c# - 编写基本的表达式生成器

Question

我正在使用 Linq-to-Entities，一个查询需要动态加载一个属性。查询如下所示:

var found = Context.IntegrateViews.GroupBy(x => x.TypeOfBed)
                   .Select(type => new TypeCounts
                            { Name = type.Key, Count = type.Count() }).ToList();

运行 Group By 子句的属性是 TypeOfBed。现在我想在不同的属性上运行此查询，例如，下次我想运行它时，假设是 x.TypeOfChair 等等。为此，我需要在其中动态设置此属性。

我正在尝试编写一个像这样的表达式构建器。

public Expression<Func<LookupFilterItem>> PropertyLambda(string propertyName)
{
    var param = Expression.Parameter(typeof(LookupFilterItem), "lookupItem");
    //type of x in above example is LookupFilterItem
    var propertyExpression = Expression.Property(param, propertyName);

    var returnExpr = Expression.Lambda<Func<LookupFilterItem>>(propertyExpression);
    return returnExpr;
    //If I Pass string "TypeOfBed" to this method, I am expecting it to return something like
    // lookupItem => lookupItem.TypeOfBed
}

我打算这样使用它:

var found = Context.IntegrateViews.GroupBy(PropertyLambda("TypeOfBed"))
                   .Select(type => new TypeCounts
                            { Name = type.Key, Count = type.Count() }).ToList();

但这是行不通的，我收到一个编译时错误:

error CS0411: The type arguments for method 'System.Linq.Queryable.GroupBy(System.Linq.IQueryable, System.Linq.Expressions.Expression>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

我在这里做错了什么？

Answer 1

这似乎是一个语法问题。试试这个:

public Expression<Func<LookupFilterItem, string>> PropertyLambda(string propertyName)
    {
        var param = Expression.Parameter(typeof(LookupFilterItem), "lookupItem");
        var propertyExpression = Expression.Property(param, propertyName);
        var returnExpr = Expression.Lambda<Func<LookupFilterItem, string>>(propertyExpression, param);
        return returnExpr;
    }

以及用法:

var found = Context.IntegrateViews.GroupBy(PropertyLambda("TypeOfBed").Compile())
               .Select(type => new TypeCounts
                        { Name = type.Key, Count = type.Count() }).ToList();