从对象列表中选择匹配的ID

Question

Okay so i have a list filled with objects which have the following properties:

category_id and category_title

In the code it looks like this:

class CategoryModel
{
    public int category_id { get; private set; }
    public string category_title { get; private set; }

    //Constructor
    public CategoryModel(int id, string title)
    {
        category_id = id;
        category_title = title;
    }

I have multiple objects of this class stored in a list

    static List<CategoryModel> CategoryList = new List<CategoryModel>();

From this list i am trying to select the category_title where it matches a certain category_id

    public static string GetCategoryName(int categoryNumber)
    {
        IEnumerable<string> title = from c in CategoryList
                    where c.category_id.Equals(categoryNumber)
                    select c.category_title;
        return title.ToString();
    }

So let's say GetCategoryName(4), it's now supposed to select from the list where category_id equals 4 and return category_title

However it seems that i'm not getting the name back but a syntax of the query instead!

Output: system.linq.enumerable.whereselectlistiterator'2[project.CategoryModel,System.string]

Answer 1

Just use Linq to do it:

return CategoryList.Single(c => c.category_id == categoryNumber).category_title;

Single will throw an exception if the category list does not have any matching ID.

Answer 2

Your query returns list not a single category.

Please try this

public static string GetCategoryName(int categoryNumber)
{
    IEnumerable<string> title = from c in CategoryList
                where c.category_id.Equals(categoryNumber)
                select c.category_title;
    return title.FirstOrDefault();
}

Answer 3

The IEnumerable class does not implement some sort of fancy format of its elements in the ToString function. I think the simplest way to approach this would be to use the String.Join function if there are supposed to be multiple results:

return String.Join(",", title);

Answer 4

You should use Linq to select first or default value :

public static string GetCategoryName(int categoryNumber) {
    return CategoryList.FirstOrDefault( category => category.category_id == categoryNumber).category_title;
}

But this can throw exception when there's no such category id because of .category_title so the best solution would be :

public static string GetCategoryName(int categoryNumber) {
    CategoryModel category = CategoryList.FirstOrDefault( category => category.category_id == categoryNumber);
    return category == null ? string.Empty : category.category_title;
}

Which will return empty string when no such category was found.

About the error you're getting back. It is because your query returns "many" objects of CategoryModel ie. IEnumerable<string> which then you try to convert to string.

Doing ToString() on any kind of table ( array, list, etc. ) will return something like system.string[] or in other words same result as you would do call IEnumerable<string> .ToString() .

Answer 5

public static string GetCategoryName(int categoryNumber)
{
    CategoryModel result = CategoryList.Where(x => x.category_id == categoryNumber).FirstOrDefault();
    if (result != null)
        return result.category_name;
    else
        //no matches, do something else
}

This will take the item from CategoryList where the category_id matches the number you passed in, and return category_name if there was a match.

Answer 6

You're ToString'ing the Enumerable, which hasn't been evaluated, which is why you're getting the query implementation object.

Assuming you're after the only item that matches, do:

title.SingleOrDefault()?.ToString()

NOTE: the ? operator will only work on C# 6; you can just do a normal assignment and if x == null type check for below