Using LINQ to find duplicates across multiple properties

Question

Given a class with the following definition:

public class MyTestClass
{
    public int ValueA { get; set; }
    public int ValueB { get; set; }
}

How can duplicate values be found in a MyTestClass[] array?

For example,

MyTestClass[] items = new MyTestClass[3];
items[0] = new MyTestClass { ValueA = 1, ValueB = 1 };
items[1] = new MyTestClass { ValueA = 0, ValueB = 1 };
items[2] = new MyTestClass { ValueA = 1, ValueB = 1 };

Contains a duplicate as there are two MyTestClass objects where ValueA and ValueB both = 1

Answer 1

You can find your duplicates by grouping your elements by ValueA and ValueB. Do a count on them afterwards and you will find which ones are duplicates.

This is how you would isolate the dupes :

var duplicates = items.GroupBy(i => new {i.ValueA, i.ValueB})
  .Where(g => g.Count() > 1)
  .Select(g => g.Key);

Answer 2

You could just use Jon Skeet's DistinctBy and Except together to find duplicates. See this Response for his explanation of DistinctBy.

MyTestClass[] items = new MyTestClass[3];
items[0] = new MyTestClass { ValueA = 1, ValueB = 1 };
items[1] = new MyTestClass { ValueA = 0, ValueB = 1 };
items[2] = new MyTestClass { ValueA = 1, ValueB = 1 };

MyTestClass [] distinctItems = items.DistinctBy(p => new {p.ValueA, p.ValueB}).ToArray();
MyTestClass [] duplicates = items.Except(distinctItems).ToArray();

It will only return one item and not both duplicates however.

Answer 3

MyTestClass should implement the Equals method.

public bool Equals(MyTestClass x, MyTestClass y)
{
    if (Object.ReferenceEquals(x, y)) return true;

    if (Object.ReferenceEquals(x, null) ||
        Object.ReferenceEquals(y, null))
            return false;

        return x.ValueA == y.ValueA && y.ValueB == y.ValueB;
}

Here you have a good article about it.

After that you can get a "clean" list of MyTestClass with "Distinct" method.