Get the number of distinct property values in List using LINQ in C#

Question

In a list List<MyClass> ListOfMyClasses, how can one get the how many distinct GroupId property values there are using LINQ?

public class MyClass
{
     public int GroupId;
}

For example let's say we have this list:

ListOfMyClasses: {MyClass1 (GroupId = 1), MyClass2 (GroupId = 3), MyClass3 (GroupId = 1)}

Here we should get the result as 2 (Two distinct numbers for GroupId).

Look at [this](http://stackoverflow.com/questions/18565004/using-linq-to-group-a-list-of-objects) SO question. — Jeroen Heier, Jun 01 '16 at 20:32

score 16 · Accepted Answer · answered Jun 01 '16 at 20:30

16

Here is one way to do it using Distinct:

ListOfMyClasses.Select(t => t.GroupId).Distinct().Count()

Or you can also use GroupBy:

ListOfMyClasses.GroupBy(t => t.GroupId).Count()

answered Jun 01 '16 at 20:30

sstan

35,425
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score 4 · Answer 2 · answered Jun 01 '16 at 20:31

4

This should work for you.

var result = list.Select(x => x.GroupId).Distinct().Count();

First you are selecting out all the GroupIds. Then you are filtering them to be distinct. Finally you are getting the count of those values.

answered Jun 01 '16 at 20:31

Matt Rowland

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score 0 · Answer 3 · answered Jun 01 '16 at 23:15

In addition, just want to share some extension which I use in all my projects and it also solves your task

public class EqualityComparer<TEntity, TProperty>: IEqualityComparer<TEntity>
{
    private readonly Func<TEntity, TProperty> _property;

    public EqualityComparer(Func<TEntity, TProperty> property)
    {
        _property = property;
    }

    public bool Equals(TEntity x, TEntity y)
    {
        return _property(x).Equals(_property.Invoke(y));
    }

    public int GetHashCode(TEntity obj)
    {
        return _property(obj).GetHashCode();
    }
}

public static class Extensions
{
    public static IEnumerable<T> DistinctByProperty<T, TProp>(this IEnumerable<T> source, Func<T, TProp> propertyFunc)
    {
        return source.Distinct(new EqualityComparer<T, TProp>(propertyFunc));
    }
}

It allows you to write ListOfMyClasses.DistinctByProperty(x => x.GroupId).Count()

Get the number of distinct property values in List using LINQ in C#

3 Answers3