Sequence Contains No Matching Element

Sequence contains no matching element

Well, I'd expect it's this line that's throwing the exception:

var documentRow = _dsACL.Documents.First(o => o.ID == id)

First() will throw an exception if it can't find any matching elements. Given that you're testing for null immediately afterwards, it sounds like you want FirstOrDefault(), which returns the default value for the element type (which is null for reference types) if no matching items are found:

var documentRow = _dsACL.Documents.FirstOrDefault(o => o.ID == id)

Other options to consider in some situations are Single() (when you believe there's exactly one matching element) and SingleOrDefault() (when you believe there's exactly one or zero matching elements). I suspect that FirstOrDefault is the best option in this particular case, but it's worth knowing about the others anyway.

On the other hand, it looks like you might actually be better off with a join here in the first place. If you didn't care that it would do all matches (rather than just the first) you could use:

var query = from target in _lstAcl.Documents
            join source in _dsAcl.Document
            where source.ID.ToString() equals target.ID
            select new { source, target };
foreach (var pair in query)
{
    target.Read = source.Read;
    target.ReadRule = source.ReadRule;
    // etc
}

That's simpler and more efficient IMO.

Even if you do decide to keep the loop, I have a couple of suggestions:

• Get rid of the outer if. You don't need it, as if Count is zero the for loop body will never execute

• Use exclusive upper bounds in for loops - they're more idiomatic in C#:

  for (i = 0; i < _lstAcl.Documents.Count; i++)
  

• Eliminate common subexpressions:

  var target = _lstAcl.Documents[i];
  // Now use target for the rest of the loop body
  

• Where possible use foreach instead of for to start with:

  foreach (var target in _lstAcl.Documents)
  

System.InvalidOperationException: 'Sequence contains no matching element'

Try using FirstOrDefault and LastOrDefault instead of First and Last, these methods will return the default value of the type they are invoked for if no elements match the lambda expression you provide as a parameter.

In your project, You just use filter.MainFilter.First(xxxx) to select the data, So if no elements match the lambda expression you provide as a parameter,First() will throw an exception, So here will report this error.

Sequence contains no matching element error in Linq Statement

var listOfWorkWeeks = daysInMonth
        .Select(day => new DateTime(year, month, day))
        .GroupBy(d => cal.GetWeekOfYear(d, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday))
        .Select(g => Tuple.Create(g.Key, g.FirstOrDefault(), g.LastOrDefault(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday)))
        .ToList();

Try using FirstOrDefault and LastOrDefault instead of First and Last, these methods will return the default value of the type they are invoked for if no elements match the lambda expression you provide as a parameter.

In case of g.FirstOrDefault(), the default value will be returned if g is empty and in case of g.LastOrDefault(d => d.DayOfWeek != DayOfWeek.Saturday && d.DayOfWeek != DayOfWeek.Sunday) the default value will be returned if all days are either saturday or sunday.

Sequence contains no matching element error when returning class Type of Interface using IsAssignableFrom

I wasn't able to see this in my answer on your original question. The assembly in which the "client" resides and the assembly in which the page models reside are different. That means the PageModelFactory will need a second constructor parameter to know which assembly to search when initializing new page models:

public class PageModelFactory
{

    private readonly object client;

    private Assembly Assembly => GetType().Assembly;

    public PageModelFactory(object client)
    {
        this.client = client;
    }

    // Create Page Objects
    public ILogin CreateLoginPage()
    {
        var pageModelTypes = GetPageModelTypes<ILogin>();
        var constructorSignature = new Type[] { client.GetType() };

        foreach (var type in pageModelTypes)
        {
            var constructor = type.GetConstructor(constructorSignature);

            if (constructor != null)
                return (ILogin)constructor.Invoke(new object[] { client });
        }

        throw new InvalidOperationException($"No class found implementing ILogin with a constructor that accepts {client.GetType().FullName} as an argument in assembly {Assembly.Name}");
    }

    private IEnumerable<Type> GetPageModelTypes<TPageModelInterface>()
    {
        return Assembly.GetTypes()
                       .Where(type => type.IsClass
                                   && typeof(TPageModelInterface).IsAssignableFrom(type));
    }

}

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