HttpHeader 在 key 不存在的时候会返回什么呢？

背景

这篇文章主要是为了说明 ChatGPT 使用有风险 😅，还是得实践出真知。👊

我们有一段 code 如下，而我想知道在 headerKey 不存在的情况下，会返回 null 还是 empty string 呢。

public string GetHttpRequestHeader(string headerKey)
{
    return this.HttpContext?.Request.Headers[headerKey];
}

在想尝试使用优秀的 ChatGPT 的情况下（才不是懒 🥺），我直接跟 ChatGPT 进行了如下对话。

Q: When using Microsoft.AspNetCore.Http.HttpRequest.Headers, what will return if header key doesn't exist

ChatGPT: When using the Microsoft.AspNetCore.Http.HttpRequest.Headers property to access the headers of an HTTP request in .NET Core, attempting to access a header key that doesn't exist will return an empty string ("").

For example, let's say you want to retrieve the value of a header named X-Custom-Header from an HTTP request using the Headers property. You can use the following code:

string customHeaderValue = httpRequest.Headers["X-Custom-Header"];

If the X-Custom-Header key exists in the Headers collection, customHeaderValue will contain its value. However, if the X-Custom-Header key doesn't exist, customHeaderValue will be an empty string (""). It's important to check for empty strings when working with header values to avoid errors caused by null or undefined values.

Q: will it be null?

ChatGPT: No, when using the Microsoft.AspNetCore.Http.HttpRequest.Headers property to access the headers of an HTTP request in .NET Core, attempting to access a header key that doesn't exist will not return null. Instead, it will return an empty string ("").

In .NET Core, accessing a non-existent key in a dictionary-like object (such as the Headers property of the HttpRequest class) will not throw a KeyNotFoundException but will instead return the default value for the type of the values in the dictionary. For strings, the default value is an empty string.

Therefore, if you attempt to retrieve the value of a non-existent header key using Headers[key], it will not be null, but rather an empty string. You should keep this in mind when checking for the existence of headers or parsing header values.

Q: could you share the authority doc for this one?

ChatGPT: Sure! Here is the official documentation for the HttpRequest.Headers property in .NET Core:

• Microsoft.AspNetCore.Http.HttpRequest.Headers Property
  • https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.httprequest.headers?view=aspnetcore-5.0

According to this documentation, the Headers property returns an instance of the IHeaderDictionary interface, which is a read-only dictionary of header names and values. When you access the dictionary using the indexer Headers[key], it returns the value associated with the given key or an empty string ("") if the key is not found. This behavior is consistent with the behavior of the IDictionary<TKey,TValue> interface in .NET Core.

I hope this helps!

于是我对着如下这段 code 自信满满地去掉了 null check 后的 default value XXX。想着 check in 前测一下的，结果还是忘记了。所以最后上线之后被同事过来 challenge。😭

string scenario = executionContext.GetHttpRequestHeader(HeaderUtils.Scenario) ?? "XXX";

然后我去看了源码，Headers 这个 property 是类型 IHeaderDictionary，而这个接口的定义写得很明确，IHeaderDictionary override 了 IDictionary 的 indexer，所以不会抛异常 KeyNotFoundException，而是默认返回 StringValues.Empty。

namespace Microsoft.AspNetCore.Http
{
    public abstract class HttpRequest
    {
        public abstract IHeaderDictionary Headers { get; }
    }
}

namespace Microsoft.AspNetCore.Http
{
    /// <summary>
    /// Represents HttpRequest and HttpResponse headers
    /// </summary>
    public partial interface IHeaderDictionary : IDictionary<string, StringValues>
    {
        /// <summary>
        /// IHeaderDictionary has a different indexer contract than IDictionary, where it will return StringValues.Empty for missing entries.
        /// </summary>
        /// <param name="key"></param>
        /// <returns>The stored value, or StringValues.Empty if the key is not present.</returns>
        new StringValues this[string key] { get; set; }

        /// <summary>
        
        /// Strongly typed access to the Content-Length header. Implementations must keep this in sync with the string representation.
        /// </summary>
        long? ContentLength { get; set; }
    }
}

namespace System.Collections.Generic
{
    [DefaultMember("Item")]
    public interface IDictionary<TKey, TValue> : ICollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>, IEnumerable
    {
        //
        // Summary:
        //     Gets or sets the element with the specified key.
        //
        // Parameters:
        //   key:
        //     The key of the element to get or set.
        //
        // Returns:
        //     The element with the specified key.
        //
        // Exceptions:
        //   T:System.ArgumentNullException:
        //     key is null.
        //
        //   T:System.Collections.Generic.KeyNotFoundException:
        //     The property is retrieved and key is not found.
        //
        //   T:System.NotSupportedException:
        //     The property is set and the System.Collections.Generic.IDictionary`2 is read-only.
        TValue this[TKey key] { get; set; }
    }
}

需要注意的是，StringValues.Empty 并不是 string.Empty，而是 default null，下面的输出可以帮助大家理解。（感觉 ChatGPT 就是把这个搞混了...？🤭

using System;
using Microsoft.Extensions.Primitives;
					
public class Program
{
	public static void Main()
	{
		Console.WriteLine(StringValues.Empty);
		Console.WriteLine((string)null);
		Console.WriteLine(StringValues.Empty == (string)null);
		Console.WriteLine(StringValues.Empty == default(string));
		Console.WriteLine(StringValues.Empty == string.Empty);
	}
}

// Output:
//
//
// True
// True
// False