LINQ按错误分组：“不包含'GroupBy'的定义”

Question

I have a List that I need to group by. In the list I add this object:

private class MyObject {

        private System.Drawing.Color color;
        private string title;            

        public MyObject(System.Drawing.Color color, string title)
        {
            this.color = color;
            this.title = title;
        }

        public string Title
        {
            get { return title; }
            set { title = value; }
        }

        public System.Drawing.Color Color
        {
            get { return color; }
            set { color = value; }
        }

}

I create my List as:

System.Collections.Generic.List<MyObject> mList = new       System.Collections.Generic.List<MyObject >();

Then I try to group the content:

var groupedList = mList.GroupBy(c => new {c.Title} ).ToList();

And that is when I get the error. Any ideas? How can I do this? Thanks!

Answer 1

Those kinds of errors typically mean you aren't including System.Linq in your using list. Try throwing this at the top of your code file:

using System.Linq;

Answer 2

您是否有对System.Linq命名空间的引用？

Answer 3

You need to add using System.Linq to the file, at the top or inside of the namespace.

GroupBy isn't actually a method of the List class, it's an extension method that is defined in System.Linq . When you add a using statement to that namespace it allows all extension methods in that namespace to be resolve to any types they extend for the file the using is in.

Some unrelated notes:

1. you probably also want to add a using System.Collections.Generic , as well as several other common namespaces to the top of that file, and likely most others. Typing out System.Collections.Generic.List<MyObject> mList = new System.Collections.Generic.List<MyObject >(); just makes me cringe. Once you add that using you'll only need: List<MyObject> mList = new List<MyObjecT>(); .
2. You probably don't want to create a new anonymous class for your grouping. It will work, but it'll make using the results that much more annoying without adding anything useful. Just do: mList.GroupBy(c => c.Title ) if you only want to group on the Title .