Request.Headers returns Microsoft.AspNetCore.Http.IHeaderDictionary interface that define next property:
StringValues this[string key] { get; set; }
IHeaderDictionary has a different indexer contract than IDictionary, where it will return StringValues.Empty for missing entries.
Return type: Microsoft.Extensions.Primitives.StringValues
Returns: The stored value, or StringValues.Empty if the key is not present.
So, you can simply use Request.Headers["environment"] to get value of “environment” header