Linq条件选择带有空成员集合

Question

I have searched through a lot of similar questions, but cannot find one that solves this problem. Good chance I am overlooking something obvious, but please suggest how to do this with a single iteration.

var dataWithResults =
    from d in data
    where d.QCResults != null
    from q in d.QCResults
    select new QCAMaterial
    {
        Name = q == null ? nodata : q.ComponentName,
        Value = q == null ? nodata : q.Result
    };

var dataWithNullResults =
    from d in data
    where d.QCResults == null
    select new QCAMaterial
    {
        Name = nodata,
        Value = nodata
    };

Answer 1

Since both the classes without QCResults as well as QCResults with no specific data in their properties get the nodata value, you could omit the 'where' and combine them as:

    IEnumerable<QCResult> dummycol = new[] {new QCResult()}; //assuming QCResult is the class exposed by the QCResults property
    var res = from d in data                
            from q in d.QCResults ?? dummycol
            select new QCAMaterial
            {
                Name = q == null ? nodata: q.ComponentName, //side note: can also be written as q?.ComponentName ?? nodata
                Value = q == null ? nodata : q.Result
            };

If QCResults is empty, a dummy collection is used with a single entry. The dummy-collection could be created inline as well, but this should be more efficient.

Answer 2

You can do a combination of Enumerable.Empty and Enumerable.DefaultIfEmpty :

var allData =
            from d in data
            from q in d.QCResults ?? Enumerable.Empty<QCResult>().DefaultIfEmpty()
            select new QCAMaterial
            {
                Name = q == null ? nodata : q.ComponentName,
                Value = q == null ? nodata : q.Result
            };

In case you want to treat an empty list ( QCResults ) the same as a null list, you can modify the query:

var allData =
            from d in data
            from q in (d.QCResults ?? Enumerable.Empty<QCResult>()).DefaultIfEmpty()
            select new QCAMaterial
            {
                Name = q == null ? nodata : q.ComponentName,
                Value = q == null ? nodata : q.Result
            };

Answer 3

You can use a SelectMany:

    var result = data.SelectMany(d => 
                        {
                            return d.QCResults != null
                                ? d.QCResults.Select(q => new QCAMaterial { Name = q.ComponentName, Value = q.Result})
                                : new List<QCAMaterial>{new QCAMaterial { Name = "no data", Value = "no data" }};
                        });