If you’ve ever struggled with verbose LINQ queries when trying to write a LEFT JOIN, you’re not alone. For years, developers had to hack together GroupJoin + SelectMany + DefaultIfEmpty() just to mimic a SQL LEFT JOIN.

But now, with .NET 10 and EF Core 10, Microsoft is finally giving us something we’ve all been waiting for:
👉 First-class LeftJoin and RightJoin LINQ methods.

LINQ_LeftJoin_Method

Let’s dive into what they are, why they matter, how to use them, their requirements, limitations, and real-life examples.

What is LeftJoin in .NET 10?

The new LeftJoin operator in LINQ allows you to directly perform an outer join between two collections or EF Core entities.

No more workarounds. No more boilerplate. Just clean, readable code.

Where Can You Use It?

You’ll use LeftJoin in any scenario where:

When Was It Introduced?

Why Is It Important?

Before .NET 10, a Left Join in LINQ looked like this:

var query = context.Students
    .GroupJoin(
        context.Departments,
        s => s.DepartmentID,
        d => d.ID,
        (s, deptGroup) => new { s, deptGroup }
    )
    .SelectMany(
        x => x.deptGroup.DefaultIfEmpty(),
        (x, dept) => new {
            StudentName = x.s.FirstName + " " + x.s.LastName,
            DepartmentName = dept != null ? dept.Name : "[NONE]"
        }
    );

Messy, right? 👎

Now, with .NET 10’s LeftJoin, it becomes:

var query = context.Students
    .LeftJoin(
        context.Departments,
        student => student.DepartmentID,
        dept => dept.ID,
        (student, dept) => new {
            StudentName = student.FirstName + " " + student.LastName,
            DepartmentName = dept?.Name ?? "[NONE]"
        }
    );

  1. Much cleaner

  2. Easier to maintain

  3. Matches SQL more naturally

How to Use LeftJoin in EF Core 10

Example: Customers & Orders

Entities

public class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Order
{
    public int Id { get; set; }
    public int CustomerId { get; set; }
    public DateTime OrderDate { get; set; }
}

Query with LeftJoin

var query = context.Customers
    .LeftJoin(
        context.Orders,
        c => c.Id,
        o => o.CustomerId,
        (c, o) => new {
            CustomerName = c.Name,
            OrderId = o?.Id ?? 0,
            OrderDate = o?.OrderDate
        }
    );

foreach (var item in query)
{
    Console.WriteLine(
        $"Customer: {item.CustomerName}, OrderId: {item.OrderId}, Date: {item.OrderDate}"
    );
}

This will list all customers, even those who never placed an order.

Requirements

Limitations

Real-Life Example

Imagine building a customer dashboard where you must show all customers — even those without orders.

Without LeftJoin, your LINQ would be cluttered with GroupJoin + DefaultIfEmpty.
With LeftJoin in .NET 10, it’s a single, readable method call.

SQL Equivalent

SELECT c.Name, o.Id AS OrderId, o.OrderDate
FROM Customers c
LEFT JOIN Orders o ON c.Id = o.CustomerId;

C# Equivalent with .NET 10

var dashboardData = context.Customers
    .LeftJoin(
        context.Orders,
        c => c.Id,
        o => o.CustomerId,
        (c, o) => new {
            c.Name,
            OrderId = o?.Id,
            o?.OrderDate
        }
    );

This generates the exact same SQL under the hood.

Conclusion

For over 20 years, LINQ developers have been writing verbose workarounds for LEFT JOIN. Now, with .NET 10 and EF Core 10, we finally have:

The LeftJoin (and RightJoinMethods are small additions — but huge wins for productivity.

If you’re adopting .NET 10, this is one of the must-try features that will simplify your data queries instantly.

For More Details, Read the Official Microsoft Docs