使用字符串的嵌套属性动态构建linq groupby表达式

Question

I use nhibernate mapping by code, I want to make this expression dynamicllay (with a nested object)

I have a class event that has a relation many to one with Event state/and I want to grouping by code in the table EventState

var grouping = query.GroupBy(x => x.EventState.Code)

It works for me with a simple property, here is my code:

var arg = Expression.Parameter(type, categoryColumnName);
var bodyy = Expression.Convert(Expression.Property(arg, categoryColumnName), typeof (object));
var lambdaGroupBy = Expression.Lambda<Func<Operation, object>>(bodyy, arg);

var keySelector = lambdaGroupBy.Compile();
var grouping = query.GroupBy(keySelector);
return grouping.Select(a => new PieChartObject { Category = a.Key.ToString(), Value = a.Count().ToString() }).ToList();

But I can't do it with nested object.

Answer 1

GroupBy will partition your query by what you provide as key selector. To determine whether two items in your query have the same key, it uses the default comparer of the given type. For object, this is uses the Equals and GetHashCode methods which in turn for strings mean that the contents of the strings are identical. If you use a class, by default the reference identity is used, so I think that GroupBy isn't doing anything in your case because the keys you provided are not identical, even though they may have the same values.

So there are two valid solutions: You can either override Equals and GetHashCode in your nested object class, or you can provide a custom key comparer to GroupBy, if you want this behavior only for this particular query. But I guess, as you want to be generic, implementing Equals and GetHashCode would be a better option. The only exception is of course when you cannot do this, eg because it is a compiler-generated class. In that case, there is few things you can do about that.