# Building .NET Lambda functions with Native AOT compilation in AWS SAM
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Build and package your .NET 8 AWS Lambda functions with the AWS Serverless Application Model (AWS SAM), utilizing Native Ahead-of-Time (AOT) compilation to improve AWS Lambda cold-start times.

**Topics**
+ [.NET 8 Native AOT overview](#build-dotnet7-overview)
+ [Using AWS SAM with your .NET 8 Lambda functions](#build-dotnet7-sam)
+ [Install prerequisites](#build-dotnet7-prerequisites)
+ [Define .NET 8 Lambda functions in your AWS SAM template](#build-dotnet7-sam-define)
+ [Build your application with the AWS SAM CLI](#build-dotnet7-sam-build)
+ [Learn more](#build-dotnet7-learn-more)

## .NET 8 Native AOT overview
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Historically, .NET Lambda functions have cold-start times which impact user experience, system latency, and usage costs of your serverless applications. With .NET Native AOT compilation, you can improve cold-start times of your Lambda functions. To learn more about Native AOT for .NET 8, see [Using Native AOT](https://github.com/dotnet/runtime/tree/main/src/coreclr/nativeaot#readme) in the *Dotnet GitHub repository*.

## Using AWS SAM with your .NET 8 Lambda functions
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Do the following to configure your .NET 8 Lambda functions with the AWS Serverless Application Model (AWS SAM):
+ Install prerequisites on your development machine.
+ Define .NET 8 Lambda functions in your AWS SAM template.
+ Build your application with the AWS SAM CLI.

## Install prerequisites
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The following are required prerequisites:
+ The AWS SAM CLI
+ The .NET Core CLI
+ The Amazon.Lambda.Tools .NET Core Global Tool
+ Docker

**Install the AWS SAM CLI**

1. To check if you already have the AWS SAM CLI installed, run the following:

   ```
   sam --version
   ```

1. To install the AWS SAM CLI, see [Install the AWS SAM CLI](install-sam-cli.md).

1. To upgrade an installed version of the AWS SAM CLI, see [Upgrading the AWS SAM CLI](manage-sam-cli-versions.md#manage-sam-cli-versions-upgrade).

**Install the .NET Core CLI**

1. To download and install the .NET Core CLI, see [Download .NET](https://dotnet.microsoft.com/download) from Microsoft's website.

1. For more information on the .NET Core CLI, see [.NET Core CLI](https://docs.aws.amazon.com/lambda/latest/dg/csharp-package-cli.html) in the *AWS Lambda Developer Guide*.

**Install the Amazon.Lambda.Tools .NET Core Global Tool**

1. Run the following command:

   ```
   dotnet tool install -g Amazon.Lambda.Tools
   ```

1. If you already have the tool installed, you can make sure that it is the latest version using the following command:

   ```
   dotnet tool update -g Amazon.Lambda.Tools
   ```

1. For more information about the Amazon.Lambda.Tools .NET Core Global Tool, see the [AWS Extensions for .NET CLI](https://github.com/aws/aws-extensions-for-dotnet-cli) repository on GitHub.

**Install Docker**
+ Building with Native AOT, requires Docker to be installed. For installation instructions, see [Installing Docker to use with the AWS SAM CLI](install-docker.md).

## Define .NET 8 Lambda functions in your AWS SAM template
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To define a .NET8 Lambda function in your AWS SAM template, do the following:

1. Run the following command from a starting directory of your choice::

   ```
   sam init
   ```

1. Select `AWS Quick Start Templates` to choose a starting template.

1. Choose the `Hello World Example` template.

1. Choose to not use the most popular runtime and package type by entering `n`.

1. For runtime, choose `dotnet8`.

1. For package type, choose `Zip`.

1. For your starter template, choose `Hello World Example using native AOT`.

**Install Docker**
+ Building with Native AOT, requires Docker to be installed. For installation instructions, see [Installing Docker to use with the AWS SAM CLI](install-docker.md).

```
Resources:
HelloWorldFunction:
  Type: AWS::Serverless::Function
  Properties:
    CodeUri: ./src/HelloWorldAot/
    Handler: bootstrap
    Runtime: dotnet8
    Architectures:
      - x86_64
    Events:
      HelloWorldAot:
        Type: Api 
        Properties:
          Path: /hello
          Method: get
```

**Note**  
When the `Event` property of an `AWS::Serverless::Function` is set to `Api`, but the `RestApiId` property is not specified, AWS SAM generates the `AWS::ApiGateway::RestApi` CloudFormation resource.

## Build your application with the AWS SAM CLI
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 From your project's root directory, run the `sam build` command to begin building your application. If the `PublishAot` property has been defined in your .NET 8 project file, the AWS SAM CLI will build with Native AOT compilation. To learn more about the `PublishAot` property, see [Native AOT Deployment](https://learn.microsoft.com/en-us/dotnet/core/deploying/native-aot/) in Microsoft's *.NET documentation*.

To build your function, the AWS SAM CLI invokes the .NET Core CLI which uses the Amazon.Lambda.Tools .NET Core Global Tool.

**Note**  
When building, if a `.sln` file exists in the same or parent directory of your project, the directory containing the `.sln` file will be mounted to the container. If a `.sln` file is not found, only the project folder is mounted. Therefore, if you are building a multi-project application, ensure the `.sln` file is property located.

## Learn more
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For more information on building .NET 8 Lambda functions, see [Introducing the .NET 8 runtime for AWS Lambda](https://aws.amazon.com/blogs/compute/introducing-the-net-8-runtime-for-aws-lambda/).

For a reference of the **sam build** command, see [sam build](sam-cli-command-reference-sam-build.md).