How to Enable CORS in ASP.NET Core Web API for Cross-Domain Requests, Especially on Azure Free Plan?

Solving Cross-Domain Request Issues with CORS in ASP.NET Core Web API

Cross-Origin Resource Sharing (CORS) is crucial for web applications with separately hosted front-end and back-end components. This guide addresses a common CORS configuration problem in ASP.NET Core Web APIs, particularly when deploying to the Azure Free Plan.

The Challenge:

A client website hosted on a different domain experienced CORS restrictions when attempting to access a Web API. Standard Microsoft guidance didn't fully resolve the issue for all API endpoints.

The Solution: A Streamlined CORS Configuration

The solution involves a simplified CORS setup:

1. Install the Necessary Package: Begin by installing the CORS NuGet package:

   <code class="language-bash">Install-Package Microsoft.AspNetCore.Cors</code>

2. Configure CORS in Startup.cs: Add the CORS service within the ConfigureServices method:

   <code class="language-csharp">public void ConfigureServices(IServiceCollection services)
   {
       services.AddCors();
       services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
   }</code>

3. Enable CORS in the Configure Method: Use the UseCors middleware to enable CORS in the Configure method. This example allows requests from http://example.com:

   <code class="language-csharp">public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
   {
       app.UseCors(options => options.WithOrigins("http://example.com").AllowAnyMethod());
       app.UseMvc();
   }</code>

Important Considerations:

• For straightforward CORS policies (allowing specific origins), a custom policy isn't required.
• Replace "http://example.com" with your actual client domain.
• AllowAnyMethod() grants access to all HTTP methods. For enhanced security, specify allowed methods explicitly.
• Consult the official Microsoft documentation for advanced CORS configurations and more granular control.

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