具有通用参数的表达属性

Question

Here goes my first question in Stack Overflow. So, please bear with me and I hope I can point you right to my problem... (sorry if this is a repeat, but I searched a lot and I couldn't find an answer for the exact issue I'm having nor I could figure it out from other answers).

Let me show you the code first so you can get some context (spoiler: the issue I'm having is with keySelector ).

I have this method in a class:

public class BaseTableDataStore<T> : BaseTableDataStore, ITableDataStore<T> where T : Models.EntityData
{
    ...
    public virtual async Task<IEnumerable<T>> GetItemsQueryAsync<Tkey>(Expression<Func<T, bool>> predicate, Expression<Func<T, Tkey>> keySelector)
    {
        ...
        return await Table.Where(predicate).OrderBy(keySelector).ToEnumerableAsync();
        ...

    }
    ...
}

Then, I have this other class:

public class ConnectedObservableCollection<T> : ObservableRangeCollection<T> where T : EntityData
{
    ITableDataStore<T> table;
    public Expression<Func<T, bool>> Predicate { get; set; }
    public Expression<Func<T, long?>> KeySelector { get; set; }
    ...
    public async Task Refresh() 
    {
        ...
        _items = await table.GetItemsQueryAsync(Predicate, KeySelector);
        ...
    }
    ...
}

Finally, I have the following code in another class:

ConnectedObservableCollection<MeterEntry> meterEntries;
...
meterEntries.Predicate = (s => s.Type == 1);
meterEntries.KeySelector = (s => s.Value);

await MeterEntries.Refresh(wait);
...

That will of course work fine when s.Value is long? . But I would like to make this property generic so I can use any type of parameter instead of just long?

public Expression<Func<T, long?>> KeySelector { get; set; }

I hope it's clear enough...

Thanks!

Answer 1

If you want to change this:

public Expression<Func<T, long?>> KeySelector { get; set; }

to this (replace long with a second generic type)

public Expression<Func<T, T2>> KeySelector { get; set; }

then that second generic type must be included in your class definition.

Change

public class ConnectedObservableCollection<T> 
    : ObservableRangeCollection<T> where T : EntityData

to

public class ConnectedObservableCollection<T, T2> 
    : ObservableRangeCollection<T> where T : EntityData