使用 LINQ 在 List 中查找特定值

Question

I'm trying to use LINQ to find a specific string value "ConstantA" in a List.

Variable "stuff" is a type of:

List<KeyValuePair<string, string>> 

The first value in the List (StuffName) is the one that I'm looking for:

How do I use LINQ to find the correct StuffName = "ConstantA"?

This is my code below:

var listOfStuff = CalculateStuff();
foreach (var stuff in listOfStuff)
{
  if (stuff.Constants.FindAll("ConstantA") //**I’m having problem with LINQ here**
  {
   …
  }
}


private static List<Stuff> CalculateStuff()
{
…
}

public class Stuff : IHasIntInst
{
    public List<KeyValuePair<string, string>> Constants { get; set; }
}

Answer 1

With your current version of code:

var listOfStuff = CalculateStuff();
foreach (var stuff in listOfStuff)
{
    var items = stuff.Constants.FindAll((keyValuePair) => keyValuePair.Key == "ConstantA");
    if (items.Any())
    {
        //**I’m having problem with LINQ here**
    }
}

---

In case if you don't want items but only want to check condition, use LINQ Any method:

foreach (var stuff in listOfStuff)
{
    if (stuff.Constants.Any((keyValuePair) => keyValuePair.Key == "ConstantA"))
    {
        {
            //**I’m having problem with LINQ here**
        }
    }
}

---

In case if your Stuff class is defined using Dictionary :

public class Stuff
{
    public Dictionary<string, string> Constants { get; set; }
}

and usage:

var listOfStuff = CalculateStuff();
foreach (var stuff in listOfStuff)
{
    var items = stuff.Constants.Where((kvp) => kvp.Key == "ConstantA");

    if (items.Any())
    {
        //**I’m having problem with LINQ here**
    }
}

Note that with both cases usage is same, which means changing List<KeyValuePair<string, string>> to Dictionary<string, string> will not affect much code.

---

And finally a version I prefer the most )
The Stuff class would be:

public class Stuff
{
    public string StuffName { get; set; }
    public int StuffValue { get; set; }
}

Next, the calculate method would be:

private static List<Stuff> CalculateStuff()
{
    return new List<Stuff>()
    {
        new Stuff{StuffName = "ConstantA", StuffValue = 100},
        new Stuff{StuffName = "ConstantB",StuffValue = 200}

    };
}

And the usage:

var listOfStuff = CalculateStuff().Where(st => 
                                         st.StuffName == "ConstantA");

foreach (var stuff in listOfStuff)
{
    Console.WriteLine($"Name: {stuff.StuffName}, Value: {stuff.StuffValue}");
}

Answer 2

If you're trying to just check for at least one instance of ConstantA then just use 'Any()' like this:

if (stuff.Constants.Any(x => x.Key == "ConstantA")
{
    //do something....
}

Answer 3

Concerning the problem raised, I propose two different approaches both of them using Linq hoping that you will be useful.

1 - First Approache:

var result = listOfStuff.SelectMany(e => e.Constants.Select(d => d))
                        .Where(e=> e.Key == "ConstantA");

2 - Second Approache:

 var result = from item in listOfStuff.SelectMany(e => e.Constants)
             where item.Key =="ConstantA"
             select item ;