Using LINQ Concat to combine data

What you'll learn

In this article, let's learn about how to use Concat in LINQ in .NET.

Note: If you have not done so already, I recommend you read the article on Using LINQ Union to combine data.

When working with two collections, we can combine them using LINQ Concat() method. This will combine two collections and gives a single collection with duplicates.

Why it matters

LINQ Concat() is used to answer questions about collection such as

• Combining multiple data set from different sources for analysis
• Git merge and combine files with changes from both commits
• Append lines to files

Implementation

Using LINQ Concat to combine primitive types

Primitive data types like int, decimal, string, etc can just compare the values against other value in the collection

List<int> list = new() { 1, 2, 3, 4, 5 };
List<int> anotherList = new() { 4, 5, 6 };

//Method Syntax
List<int> result = list
                   .Concat(anotherList)
                   .ToList();
                    
//Query Syntax
List<int> result = (from number in list select number)
                   .Concat(anotherList)
                   .ToList();
        

Using LINQ Concat to combine with Equality Comparer

So, combining primitive data types with Concat() is easy and straight forward. The same goes with objects. There is no need for comparer as Concat() is going to simply combine without comparing.

public class ProductComparer : EqualityComparer<Product>
{
    public override bool Equals(Product? product, Product? anotherProduct)
    {
        return (product?.Id == anotherProduct?.Id &&
                product?.Name == anotherProduct?.Name &&
                product?.Price == anotherProduct?.Price &&
                product?.Color == anotherProduct?.Color &&
                product?.Size == anotherProduct?.Size);
    }

    public override int GetHashCode([DisallowNull] Product product)
    {
        return $"{product.Id}{product.Name}{product.Price}{product.Color}{product.Size}".GetHashCode();
    }
}
        

List<Product> products = GetProducts();
List<Product> anotherProducts = GetProducts();
ProductComparer productComparer = new();

//Method Syntax
List<Product> result = products
                       .Concat(anotherProducts, productComparer)
                       .ToList();
                    
//Query Syntax
List<Product> result = (from product in products select product)
                       .Concat(anotherProducts, productComparer)
                       .ToList();
        

Summary

In this article we learn't how to combine data between collections using Concat. This can be used to combine items between collection and return a unified collection with duplicates. All these can be used with any IEnumerable or IQueryable types.