Trying to understand my IEnumerable and why I can't use .ToList()

John DK 0
Viewed 63

I currently have the following method for one of my data objects repo patterns:

public async Task<IEnumerable<DropDownList>> GetDropDownListNoTracking()
{
    return await context.TouchTypes
       .AsNoTracking()
       .Where(s => s.IsActive)
       .Select(s => new DropDownList()
       {
           Id = s.Id,
           Name = s.Description
       }).ToListAsync();
}

When I call it in my page view :

private IList<DropDownList> TouchTypeList { get; set; }   
private async Task LoadDropDownAsync()
{  
    TouchTypeList = await _unitOfWork.TouchType.GetDropDownListNoTracking();
}

I'm trying to understand why I can't just do a GetDropDownListNoTracking().ToList() but instead, it wants me to cast : (IList<DropDownList>).

I can easily just change the property to fix this but I would assume .ToList would work here?

I'm mostly just trying to understand this so I can do it the correct way.

.net-core linq async-await .net c#
2 Answers

GetDropDownListNoTracking returns Task<IEnumerable<DropDownList>>, not IEnumerable<DropDownList>, so you'd have to do:

private async Task LoadDropDownAsync()
{  
    TouchTypeList = (await _unitOfWork.TouchType.GetDropDownListNoTracking()).ToList();
}
codeMonkey0

Calling ToList on an enumerable that you know is really always a list is a waste of CPU cycles, and points to a poor choice of return type for your method. The simplest solution is to change your method to:

public async Task<IList<DropDownList>> GetDropDownListNoTracking()
{
    return await context.TouchTypes
       .AsNoTracking()
       .Where(s => s.IsActive)
       .Select(s => new DropDownList()
       {
           Id = s.Id,
           Name = s.Description
       }).ToListAsync();
}

Although I personally would use IReadOnlyList instead.

mjwills0
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