为什么 LINQ 找不到具有包含特定子字符串的属性的项目？

Question

My application receive Messages list from server. Message object has 'Text' property. From this list I was trying to find first item where 'Text' contains specific substring. For example response could contains such elements in list(sequence saved):

Text = "mye:service?resp=out&text=some_text&id=854217";
Text = "mye:service?resp=inj&text=some_text&id=854219";
Text = "mye:service?resp=corr&text=some_text&id=854220";
Text = "mye:service?resp=out&text=some_text&id=854223";
Text = "mye:service?resp=inj&text=some_text&id=854227";
Text = "mye:service?resp=corr&text=test&id=854230";

Here is how I attempting to get the right item:

Message suitableMessage = messagesList.FirstOrDefault(m => m.Text.Contains("param") || m.Text.Contains("corr"));

It always returns first element, this one:

Text = "mye:service?resp=out&text=some_text&id=854217";

Viewing list items in the debugger, I see that there are suitable elements.

What am I doing wrong?

Answer 1

You are using .FirstOrDefault() (First or null if not found in your case), why do you excpeted to get something different than the first match?

If you want to get an array (or more accurately an IEnumerable ) of all matches, use .Where() instead.

Answer 2

If you run this example you will see that LinQ works as expected:

    static void Main(string[] args)
    {
        var messagesList = new List<Message>
        {
            new Message {Text = "mye:service?resp=out&text=some_text&id=854217"},
            new Message {Text = "mye:service?resp=inj&text=some_text&id=854219"},
            new Message {Text = "mye:service?resp=corr&text=some_text&id=854220"},
            new Message {Text = "mye:service?resp=out&text=some_text&id=854223"},
            new Message {Text = "mye:service?resp=inj&text=some_text&id=854227"},
            new Message {Text = "mye:service?resp=corr&text=test&id=854230"}
        };

        Message suitableMessage = messagesList.FirstOrDefault(m => m.Text.Contains("param") || m.Text.Contains("corr"));
        //result:
        //suitableMessage == "mye:service?resp=corr&text=some_text&id=854220"

        //verification
        if (suitableMessage.Text.Contains("param"))
        {
            Console.WriteLine("param");
        }

        if (suitableMessage.Text.Contains("corr"))
        {
            Console.WriteLine("corr");
        }

        //verification 2 - you can replace your LinQ with this code and debug
        Message suitableMessage2;
        string reason;
        string text; // just to make sure that value is not changed by some other thread ...
        foreach (var m in messagesList)
        {
            if (m.Text.Contains("param"))
            {
                suitableMessage2 = m;
                text = m.Text;
                reason = "param";
                break;
            }

            if (m.Text.Contains("corr"))
            {
                suitableMessage2 = m;
                text = m.Text;
                reason = "corr";
                break;
            }
        }

        // verification 3 - replace lambda with method if not sure how to add break point, write logs if cant debug runtime
        Message suitableMessage3 = messagesList.FirstOrDefault(DebugLinq);


        Console.ReadKey();
    }

    private static bool DebugLinq(Message m)
    {
        if (m.Text.Contains("param"))
        {
            // log Text, "param"
            return true;
        }

        if (m.Text.Contains("corr"))
        {
            // log Text, "corr"
            return true;
        }

        return false;
    }

Answer 3

Thanks to all involved. All because of my inattention, and incorrect names of extension methods. In one of the libraries used in the project there is an extension method for the list which is called "FirstOrDefult". This method, apparently, in my case always returns the first element. Including LINQ namespace did the trick, and now all works as expected