使用Linq进行反对，我如何才能基于同一列表中的另一个字段来获取一个字段的值

Question

I have a class that is defined as following:

public class MyClass
{
    public int ID { get; set; }
    public string Name { get; set; }
}

Then I have a logic where I build a list of that class:

MyList = new List<MyClass>;
foreach(MyClassData myClass in Service.MyClassList)
{
    MyList.Add(new MyClass{ID = myClass.ID, Name = myClass.Name});
}

Now, I need to get an 'ID' based on the 'Name'.

I'm trying the following:

int id01 = MyList.Where(x => x.FileName.Equals("File01")).Single(x => x.FileID)

Doing that I'm getting an error message:

Cannot implicitly convert type 'int' to 'bool'

How would I do that using LINQ?

Answer 1

The Single method's is:

 public static TSource Single<TSource>( this IEnumerable<TSource> source, Func<TSource, bool> predicate ) 

It gets a predicate that should return a boolean but you are returning an integer. It is used for filtering and not projecting.

What you are looking for is:

int id01 = MyList.Single(x => x.FileName.Equals("File01")).FileID;

Or better use SingleOrDefault :

int id01 = MyList.SingleOrDefault(x => x.FileName.Equals("File01"))?.FileID;

Also see if you want the SingleOrDefault or the FirstOrDefault