我正在尝试使用Linq表达式为IQueryable数据源创建动态where子句。我无法使TryParse函数在其中一个表达式中工作。这是我正在尝试做的事情:

IQueryable<trial_global> globalTrials = _trialsRepository.GlobalDataFiltered(filterId).AsQueryable();

BindingFlags bindingFlags = BindingFlags.Public | BindingFlags.Static;
MethodInfo tryParseMethod = typeof(double).GetMethod("TryParse", bindingFlags, null, new Type[] { typeof(string), typeof(double).MakeByRefType() }, null);
Expression tempN = Expression.Parameter(typeof(double), "tempN");
Expression left = Expression.Call(tryParseMethod, new[] { metricReference, tempN });

Expression right = Expression.Constant(true);
Expression predicateBody = Expression.Equal(left, right);

MethodCallExpression whereCallExpression = Expression.Call(
                            typeof(Queryable),
                            "Where",
                            new Type[] { globalTrials.ElementType },
                            globalTrials.Expression,
                            Expression.Lambda<Func<trial_global, bool>>(predicateBody, new ParameterExpression[] { pe })
                            );

var results = globalTrials.Provider.CreateQuery<trial_global>(whereCallExpression);

一切正常,直到分配了results,然后出现以下错误:
variable 'tempN' of type 'System.Double' referenced from scope '', but it is not defined

我在这里想念什么?我怀疑这与double.TryParse函数中的第二个参数是out参数有关。

更新:

我通过创建一个执行TryParse的静态函数并从Expression中调用此静态函数来解决了这个问题:
public static bool IsStringNumeric(string checkStr)
{
    double num = 0;
    return double.TryParse(checkStr, out num);
}

public IQueryable<trial_global> GetTrials(IQueryable<trial_global> globalTrials, Metric metric)
{
    ParameterExpression pe = Expression.Parameter(typeof(trial_global), "trial_global");
    MemberExpression metricReference = Expression.Property(pe, metric.metric_name);

    Expression predicateBody = Expression.Call(typeof(GlobalTrialsRepository).GetMethod("IsStringNumeric", new Type[] { typeof(string) }), metricReference);

    MethodCallExpression whereCallExpression = Expression.Call(
                typeof(Queryable),
                "Where",
                new Type[] { globalTrials.ElementType },
                globalTrials.Expression,
                Expression.Lambda<Func<trial_global, bool>>(predicateBody, new ParameterExpression[] { pe })
                );

    return globalTrials.Provider.CreateQuery<trial_global>(whereCallExpression);
}

这种方法行吗?有人看到这样做有什么缺点吗?

最佳答案

Tryparse使用out参数进行检查。使用tryparse创建扩展方法,然后从linq调用扩展方法

关于c# - 无法获得double.TryParse在Linq表达式树中工作,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/31350838/

10-15 21:22