更新自定义标头值作为HttpClient的DefaultRequestHeaders添加

我有一个跨请求共享的静态httpclient,我想为它添加一个自定义标头.

httpClient.DefaultRequestHeaders.Add("customHeader", somevalue.ToString());

但我注意到,在每个请求中,该值都会添加到我打算在每个请求上替换的标头中.我尝试删除标头,如果它已经存在并再次添加,但它在负载测试时给我一个错误.

if (httpClient.DefaultRequestHeaders.Contains("customHeader"))
        {
            httpClient.DefaultRequestHeaders.Remove("customHeader");
        }
httpClient.DefaultRequestHeaders.Add("customHeader",somevalue.ToString());

错误 -

System.ArgumentException: An item with the same key has already been added.
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
System.ArgumentNullException: Value cannot be null.

如何更新每个请求的自定义标头值？