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Extism .NET Host SDK

This repo houses the .NET SDK for integrating with the Extism runtime. Install this library into your host .NET applications to run Extism plugins.

Installation

This library depends on the native Extism runtime, we provide native runtime packages for all supported operating systems. You can install with:

dotnet add package Extism.runtime.all

Then, add the Extism.Sdk NuGet package to your project:

dotnet add package Extism.Sdk

Getting Started

This guide should walk you through some of the concepts in Extism and this .NET library.

First you should add a using statement for Extism:

C#:

using System;

using Extism.Sdk;

F#:

open System

open Extism.Sdk

Creating A Plug-in

The primary concept in Extism is the plug-in. You can think of a plug-in as a code module stored in a .wasm file.

Since you may not have an Extism plug-in on hand to test, let's load a demo plug-in from the web:

C#:

var manifest = new Manifest(new UrlWasmSource("https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm"));

using var plugin = new Plugin(manifest, new HostFunction[] { }, withWasi: true);

F#:

let uri = Uri("https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm")
let manifest = Manifest(new UrlWasmSource(uri))

let plugin = new Plugin(manifest, Array.Empty<HostFunction>(), withWasi = true)

Note: The schema for this manifest can be found here: https://extism.org/docs/concepts/manifest/

Calling A Plug-in's Exports

This plug-in was written in Rust and it does one thing, it counts vowels in a string. As such, it exposes one "export" function: count_vowels. We can call exports using Plugin.Call:

C#:

var output = plugin.Call("count_vowels", "Hello, World!");
Console.WriteLine(output);
// => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

F#:

let output = plugin.Call("count_vowels", "Hello, World!")
printfn "%s" output
// => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

All exports have a simple interface of optional bytes in, and optional bytes out. This plug-in happens to take a string and return a JSON encoded string with a report of results.

Plug-in State

Plug-ins may be stateful or stateless. Plug-ins can maintain state b/w calls by the use of variables. Our count vowels plug-in remembers the total number of vowels it's ever counted in the "total" key in the result. You can see this by making subsequent calls to the export:

C#:

var output = plugin.Call("count_vowels", "Hello, World!");
Console.WriteLine(output);
// => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}

output = plugin.Call("count_vowels", "Hello, World!");
Console.WriteLine(output);
// => {"count": 3, "total": 9, "vowels": "aeiouAEIOU"}

F#:

let output1 = plugin.Call("count_vowels", "Hello, World!")
printfn "%s" output1
// => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}

let output2 = plugin.Call("count_vowels", "Hello, World!")
printfn "%s" output2
// => {"count": 3, "total": 9, "vowels": "aeiouAEIOU"}

These variables will persist until this plug-in is freed or you initialize a new one.

Configuration

Plug-ins may optionally take a configuration object. This is a static way to configure the plug-in. Our count-vowels plugin takes an optional configuration to change out which characters are considered vowels. Example:

C#:

var manifest = new Manifest(new UrlWasmSource("<https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm>"));

using var plugin = new Plugin(manifest, new HostFunction[] { }, withWasi: true);

var output = plugin.Call("count_vowels", "Yellow, World!");
Console.WriteLine(output);
// => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

manifest = new Manifest(new UrlWasmSource("<https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm>"))
{
    Config = new Dictionary<string, string>
    {
        { "vowels", "aeiouyAEIOUY" }
    },
};

using var plugin2 = new Plugin(manifest, new HostFunction[] { }, withWasi: true);

var output2 = plugin2.Call("count_vowels", "Yellow, World!");
Console.WriteLine(output2);
// => {"count": 4, "total": 4, "vowels": "aeiouAEIOUY"}

