Generate a UUID (GUID) in C#

The standard way: Guid.NewGuid

Guid id = Guid.NewGuid();      // version 4, random
string s = id.ToString();      // "f47ac10b-58cc-4372-a567-0e02b2c3d479"

In .NET a UUID is called a Guid. It is the same 128 bit value in the same RFC layout. NewGuid uses the operating system's secure random source and produces a v4.

Time ordered v7 on .NET 9

Guid id = Guid.CreateVersion7();                      // now
Guid id = Guid.CreateVersion7(DateTimeOffset.UtcNow); // explicit timestamp

id.Version;   // 7, also new in .NET 9

On .NET 8 and earlier use the UUIDNext package: UuidToolkit.CreateUuidV7(). Use v7 for entity keys so inserts stay sequential, with one important exception covered below.

Format strings

id.ToString("D");   // f47ac10b-58cc-4372-a567-0e02b2c3d479   default, hyphens
id.ToString("N");   // f47ac10b58cc4372a5670e02b2c3d479       32 hex, no hyphens
id.ToString("B");   // {f47ac10b-58cc-4372-a567-0e02b2c3d479} braces
id.ToString("P");   // (f47ac10b-58cc-4372-a567-0e02b2c3d479) parentheses
id.ToString("D").ToUpperInvariant();                  // uppercase, the Windows habit

Parse and validate

if (Guid.TryParse(input, out Guid parsed))
{
    // valid, any of the D, N, B, P forms accepted, either case
}

Guid.TryParseExact(input, "D", out parsed);   // only the hyphenated form
Guid.Empty;                                    // 00000000-0000-0000-0000-000000000000, the nil UUID

The SQL Server trap

SQL Server's uniqueidentifier does not sort by the first bytes. It compares the last six bytes first, then works leftwards in groups. A v7 GUID, whose timestamp is in the first bytes, is therefore not sequential to SQL Server and a clustered index on it still fragments. Options:

• Use NEWSEQUENTIALID() as the column default and let SQL Server generate ids.
• Generate ids in .NET in SQL Server's byte order. UUIDNext has Database.SqlServer for exactly this.
• Use v7 as the key but make the clustered index something else.

On PostgreSQL and MySQL, v7 sorts as expected and none of this applies.

Entity Framework Core

public class Order
{
    public Guid Id { get; set; } = Guid.CreateVersion7();
}

// or let EF generate on add
modelBuilder.Entity<Order>().Property(o => o.Id).ValueGeneratedOnAdd();

Mistakes to avoid

• new Guid() is not a new id. It is Guid.Empty, all zeros. Use Guid.NewGuid().
• Guid.ToByteArray() is not RFC byte order. The first three groups are little endian. Use ToByteArray(bigEndian: true) on .NET 9+ when talking to other systems.
• Mixing cases in string comparisons. Compare Guid values, not strings.

Frequently asked questions

Is a GUID the same as a UUID?

Yes. GUID is the Microsoft name. The value, layout and versions are identical to RFC UUIDs, and Guid.NewGuid() produces a standard v4.

How do I generate a UUID v7 in C#?

Guid.CreateVersion7() on .NET 9 or newer. On older versions install UUIDNext and call UuidToolkit.CreateUuidV7().

Why does my sequential GUID still fragment the SQL Server index?

Because SQL Server sorts uniqueidentifier by its last bytes. Use NEWSEQUENTIALID or a generator that writes the timestamp where SQL Server sorts, such as UUIDNext's SQL Server mode.