreenName
                        );
        }

        public static Expression<Func<Member, bool>> FilterMemberByDisplayNameExpression(string displayName)
        {
            return m => (
                string.IsNullOrEmpty(m.ScreenName)
                    ? (m.Name + " " + m.LastName).TrimEnd()
                    : m.ScreenName
            ) == displayName;
        }

        private static readonly Func<Member, string> GetDisplayNameExpressionCompiled = GetDisplayNameExpression().Compile();

        [NotMapped]
        public string DisplayName
        {
            get
            {
                return GetDisplayNameExpressionCompiled(this);
            }
        }

        [NotMapped]
        public string RealName
        {
             get { return (FirstName + " " + LastName).TrimEnd(); }
        }

   }

---

问题:

(1) 如何在 FilterMemberByDisplayNameExpression() 中重用 GetDisplayNameExpression()？我尝试了Expression.Invoke:

public static Expression<Func<Member, bool>> FilterMemberByDisplayNameExpression(string displayName)
{
    Expression<Func<string, bool>> e0 = s => s == displayName;
    var e1 = GetDisplayNameExpression();

    var combinedExpression = Expression.Lambda<Func<Member, bool>>(
           Expression.Invoke(e0, e1.Body), e1.Parameters);

    return combinedExpression;
}

但我从提供商处收到以下错误:

The LINQ expression node type 'Invoke' is not supported in LINQ to Entities.

(2) 在 DisplayName 属性中使用 Expression.Compile() 是一个好方法吗？有什么问题吗？

(3) 如何在 GetDisplayNameExpression() 中移动 RealName 逻辑？我想我必须创建另一个表达式和另一个编译表达式，但我不明白如何从 GetDisplayNameExpression() 内部调用 RealNameExpression。

谢谢。

Answer 1

我可以修复你的表达式生成器，并且我可以编写你的 GetDisplayNameExpression (所以1和3)

public class Member
{
    public string ScreenName { get; set; }
    public string Name { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public static Expression<Func<Member, string>> GetRealNameExpression()
    {
        return m => (m.Name + " " + m.LastName).TrimEnd();
    }

    public static Expression<Func<Member, string>> GetDisplayNameExpression()
    {
        var isNullOrEmpty = typeof(string).GetMethod("IsNullOrEmpty", BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(string) }, null);

        var e0 = GetRealNameExpression();
        var par1 = e0.Parameters[0];

        // Done in this way, refactoring will correctly rename m.ScreenName
        // We could have used a similar trick for string.IsNullOrEmpty,
        // but it would have been useless, because its name and signature won't
        // ever change.
        Expression<Func<Member, string>> e1 = m => m.ScreenName;

        var screenName = (MemberExpression)e1.Body;
        var prop = Expression.Property(par1, (PropertyInfo)screenName.Member);
        var condition = Expression.Condition(Expression.Call(null, isNullOrEmpty, prop), e0.Body, prop);

        var combinedExpression = Expression.Lambda<Func<Member, string>>(condition, par1);
        return combinedExpression;
    }

    private static readonly Func<Member, string> GetDisplayNameExpressionCompiled = GetDisplayNameExpression().Compile();

    private static readonly Func<Member, string> GetRealNameExpressionCompiled = GetRealNameExpression().Compile();

    public string DisplayName
    {
        get
        {
            return GetDisplayNameExpressionCompiled(this);
        }
    }

    public string RealName
    {
        get
        {
            return GetRealNameExpressionCompiled(this);
        }
    }

    public static Expression<Func<Member, bool>> FilterMemberByDisplayNameExpression(string displayName)
    {
        var e0 = GetDisplayNameExpression();
        var par1 = e0.Parameters[0];

        var combinedExpression = Expression.Lambda<Func<Member, bool>>(
            Expression.Equal(e0.Body, Expression.Constant(displayName)), par1);

        return combinedExpression;
    }

请注意我如何重用 GetDisplayNameExpression 的相同参数表达e1.Parameters[0] (输入 par1 )这样我就不必重写表达式(否则我需要使用表达式重写器)。

我们可以使用这个技巧，因为我们只有一个表达式要处理，我们必须附加一些新代码。完全不同的情况(我们需要一个表达式重写器)是尝试组合两个表达式的情况(例如执行 GetRealNameExpression() + " " + GetDisplayNameExpression() ，两者都需要作为参数 a Member ，但它们的参数是分开的......可能这个https://stackoverflow.com/a/5431309/613130会起作用...

对于2，我没有发现任何问题。您正确使用static readonly 。但是请看GetDisplayNameExpression并思考“是一些业务代码重复支付更好还是那个更好？”

