Server cannot clear headers after http headers have been sent

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C#

ASP

.NET

I have an .asmx web service in same solution with API controllers.

ASMX service had function void and returning customized json response.

Problem is that I am getting error "server cannot clear headers after http headers have been sent".

What I have tried:

private void Json(object obj, int httpcode) 
{
	string objJson = JsonConvert.SerializeObject(obj);

	HttpContext.Current.Response.Clear();
	HttpContext.Current.Response.StatusCode = httpcode;
	HttpContext.Current.Response.TrySkipIisCustomErrors = true;
	HttpContext.Current.Response.ContentType = "application/json; charset=utf-8";
	HttpContext.Current.Response.Write(objJson);
	HttpContext.Current.Response.Flush();

	// Gets or sets a value indicating whether to send HTTP content to the client.
	HttpContext.Current.Response.SuppressContent = true;

	// Causes ASP.NET to bypass all events and filtering in the HTTP pipeline chain of execution and directly execute the EndRequest event.
	HttpContext.Current.ApplicationInstance.CompleteRequest();
}