---
title: "ASP0019: Suggest using IHeaderDictionary.Append or the indexer"
description: "Learn about analysis rule ASP0019: Suggest using IHeaderDictionary.Append or the indexer"
author: tdykstra
monikerRange: '>= aspnetcore-8.0'
ms.author: tdykstra
ms.date: 11/22/2022
uid: diagnostics/asp0019
---
# ASP0019: Suggest using IHeaderDictionary.Append or the indexer

| | Value |
|-|-|
| **Rule ID** |ASP0019|
| **Category** |Usage|
| **Fix is breaking or non-breaking** |Non-breaking|

## Cause

`IDictionary.Add` isn't recommended for setting or appending headers. [`IDictionary.Add`](xref:System.Collections.IDictionary.Add%2A) throws an `ArgumentException` when attempting to add a duplicate key.

## Rule description

`IDictionary.Add` isn't recommended for setting or appending headers.

## How to fix violations

To fix a violation of this rule, use `IHeaderDictionary.Append` or the indexer to append or set headers.

## When to suppress warnings

Do not suppress a warning from this rule.