聚合 Linq 表达式：获取从范围引用的变量的异常

Question

我有一个表达式列表，类型为：Expression<Func<Person, bool>>，我想聚合它们，然后将聚合结果编译成一个 Func<Person, bool>。我能够创建聚合表达式，但编译结果聚合表达式的部分抛出异常。任何帮助，将不胜感激。谢谢。

Expression<Func<Person, bool>> expr1 = x => x.Age > 10;
Expression<Func<Person, bool>> expr2 = x => x.LastName == "some firstname";
Expression<Func<Person, bool>> expr3 = x => x.FirstName == "some lastname";
Expression<Func<Person, bool>> expr4 = x => x.Initial == 'a';
Expression<Func<Person, bool>> expr5 = x => x.DateOfBirth == DateTime.Now;
Expression<Func<Person, bool>> expr6 = x => x.Height > 10;

var exprList = new List<Expression<Func<Person, bool>>>()
{
    expr1, expr2, expr3, expr4, expr5
};


var list = exprList
        .Select(x => x.Body)
        .Aggregate(Expression.AndAlso);

// this works, apparently?!
var aggregatedExpression = Expression.Lambda<Func<Person, bool>>(list, Expression.Parameter(typeof(Person), "x"));

// fails here! it cannot compile
var result = aggregatedExpression.Compile();

这是例外情况：

Unhandled Exception: System.InvalidOperationException: variable 'x' of type 'TestAggregateExpression.Person' referenced from scope '', but it is not defined

at System.Linq.Expressions.Compiler.VariableBinder.Reference(ParameterExpression node, VariableStorageKind storage)

Answer 1

我相信您需要访问列表中的所有表达式并替换参数。使用这个助手：

internal sealed class ParameterReplacer : ExpressionVisitor
{
    private readonly ParameterExpression _param;

    private ParameterReplacer(ParameterExpression param)
    {
        _param = param;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return node.Type == _param.Type ?
            base.VisitParameter(_param) : // replace
            node; // ignore
    }

    public static T Replace<T>(ParameterExpression param, T exp) where T : Expression
    {
        return (T)new ParameterReplacer(param).Visit(exp);
    }
}

用法：

var param = Expression.Parameter(typeof(Person), "x"); // I'd use 'p' by the way
exp = ParameterReplacer.Replace(param, exp);

你的情况：

var list = exprList.Select(x => x.Body)
                   .Select(exp => ParameterReplacer.Replace(param, exp))
                   .Aggregate(Expression.AndAlso);

聚合 Linq 表达式：获取从范围引用的变量的异常

Aggregate Linq Expressions: getting an exception of variable referenced from scope

c#

linq

expression-trees