c# - 如何使用动态 LINQ 进行求和

Question

我有以下与动态 linq 库完美配合的：

string where = "Price < 5";
string orderby = "BookID ASC";
IQueryable<T> MyDataQueryable = _DataRawBase.AsQueryable<T>();
MyDataQueryable = MyDataQueryable.Where(where).OrderBy(orderby);

现在我想查询 MyDataQueryable 以对某些字段进行 SUM（也许是平均值）。

我该怎么办？

就像是：

double mysum = MyDataQueryable.Sum("Price");  

会好的...

Answer 1

由于一切都是stringly-typed，您可能想尝试：

var myDataQueryable = _DataRawBase.AsQueryable<T>()
    .Sum("Price");

使用以下扩展方法：

public static object Sum(this IQueryable source, string member)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (member == null) throw new ArgumentNullException(nameof(member));

    // The most common variant of Queryable.Sum() expects a lambda.
    // Since we just have a string to a property, we need to create a
    // lambda from the string in order to pass it to the sum method.

    // Lets create a ((TSource s) => s.Price ). First up, the parameter "s":
    ParameterExpression parameter = Expression.Parameter(source.ElementType, "s");

    // Followed by accessing the Price property of "s" (s.Price):
    PropertyInfo property = source.ElementType.GetProperty(member);
    MemberExpression getter = Expression.MakeMemberAccess(parameter, property);

    // And finally, we create a lambda from that. First specifying on what
    // to execute when the lambda is called, and finally the parameters of the lambda.
    Expression selector = Expression.Lambda(getter, parameter);

    // There are a lot of Queryable.Sum() overloads with different
    // return types  (double, int, decimal, double?, int?, etc...).
    // We're going to find one that matches the type of our property.
    MethodInfo sumMethod = typeof(Queryable).GetMethods().First(
        m => m.Name == "Sum"
             && m.ReturnType == property.PropertyType
             && m.IsGenericMethod);

    // Now that we have the correct method, we need to know how to call the method.
    // Note that the Queryable.Sum<TSource>(source, selector) has a generic type,
    // which we haven't resolved yet. Good thing is that we can use copy the one from
    // our initial source expression.
    var genericSumMethod = sumMethod.MakeGenericMethod(new[] { source.ElementType });

    // TSource, source and selector are now all resolved. We now know how to call
    // the sum-method. We're not going to call it here, we just express how we're going
    // call it.
    var callExpression = Expression.Call(
        null,
        genericSumMethod,
        new[] {source.Expression, Expression.Quote(selector)});

    // Pass it down to the query provider. This can be a simple LinqToObject-datasource,
    // but also a more complex datasource (such as LinqToSql). Anyway, it knows what to
    // do.
    return source.Provider.Execute(callExpression);
}

Answer 2

这段代码对我有用：

漂浮？fSum = qryQueryableData.Select("价格").Cast<float?>().Sum();

其中“价格”是“浮动”类型的列？

可在此处获得说明。

Answer 3

我还需要扩展 Caramiriel 的答案以处理字段：

    public static object Sum(this IQueryable source, string member)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (member == null) throw new ArgumentNullException("member");

        // Properties
        PropertyInfo property = source.ElementType.GetProperty(member);
        FieldInfo field = source.ElementType.GetField(member);

        ParameterExpression parameter = Expression.Parameter(source.ElementType, "s");
        Expression selector = Expression.Lambda(Expression.MakeMemberAccess(parameter, (MemberInfo)property ?? field), parameter);
        // We've tried to find an expression of the type Expression<Func<TSource, TAcc>>,
        // which is expressed as ( (TSource s) => s.Price );

        // Method
        MethodInfo sumMethod = typeof(Queryable).GetMethods().First(
            m => m.Name == "Sum"
                && (property != null || field != null)
                && (property == null || m.ReturnType == property.PropertyType) // should match the type of the property
                && (field == null || m.ReturnType == field.FieldType) // should match the type of the property
                && m.IsGenericMethod);

        return source.Provider.Execute(
            Expression.Call(
                null,
                sumMethod.MakeGenericMethod(new[] { source.ElementType }),
                new[] { source.Expression, Expression.Quote(selector) }));
    }

Answer 4

尝试使用 lambda 表达式：

double mysum = MyDataQueryable.Sum(mdq => mdq.Price);