I am trying to compare two lists of objects in C# and return a new list that contains only the items that match certain criteria. Each list contains objects with the following structure:

public class MyObject
{
    public string Name { get; set; }
    public int Value { get; set; }
    public bool IsActive { get; set; }
}

I want to compare the Name and Value properties of each object in both lists, and return a new list of objects that meet the following criteria:

• The Name property of the object in list1 is equal to the Name property of the object in list2.
• The Value property of the object in list1 is greater than the Value property of the object in list2.

I have tried using LINQ's Where method and Join clauses, but I am having trouble getting the desired results. Here is the code I have so far:

var results = list1.Where(x => x.Name == list2.Where(y => y.Name).FirstOrDefault() &&
                               x.Value > list2.Where(y => y.Value).FirstOrDefault());

This code is only returning a list of the first object in list1 that meets the criteria, rather than a list of all matching objects. Can you help me understand what is going wrong and suggest a solution to get the desired results?

Thank you in advance for your help!

CodePudding user response：

How about performing a join to match the name between list1 and list2, then filtering for greater value with a Select?

void Main()
{
    var list1 = new List<MyObject> { 
        new MyObject { Name = "A", Value = 2 },
        new MyObject { Name = "A", Value = 1 },
        new MyObject { Name = "B", Value = 1 } 
    };

    var list2 = new List<MyObject> {
        new MyObject { Name = "A", Value = 2 },
        new MyObject { Name = "A", Value = 1 },
        new MyObject { Name = "C", Value = 1 }
    };

    var result = list1
        .Join(list2, l1 => l1.Name, l2 => l2.Name, (l1, l2) => new {l1, l2})
        .Where(x => x.l1.Value > x.l2.Value)
        .Select(x => x.l1)
        .ToList();

    result.ForEach(r => Console.WriteLine($"Name:{r.Name} Value:{r.Value} IsActive:{r.IsActive}"));
    
}

public class MyObject
{
    public string Name { get; set; }
    public int Value { get; set; }
    public bool IsActive { get; set; }
}

This returns only the item from list that has a match with list2 and where the value is greater than list2:

Name:A Value:2 IsActive:False