具有动态构建 linq 谓词的扩展方法

Question

我有这个扩展方法。

        public static IQueryable<T> SearchBy<T>(this IQueryable<T> source, SearchCriteria criteria)
    {
        //throw an error if the source is null
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }

        ParameterExpression parameter = Expression.Parameter(source.ElementType, string.Empty);
        var property = Expression.Equal(Expression.PropertyOrField(parameter, criteria.Property), Expression.Constant(criteria.PropertyValue));
        LambdaExpression lambda = Expression.Lambda(property, parameter);

        Expression methodCallExpression = Expression.Call(typeof(Queryable), "SelectMany",
                                            new Type[] { source.ElementType, property.Type },
                                            source.Expression, Expression.Quote(lambda));

        return source.Provider.CreateQuery<T>(methodCallExpression);
    }

出现此错误，

 No method 'SelectMany' on type 'System.Linq.Queryable' is compatible with the         supplied arguments.
    Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: No method 'SelectMany' on type 'System.Linq.Queryable' is compatible with the supplied arguments.

Source Error:

Line 67:             LambdaExpression lambda = Expression.Lambda(property, parameter);
Line 68: 
Line 69:             Expression methodCallExpression = Expression.Call(typeof(Queryable), "SelectMany",
Line 70:                                                 new Type[] { source.ElementType, property.Type },
Line 71:                                                 source.Expression, Expression.Quote(lambda));

当我从实体框架调用此方法时

this.grdReservations.DataSource = dataContext.CustomerSet.SearchBy(crit);

就像这样，这就是 SearchCriteria

        SearchCriteria crit = new SearchCriteria();
        crit.Property = "UserName";
        crit.PropertyValue = "new_user";
        crit.Search = SearchType.Equal;

如果有人可以看一下并给我一个正确的方向推动，我会非常高兴。

感谢您抽出时间。

编辑：我在家，所以我无法测试，但我尝试的任何方法（“Select”、“Where”、“SelectMany”）都返回了此错误，所以我假设我做错了其他事情。

Answer 1

您想调用 SelectMany 还是只调用 Select？

SelectMany 期望 lambda 返回 IEnumerable[TResult] 而不是 TResult。

Answer 2

 this works

public static IQueryable<T> SearchBy<T>(this IQueryable<T> source, SearchCriteria criteria)
    {
        //throw an error if the source is null
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }

        ParameterExpression parameter = Expression.Parameter(source.ElementType, string.Empty);
        BinaryExpression property = Expression.Equal(Expression.PropertyOrField(parameter, criteria.Property), Expression.Constant(criteria.PropertyValue));
        LambdaExpression lambda = Expression.Lambda(property, parameter);

        Expression methodCallExpression = Expression.Call(typeof(Queryable), "Where",
                                            new Type[] { source.ElementType },
                                            source.Expression, Expression.Quote(lambda));

        return source.Provider.CreateQuery<T>(methodCallExpression);
    }

我需要将 Expression.Call 类型数组更改为此

new Type[] { source.ElementType }

此示例 有所帮助非常。

谢谢你们的宝贵时间

Answer 3

lambda 似乎采用 T 类型的参数值并返回 bool。看起来它应该返回一个 IEnumerable 。

或者，如果将“SelectMany”更改为“Select”，则 lambda 需要返回 T 类型的值。