33 lines
996 B
Markdown
33 lines
996 B
Markdown
---
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title: "ASP0019: Suggest using IHeaderDictionary.Append or the indexer"
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description: "Learn about analysis rule ASP0019: Suggest using IHeaderDictionary.Append or the indexer"
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author: tdykstra
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monikerRange: '>= aspnetcore-8.0'
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ms.author: tdykstra
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ms.date: 11/22/2022
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uid: diagnostics/asp0019
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---
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# ASP0019: Suggest using IHeaderDictionary.Append or the indexer
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| | Value |
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| **Rule ID** |ASP0019|
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| **Category** |Usage|
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| **Fix is breaking or non-breaking** |Non-breaking|
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## Cause
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`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.
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## Rule description
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`IDictionary.Add` isn't recommended for setting or appending headers.
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## How to fix violations
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To fix a violation of this rule, use `IHeaderDictionary.Append` or the indexer to append or set headers.
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## When to suppress warnings
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Do not suppress a warning from this rule.
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