Register document names in a case-insensitive manner

Author: sander1095

src/OpenApi/src/Extensions/OpenApiEndpointRouteBuilderExtensions.cs

@@ -30,16 +30,22 @@ public static IEndpointConventionBuilder MapOpenApi(this IEndpointRouteBuilder e
         var options = endpoints.ServiceProvider.GetRequiredService<IOptionsMonitor<OpenApiOptions>>();
         return endpoints.MapGet(pattern, async (HttpContext context, string documentName = OpenApiConstants.DefaultDocumentName) =>
             {
+                // We need to retrieve the document name in a case-insensitive manner
+                // to support case-insensitive document name resolution.
+                // Keyed Services are case-sensitive by default, which doesn't work well for document names in ASP.NET Core
+                // as routing in ASP.NET Core is case-insensitive by default.
+                var lowercasedDocumentName = documentName.ToLowerInvariant();
+
                 // It would be ideal to use the `HttpResponseStreamWriter` to
                 // asynchronously write to the response stream here but Microsoft.OpenApi
                 // does not yet support async APIs on their writers.
                 // See https://github.com/microsoft/OpenAPI.NET/issues/421 for more info.
-                var documentService = context.RequestServices.GetKeyedService<OpenApiDocumentService>(documentName);
+                var documentService = context.RequestServices.GetKeyedService<OpenApiDocumentService>(lowercasedDocumentName);
                 if (documentService is null)
                 {
                     context.Response.StatusCode = StatusCodes.Status404NotFound;
                     context.Response.ContentType = "text/plain;charset=utf-8";
-                    await context.Response.WriteAsync($"No OpenAPI document with the name '{documentName}' was found.");
+                    await context.Response.WriteAsync($"No OpenAPI document with the name '{lowercasedDocumentName}' was found.");
                 }
                 else
                 {