强制.net表达式使用当前值

本文关键字：net 表达式 强制 | 更新日期: 2023-09-27 17:49:45

我想弄清楚是否有一种方法可以强制c#表达式将部分表达式转换为值。我正在调用一个接受定义查询的表达式的方法。在标识查询的对象中有一系列值。下面是一个简单的例子:

var identifier = new { id = 5 };
context.SomeMethod(i=>i.Id == identifier.Id);

这个失败。基于错误，我看到它看起来像表达式试图合并"标识符"。将Id"插入表达式，而不是解析"标识符。Id"的值，即5。下面的代码可以工作:

var id = 5;
context.SomeMethod(i=>i.Id == id)

虽然这可以工作，但这是一个简化的例子，在我的实际代码中，这将是痛苦的。所以我的问题是，是否有一些语法可以用来包装表达式的一部分，以强制它解析为一个值?

强制.net表达式使用当前值

本文讨论如何简化Expression。它采用两次传递的方法，首先将所有不是参数且不具有和参数的节点标记为子节点，然后进行另一次传递，对这些节点进行计算，以便在不依赖参数的情况下计算任何。

下面是一些小调整的代码:

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);
        }
        public 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;
        }
        public 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;
        }
    }
}

和一个额外的方法，以便它可以在IQueryable<T>而不是Expression上调用

class Query<T> : IQueryable<T>
{
    private IQueryProvider provider;
    private Expression expression;
    public Query(IQueryProvider provider, Expression expression)
    {
        this.provider = provider;
        this.expression = expression;
    }
    public IEnumerator<T> GetEnumerator()
    {
        return ((IEnumerable<T>)this.Provider.Execute(this.Expression)).GetEnumerator();
    }
    IEnumerator IEnumerable.GetEnumerator()
    {
        return ((IEnumerable)this.Provider.Execute(this.Expression)).GetEnumerator();
    }
    public Type ElementType
    {
        get { return typeof(T); }
    }
    public Expression Expression
    {
        get { return expression; }
    }
    public IQueryProvider Provider
    {
        get { return provider; }
    }
}
public static IQueryable<T> Simplify<T>(this IQueryable<T> query)
{
    return new Query<T>(query.Provider, Evaluator.PartialEval(query.Expression));
}

你现在可以写:

var identifier = new { id = 5 };
var query context.SomeMethod(i=>i.Id == identifier.Id)
    .Simplify();

并以一个有效的查询结束:

context.SomeMethod(i=>i.Id == 5)

c#不支持任何类型的匿名委托/表达式的显式捕获规则。

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