Troubleshoot ASP.NET Core Localization

By Hisham Bin Ateya

This article provides instructions on how to diagnose ASP.NET Core app localization issues.

Localization configuration issues

Localization middleware order
The app may not localize because the localization middleware isn't ordered as expected.

To resolve this issue, ensure that localization middleware is registered before MVC middleware. Otherwise, the localization middleware isn't applied.

public void ConfigureServices(IServiceCollection services)
    services.AddLocalization(options => options.ResourcesPath = "Resources");


Localization resources path not found

Supported Cultures in RequestCultureProvider don't match with registered once

Resource file naming issues

ASP.NET Core has predefined rules and guidelines for localization resources file naming, which are described in detail here.

Missing resources

Common causes of resources not being found include:

  • Resource names are misspelled in either the resx file or the localizer request.
  • The resource is missing from the resx for some languages, but exists in others.
  • If you're still having trouble, check the localization log messages (which are at Debug log level) for more details about the missing resources.

Hint: When using CookieRequestCultureProvider, verify single quotes are not used with the cultures inside the localization cookie value. For example, c='en-UK'|uic='en-US' is an invalid cookie value, while c=en-UK|uic=en-US is a valid.

Resources & Class Libraries issues

ASP.NET Core by default provides a way to allow the class libraries to find their resource files via ResourceLocationAttribute.

Common issues with class libraries include:

  • Missing the ResourceLocationAttribute in a class library will prevent ResourceManagerStringLocalizerFactory from discovering the resources.
  • Resource file naming. For more information, see Resource file naming issues section.
  • Changing the root namespace of the class library. For more information, see Root Namespace issues section.

CustomRequestCultureProvider doesn't work as expected

The RequestLocalizationOptions class has three default providers:

  1. QueryStringRequestCultureProvider
  2. CookieRequestCultureProvider
  3. AcceptLanguageHeaderRequestCultureProvider

The CustomRequestCultureProvider allows you to customize how the localization culture is provided in your app. The CustomRequestCultureProvider is used when the default providers don't meet your requirements.

  • A common reason custom provider don't work properly is that it isn't the first provider in the RequestCultureProviders list. To resolve this issue:

  • Insert the custom provider at the position 0 in the RequestCultureProviders list as the following:

options.RequestCultureProviders.Insert(0, new CustomRequestCultureProvider(async context =>
        // My custom request culture logic
        return new ProviderCultureResult("en");
options.AddInitialRequestCultureProvider(new CustomRequestCultureProvider(async context =>
        // My custom request culture logic
        return new ProviderCultureResult("en");
  • Use AddInitialRequestCultureProvider extension method to set the custom provider as initial provider.

Root Namespace issues

When the root namespace of an assembly is different than the assembly name, localization doesn't work by default. To avoid this issue use RootNamespace, which is described in detail here


This can occur when a project's name is not a valid .NET identifier. For instance my-project-name.csproj will use the root namespace my_project_name and the assembly name my-project-name leading to this error.

Resources & Build Action

If you use resource files for localization, it's important that they have an appropriate build action. They should be Embedded Resource, otherwise the ResourceStringLocalizer is not able to find these resources.

GitHub issues with helpful problem solving tips