如何记录谓词表达式？

Question

通常，我的存储库具有用于调试目的的日志记录语句，使我能够查看参数的值。最近，我沿着创建一个通用存储库的道路，该存储库将谓词表达式作为参数，以实现极大的灵活性，但是我无法找到一种将条件记录到远程有用的地方的体面方法。

示例方法：

public int GetCount<K>(Expression<Func<K, bool>> predicate) where K : class
{
    Logger.LogDebugMessage(String.Format("Parameters [predicate: {0}]", predicate == null ? string.Empty : predicate.Body.ToString()));

    ...
}

您可以看到我此时正在使用 Body.ToString()，但结果不太可读：

Parameters [predicate: (fa.SomeId == value(NameSpace.SomeClass+) >c__DisplayClass2).SomeId)]

最终我希望看到的是类似于以下内容的内容：

Parameters [predicate: (fa.SomeId == 1 && fa.Account.Name = = "MyName").SomeId)]

本质上，该日志的价值在于能够在发生故障时知道输入值。 有没有办法解决这个问题，强制 API 用户以字符串形式提供谓词？

Answer 1

Matt Warren 的这篇博客文章包含用变量名称替换局部变量引用的代码。因此，您将得到 predicate: (fa.SomeId == someValue)，而不是 predicate: (fa.SomeId == value(NameSpace.SomeClass+<>c__DisplayClass2).SomeId) ）。

博客文章：

http://blogs.msdn.com/b/mattwar/archive/2007/08/01/linq-building-an-iqueryable-provider-part-iii.aspx

来源：

public static class Evaluator
{
    /// <summary>
    /// Performs evaluation & replacement of independent sub-trees
    /// </summary>
    /// <param name="expression">The root of the expression tree.</param>
    /// <param name="fnCanBeEvaluated">A function that decides whether a given expression node can be part of the local function.</param>
    /// <returns>A new tree with sub-trees evaluated and replaced.</returns>
    public static Expression PartialEval(Expression expression, Func<Expression, bool> fnCanBeEvaluated)
    {
        return new SubtreeEvaluator(new Nominator(fnCanBeEvaluated).Nominate(expression)).Eval(expression);
    }

    /// <summary>
    /// Performs evaluation & replacement of independent sub-trees
    /// </summary>
    /// <param name="expression">The root of the expression tree.</param>
    /// <returns>A new tree with sub-trees evaluated and replaced.</returns>
    public static Expression PartialEval(Expression expression)
    {
        return PartialEval(expression, Evaluator.CanBeEvaluatedLocally);
    }

    private static bool CanBeEvaluatedLocally(Expression expression)
    {
        return expression.NodeType != ExpressionType.Parameter;
    }

    /// <summary>
    /// Evaluates & replaces sub-trees when first candidate is reached (top-down)
    /// </summary>
    class SubtreeEvaluator : ExpressionVisitor
    {
        HashSet<Expression> candidates;

        internal SubtreeEvaluator(HashSet<Expression> candidates)
        {
            this.candidates = candidates;
        }

        internal Expression Eval(Expression exp)
        {
            return this.Visit(exp);
        }

        protected override Expression Visit(Expression exp)
        {
            if (exp == null)
            {
                return null;
            }
            if (this.candidates.Contains(exp))
            {
                return this.Evaluate(exp);
            }
            return base.Visit(exp);
        }

        private Expression Evaluate(Expression e)
        {
            if (e.NodeType == ExpressionType.Constant)
            {
                return e;
            }
            LambdaExpression lambda = Expression.Lambda(e);
            Delegate fn = lambda.Compile();
            return Expression.Constant(fn.DynamicInvoke(null), e.Type);
        }
    }

    /// <summary>
    /// Performs bottom-up analysis to determine which nodes can possibly
    /// be part of an evaluated sub-tree.
    /// </summary>
    class Nominator : ExpressionVisitor
    {
        Func<Expression, bool> fnCanBeEvaluated;
        HashSet<Expression> candidates;
        bool cannotBeEvaluated;

        internal Nominator(Func<Expression, bool> fnCanBeEvaluated)
        {
            this.fnCanBeEvaluated = fnCanBeEvaluated;
        }

        internal HashSet<Expression> Nominate(Expression expression)
        {
            this.candidates = new HashSet<Expression>();
            this.Visit(expression);
            return this.candidates;
        }

        protected override Expression Visit(Expression expression)
        {
            if (expression != null)
            {
                bool saveCannotBeEvaluated = this.cannotBeEvaluated;
                this.cannotBeEvaluated = false;
                base.Visit(expression);
                if (!this.cannotBeEvaluated)
                {
                    if (this.fnCanBeEvaluated(expression))
                    {
                        this.candidates.Add(expression);
                    }
                    else
                    {
                        this.cannotBeEvaluated = true;
                    }
                }
                this.cannotBeEvaluated |= saveCannotBeEvaluated;
            }
            return expression;
        }
    }
}

Answer 2

我曾经遇到过这个问题，并通过为所有可能被记录的谓词创建一个枚举来解决它，在我的例子中，这些谓词并不多。如果您的情况不是这样，只需为每个谓词创建一个枚举条目和一个 Dictionary, YourEnum>，并记录枚举的值而不是谓词。