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Part 26 - Set operators in LINQ

The following operators belong to Set operators category
Distinct
Union
Intersect
Except

In this article we will discuss Distinct operator. This operator returns distinct elements from a given collection.

Example 1: Return distinct country names. In this example the default comparer is being used and the comparison is case-sensitive, so in the output we see country USA 2 times. 


string[] countries = { "USA", "usa", "INDIA", "UK", "UK" };
var result = countries.Distinct();
foreach (var v in result)
{
    Console.WriteLine(v);
}

Output:
distinct in linq

Example 2: For the comparison to be case-insensitive, use the other overloaded version of Distinct() method to which we can pass a class that implements IEqualityComparer as an argument. In this case we see country USA only once in the output.

string[] countries = { "USA", "usa", "INDIA", "UK", "UK" };
var result = countries.Distinct(StringComparer.OrdinalIgnoreCase);
foreach (var v in result)
{
    Console.WriteLine(v);
}

Output:
distinct in linq example

When comparing elements, Distinct() works in a slightly different manner with complex types like Employee, Customer etc. 

Example 3: Notice that in the output we don't get unique employees. This is because, the default comparer is being used which will just check for object references being equal and not the individual property values.

List<Employee> list = new List<Employee>()
{
    new Employee { ID = 101, Name = "Mike"},
    new Employee { ID = 101, Name = "Mike"},
    new Employee { ID = 102, Name = "Mary"}
};
var result = list.Distinct();
foreach (var v in result)
{
    Console.WriteLine(v.ID + "\t" + v.Name);
}

Output:
linq distinct c# example

To solve the problem in Example 3, there are 3 ways
1. Use the other overloaded version of Distinct() method to which we can pass a custom class that implements IEqualityComparer
2. Override Equals() and GetHashCode() methods in Employee class
3. Project the properties into a new anonymous type, which overrides Equals() and GetHashCode() methods

Example 4 : Using the overloaded version of Distinct() method to which we can pass a custom class that implements IEqualityComparer

Step 1 : Create a custom class that implements IEqualityComparer<T> and implement Equals() and GetHashCode() methods

 public class EmployeeComparer : IEqualityComparer<Employee>
{
    public bool Equals(Employee x, Employee y)
    {
        return x.ID == y.ID && x.Name == y.Name;
    }
    public int GetHashCode(Employee obj)
    {
        return obj.ID.GetHashCode() ^ obj.Name.GetHashCode();
    }
}

Step 2 : Pass an instance of EmployeeComparer as an argument to Distinct() method

 List<Employee> list = new List<Employee>()
{
    new Employee { ID = 101, Name = "Mike"},
    new Employee { ID = 101, Name = "Mike"},
    new Employee { ID = 102, Name = "Mary"}
};
var result = list.Distinct(new EmployeeComparer());
foreach (var v in result)
{
    Console.WriteLine(v.ID + "\t" + v.Name);
}

Output:
iequalitycomparer example

Example 5 : Override Equals() and GetHashCode() methods in Employee class

public class Employee
{
    public int ID { get; set; }
    public string Name { get; set; }
    public override bool Equals(object obj)
    {
        return this.ID == ((Employee)obj).ID && this.Name == ((Employee)obj).Name;
    }
    public override int GetHashCode()
    {
        return this.ID.GetHashCode() ^ this.Name.GetHashCode();
    }
}

Example 6 : Project the properties into a new anonymous type, which overrides Equals() and GetHashCode() methods

List<Employee> list = new List<Employee>()
{
    new Employee { ID = 101, Name = "Mike"},
    new Employee { ID = 101, Name = "Mike"},
    new Employee { ID = 102, Name = "Mary"}
};
var result = list.Select(x => new { x.ID, x.Name }).Distinct();
foreach (var v in result)
{
    Console.WriteLine(" " + v.ID + "\t" + v.Name);
}