LeftJoin in .NET 10 & EF Core 10 - A Game Changer for LINQ Developers

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.

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.

It works just like SQL’s LEFT JOIN:
- Keep all records from the left source.
- Match with the right source records when available.
- Fill with null when no match exists.

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

Where Can You Use It?

You’ll use LeftJoin in any scenario where:

You’re working with relational data (e.g., EF Core entities).
You want to preserve data from one side, even if no matches exist on the other.
Common cases include:
- Customers & Orders
- Employees & Departments
- Products & Discounts

When Was It Introduced?

Introduced in: .NET 10 preview & EF Core 10 preview (2025).
Final release expected: November 2025.
This means if you’re testing .NET 10 preview SDK, you can already try it.

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]"
        }
    );

Much cleaner
Easier to maintain
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

.NET 10 SDK/runtime (preview or later).
Entity Framework Core 10 (preview or later).
A relational database provider that supports SQL joins (SQL Server, PostgreSQL, etc.).
Only method syntax (.LeftJoin()) is supported — not query expression syntax (from … in … join).

Limitations

Currently available only in preview (things may change before final release).
Query syntax (from c in Customers …) does not support left join yet.
Need to handle null values explicitly (e.g., dept?.Name ?? "[NONE]").
Requires upgrading to .NET 10 — not usable in earlier versions.

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:

Cleaner syntax
SQL-like readability
Less error-prone queries

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

What’s New in EF Core 10 — Microsoft Learn
This describes the addition of LeftJoin and RightJoin among other EF Core 10 features. What’s New in EF Core 10

What is LeftJoin in .NET 10?