通用解决方案

现在...我非常确定它是可行的...事实上它是可行的:一个表达式“扩展器”，将“特殊属性”“扩展”到其表达式“自动”。

public static class QueryableEx
{
    private static readonly ConcurrentDictionary<Type, Dictionary<PropertyInfo, LambdaExpression>> expressions = new ConcurrentDictionary<Type, Dictionary<PropertyInfo, LambdaExpression>>();

    public static IQueryable<T> Expand<T>(this IQueryable<T> query)
    {
        var visitor = new QueryableVisitor();
        Expression expression2 = visitor.Visit(query.Expression);

        return query.Expression != expression2 ? query.Provider.CreateQuery<T>(expression2) : query;
    }

    private static Dictionary<PropertyInfo, LambdaExpression> Get(Type type)
    {
        Dictionary<PropertyInfo, LambdaExpression> dict;

        if (expressions.TryGetValue(type, out dict))
        {
            return dict;
        }

        var props = type.GetProperties(BindingFlags.Public | BindingFlags.Instance);

        dict = new Dictionary<PropertyInfo, LambdaExpression>();

        foreach (var prop in props)
        {
            var exp = type.GetMember(prop.Name + "Expression", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static).Where(p => p.MemberType == MemberTypes.Field || p.MemberType == MemberTypes.Property).SingleOrDefault();

            if (exp == null)
            {
                continue;
            }

            if (!typeof(LambdaExpression).IsAssignableFrom(exp.MemberType == MemberTypes.Field ? ((FieldInfo)exp).FieldType : ((PropertyInfo)exp).PropertyType))
            {
                continue;
            }

            var lambda = (LambdaExpression)(exp.MemberType == MemberTypes.Field ? ((FieldInfo)exp).GetValue(null) : ((PropertyInfo)exp).GetValue(null, null));

            if (prop.PropertyType != lambda.ReturnType)
            {
                throw new Exception(string.Format("Mismatched return type of Expression of {0}.{1}, {0}.{2}", type.Name, prop.Name, exp.Name));
            }

            dict[prop] = lambda;
        }

        // We try to save some memory, removing empty dictionaries
        if (dict.Count == 0)
        {
            dict = null;
        }

        // There is no problem if multiple threads generate their "versions"
        // of the dict at the same time. They are all equivalent, so the worst
        // case is that some CPU cycles are wasted.
        dict = expressions.GetOrAdd(type, dict);

        return dict;
    }

    private class SingleParameterReplacer : ExpressionVisitor
    {
        public readonly ParameterExpression From;
        public readonly Expression To;

        public SingleParameterReplacer(ParameterExpression from, Expression to)
        {
            this.From = from;
            this.To = to;
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return node != this.From ? base.VisitParameter(node) : this.Visit(this.To);
        }
    }

    private class QueryableVisitor : ExpressionVisitor
    {
        protected static readonly Assembly MsCorLib = typeof(int).Assembly;
        protected static readonly Assembly Core = typeof(IQueryable).Assembly;

        // Used to check for recursion
        protected readonly List<MemberInfo> MembersBeingVisited = new List<MemberInfo>();

        protected override Expression VisitMember(MemberExpression node)
        {
            var declaringType = node.Member.DeclaringType;
            var assembly = declaringType.Assembly;

            if (assembly != MsCorLib && assembly != Core && node.Member.MemberType == MemberTypes.Property)
            {
                var dict = QueryableEx.Get(declaringType);

                LambdaExpression lambda;

                if (dict != null && dict.TryGetValue((PropertyInfo)node.Member, out lambda))
                {
                    // Anti recursion check
                    if (this.MembersBeingVisited.Contains(node.Member))
                    {
                        throw new Exception(string.Format("Recursively visited member. Chain: {0}", string.Join("->", this.MembersBeingVisited.Concat(new[] { node.Member }).Select(p => p.DeclaringType.Name + "." + p.Name))));
                    }

                    this.MembersBeingVisited.Add(node.Member);

                    // Replace the parameters of the expression with "our" reference
                    var body = new SingleParameterReplacer(lambda.Parameters[0], node.Expression).Visit(lambda.Body);

                    Expression exp = this.Visit(body);

                    this.MembersBeingVisited.RemoveAt(this.MembersBeingVisited.Count - 1);

                    return exp;
                }
            }

            return base.VisitMember(node);
        }
    }
}

• 它是如何运作的？魔法、反射、仙尘……
• 它是否支持引用其他属性的属性？ 是
• 它需要什么？

它需要名称 Foo 的每个“特殊”属性有一个名为 FooExpression 的相应静态字段/静态属性返回 Expression<Func<Class, something>>

需要通过扩展方法Expand()对查询进行“转换”在物化/枚举之前的某个时刻。所以:

public class Member
{
    // can be private/protected/internal
    public static readonly Expression<Func<Member, string>> RealNameExpression =
        m => (m.Name + " " + m.LastName).TrimEnd();

    // Here we are referencing another "special" property, and it just works!
    public static readonly Expression<Func<Member, string>> DisplayNameExpression =
        m => string.IsNullOrEmpty(m.ScreenName) ? m.RealName : m.ScreenName;

    public string RealName
    {
        get 
        { 
            // return the real name however you want, probably reusing
            // the expression through a compiled readonly 
            // RealNameExpressionCompiled as you had done
        }  
    }

    public string DisplayName
    {
        get
        {
        }
    }
}

// Note the use of .Expand();
var res = (from p in ctx.Member 
          where p.RealName == "Something" || p.RealName.Contains("Anything") ||
                p.DisplayName == "Foo"
          select new { p.RealName, p.DisplayName, p.Name }).Expand();

// now you can use res normally.

• 限制 1:一个问题是像 Single(Expression) 这样的方法, First(Expression) , Any(Expression)类似，不返回 IQueryable 。首先使用 Where(Expression).Expand().Single() 进行更改

• 限制 2:“特殊”属性不能在循环中引用自身。因此，如果 A 使用 B，B 就无法使用 A，并且使用三元表达式等技巧将无法使其发挥作用。