结合两个Linq表达式

Question

使用来自各种SO帖子的信息,特别是这个博客(更正使用AndAlso而不是And),我设法将类似的类型linq表达式组合成一个谓词.但现在我想结合两个表达式,其中一个是另一个的输入.这是完全展开的原创Expression;

    private Expression<Func<T, bool>> ExpressionIsNamed(IEnumerable<EntityName> AccessorNames)
    {
        // works
        Expression<Func<T, bool>> Texpr = x => x.Security.Readers.Any(n => AccessorNames.ToStringArray().Contains(n.Text));

        return Texpr;
    }

请注意,至关重要的是,我需要将它们作为表达式进行管理,因为我的数据库驱动程序需要遍历树并转换为本机调用,因此使用Compile()进行组合不是一种选择.

以下是我想要与Any()上面的调用结合起来的函数.最终输出Expression必须是类型Expression<Func<T, bool>>,我需要传递x.Security.Readers给这个.

    public static Expression<Func<IEnumerable<EntityName>,bool>> AccessCheckExpression(IEnumerable<EntityName> AccessorNames)
    {
        return accessList => accessList.Any(n => AccessorNames.ToStringArray().Contains(n.Text));
    }

我得尽可能这一点,但我在努力解决如何换出accessList =>,从accessCheck并将它用accessList在一个单一的表达.到目前为止,我有这个;

    private Expression<Func<T, bool>> ExpressionIsNamed(IEnumerable<EntityName> AccessorNames)
    {
        Expression<Func<T, IEnumerable<EntityName>>> accessList = (T x) => x.Security.Readers;
        Expression<Func<IEnumerable<EntityName>, bool>> accessCheck = SecurityDescriptor.AccessCheckExpression(AccessorNames);

        // Combine?
        Expression<Func<T, bool>> Texpr = ??? accessCheck + accessList ???

        return Texpr;
    }

[更新]

所以我还有一点点;

    class ParameterUpdateVisitor : System.Linq.Expressions.ExpressionVisitor
    {
        private ParameterExpression _oldParameter;
        private ParameterExpression _newParameter;

        public ParameterUpdateVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
        {
            _oldParameter = oldParameter;
            _newParameter = newParameter;
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            if (object.ReferenceEquals(node, _oldParameter))
                return _newParameter;

            return base.VisitParameter(node);
        }
    }

    static Expression<Func<T, bool>> UpdateParameter<T>(
        Expression<Func<T, IEnumerable<EntityName>>> expr,
        ParameterExpression newParameter)
    {
        var visitor = new ParameterUpdateVisitor(expr.Parameters[0], newParameter);
        var body = visitor.Visit(expr.Body);

        return Expression.Lambda<Func<T, bool>>(body, newParameter);
    }

然后我可以编译;

UpdateParameter(accessList, accessCheck.Parameters[0]);

感谢所有这些Invoke()建议,但我的猜测是,当他们到达MongoDB驱动程序时,它不会喜欢InvocationExpression.但是,Invoke我和上面的代码现在都以完全相同的方式失败.即;

System.ArgumentException: Expression of type 
'System.Func`2[MyLib.Project,System.Collections.Generic.IEnumerable`1[MyLib.EntityName]]' 
cannot be used for parameter of type 
                            'System.Collections.Generic.IEnumerable`1[MyLib.EntityName]'

因此看起来隐含参数x.Security.Readers与普通旧参数的类型不同IEnumerable<EntityName>

Answer 1

VisitorExpression是你的朋友.这是一个简化但完整的合并类似的例子:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

class Source {
    public List<Value> Values {get;set;}
}
class Value {
    public int FinalValue {get;set;}
}
static class Program {
    static void Main() {
        Expression<Func<Source, IEnumerable<Value>>> f1 =
            source => source.Values;
        Expression<Func<IEnumerable<Value>, bool>> f2 =
            vals => vals.Any(v => v.FinalValue == 3);

        // change the p0 from f2 => f1
        var body = SwapVisitor.Swap(f2.Body, f2.Parameters[0], f1.Body);
        var lambda = Expression.Lambda<Func<Source, bool>>(body,f1.Parameters);
        // which is:
        // source => source.Values.Any(v => (v.FinalValue == 3))
    }

}
class SwapVisitor : ExpressionVisitor {
    private readonly Expression from, to;
    private SwapVisitor(Expression from, Expression to) {
        this.from = from;
        this.to = to;
    }
    public static Expression Swap(Expression body,
        Expression from, Expression to)
    {
        return new SwapVisitor(from, to).Visit(body);
    }
    public override Expression Visit(Expression node)
    {
         return node == from ? to : base.Visit(node);
    }
}

编辑:在您的情况下,这将是:

private Expression<Func<T, bool>> ExpressionIsNamed(
    IEnumerable<EntityName> AccessorNames)
{
    Expression<Func<T, IEnumerable<EntityName>>> accessList =
        (T x) => x.Security.Readers;
    Expression<Func<IEnumerable<EntityName>, bool>> accessCheck =
        SecurityDescriptor.AccessCheckExpression(AccessorNames);


    var body = SwapVisitor.Swap(accessCheck.Body,
        accessCheck.Parameters[0], accessList.Body);
    return Expression.Lambda<Func<T, bool>>(body, accessList.Parameters);
}