This is the .NET API for reading MaxMind DB files. MaxMind DB is a binary file format that stores data indexed by IP address subnets (IPv4 or IPv6).
We recommend installing this library with NuGet. To do this, type the following into the Visual Studio Package Manager Console:
install-package MaxMind.Db
Note: For accessing MaxMind GeoIP2 databases, we generally recommend using the GeoIP2 .NET API rather than using this package directly.
To use the API, you must first create a Reader object. The constructor for the
reader object takes a string with the path to the MaxMind DB file. Optionally
you may pass a second parameter with a FileAccessMode enum with the value
MemoryMapped or Memory. The default mode is MemoryMapped, which maps the
file to virtual memory. This often provides performance comparable to loading
the file into real memory with the Memory mode while using significantly less
memory.
To look up an IP address, pass a System.Net.IPAddress object to the Find<T>
method on Reader. This method will return the result as type T. T may
either be a generic collection, a class using the [MaxMind.Db.Constructor]
attribute to declare which constructor to use during deserialization, or a class
with [MaxMind.Db.MapKey("name")]-annotated init properties for
property-based activation.
We recommend reusing the Reader object rather than creating a new one for each
lookup. The creation of this object is relatively expensive as it must read in
metadata for the file.
using (var reader = new Reader("GeoIP2-City.mmdb"))
{
var ip = IPAddress.Parse("24.24.24.24");
var data = reader.Find<Dictionary<string, object>>(ip);
...
}using MaxMind.Db;
using System.Net;
namespace MyCode
{
public class Asn
{
[Constructor]
public Asn(
// The MapKey attribute tells the reader to map the database
// key to the specified constructor parameter or property.
[MapKey("autonomous_system_number")] long? autonomousSystemNumber,
[MapKey("autonomous_system_organization")] string autonomousSystemOrganization,
// The Inject attribute allows you to inject arbitrary values
// when deserializing.
[Inject("ip_address")] IPAddress ipAddress,
// The Network attribute tells the reader to set the constructor
// parameter to be the network associated with the record in the
// database.
[Network] Network network)
{
...
}
...
}
public class Program
{
private static void Main(string[] args)
{
using (var reader = new Reader("GeoLite2-ASN.mmdb"))
{
var ip = IPAddress.Parse("24.24.24.24");
var injectables = new InjectableValues();
injectables.AddValue("ip_address", ip);
var data = reader.Find<Asn>(ip, injectables);
...
}
}
}
}As an alternative to constructor-based activation, you can use init
properties. This does not require a [Constructor]-annotated constructor.
using MaxMind.Db;
using System.Net;
namespace MyCode
{
public class Asn
{
[MapKey("autonomous_system_number")]
public long? AutonomousSystemNumber { get; init; }
[MapKey("autonomous_system_organization")]
public string? AutonomousSystemOrganization { get; init; }
[Inject("ip_address")]
public IPAddress? IpAddress { get; init; }
[Network]
public Network? Network { get; init; }
}
public class Program
{
private static void Main(string[] args)
{
using (var reader = new Reader("GeoLite2-ASN.mmdb"))
{
var ip = IPAddress.Parse("24.24.24.24");
var injectables = new InjectableValues();
injectables.AddValue("ip_address", ip);
var data = reader.Find<Asn>(ip, injectables);
...
}
}
}
}This API fully supports use in multi-threaded applications. In such
applications, we suggest creating one Reader object and sharing that among
threads.
The MaxMind DB format is an open format for quickly mapping IP addresses to records. See the specification for more information on the format.
Please report all issues with this code using the GitHub issue tracker.
If you are having an issue with a MaxMind database or service that is not specific to this reader, please contact MaxMind support.
Patches and pull requests are encouraged. Please include unit tests whenever possible.
The MaxMind DB Reader API uses Semantic Versioning.
This software is Copyright (c) 2013-2026 by MaxMind, Inc.
This is free software, licensed under the Apache License, Version 2.0.