F#:

let uri = Uri("https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm")
let manifest = Manifest(new UrlWasmSource(uri))
manifest.Config <- dict [("vowels", "aeiouAEIOU")]

let plugin = new Plugin(manifest, Array.Empty<HostFunction>(), withWasi = true)

let output = plugin.Call("count_vowels", "Yellow, World!")
Console.WriteLine(output)
// => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

let manifest2 = 
    Manifest(new UrlWasmSource(Uri("https://github.com/extism/plugins/releases/latest/download/count_vowels.wasm")))
manifest2.Config <- dict [("vowels", "aeiouyAEIOUY")]

let plugin2 =
    new Plugin(manifest2, Array.Empty<HostFunction>(), withWasi = true)

let output2 = plugin2.Call("count_vowels", "Yellow, World!")
printfn "%s" output2
// => {"count": 4, "total": 4, "vowels": "aeiouAEIOUY"}

Host Functions

Let's extend our count-vowels example a little bit: Instead of storing the total in an ephemeral plug-in var, let's store it in a persistent key-value store!

Wasm can't use our KV store on it's own. This is where Host Functions come in.

Host functions allow us to grant new capabilities to our plug-ins from our application. They are simply some Go functions you write which can be passed down and invoked from any language inside the plug-in.

Let's load the manifest like usual but load up this count_vowels_kvstore plug-in:

C#:

var manifest = new Manifest(new UrlWasmSource("https://github.com/extism/plugins/releases/latest/download/count_vowels_kvstore.wasm"));

F#:

let manifest = Manifest(new UrlWasmSource(Uri("https://github.com/extism/plugins/releases/latest/download/count_vowels_kvstore.wasm")))

Note: The source code for this is here and is written in rust, but it could be written in any of our PDK languages.

Unlike our previous plug-in, this plug-in expects you to provide host functions that satisfy our its import interface for a KV store.

We want to expose two functions to our plugin, void kv_write(key string, value byte[]) which writes a bytes value to a key and byte[] kv_read(key string) which reads the bytes at the given key.

C#:

// pretend this is Redis or something :)
var kvStore = new Dictionary<string, byte[]>();

var functions = new[]
{
    HostFunction.FromMethod("kv_read", IntPtr.Zero, (CurrentPlugin plugin, long keyOffset) =>
    {
        var key = plugin.ReadString(keyOffset);
        if (!kvStore.TryGetValue(key, out var value))
        {
            value = new byte[] { 0, 0, 0, 0 };
        }

        Console.WriteLine($"Read {BitConverter.ToUInt32(value)} from key={key}");
        return plugin.WriteBytes(value);
    }),

    HostFunction.FromMethod("kv_write", IntPtr.Zero, (CurrentPlugin plugin, long keyOffset, long valueOffset) =>
    {
        var key = plugin.ReadString(keyOffset);
        var value = plugin.ReadBytes(valueOffset);

        Console.WriteLine($"Writing value={BitConverter.ToUInt32(value)} from key={key}");
        kvStore[key] = value.ToArray();
    })
};

F#:

let kvStore = new Dictionary<string, byte[]>()

let functions =
    [|
        HostFunction.FromMethod("kv_read", IntPtr.Zero, fun (plugin: CurrentPlugin) (offs: int64) ->
            let key = plugin.ReadString(offs)
            let value = 
                match kvStore.TryGetValue(key) with
                | true, v -> v
                | _ -> [| 0uy; 0uy; 0uy; 0uy |] // Default value if key not found

            Console.WriteLine($"Read {BitConverter.ToUInt32(value, 0)} from key={key}")
            plugin.WriteBytes(value)
        )

        HostFunction.FromMethod("kv_write", IntPtr.Zero, fun (plugin: CurrentPlugin) (kOffs: int64) (vOffs: int64) ->
            let key = plugin.ReadString(kOffs)
            let value = plugin.ReadBytes(vOffs).ToArray()

            Console.WriteLine($"Writing value={BitConverter.ToUInt32(value, 0)} from key={key}")
            kvStore.[key] <- value
        )
    |]

Note: In order to write host functions you should get familiar with the methods on the CurrentPlugin type. The plugin parameter is an instance of this type.

We need to pass these imports to the plug-in to create them. All imports of a plug-in must be satisfied for it to be initialized:

C#:

using var plugin = new Plugin(manifest, functions, withWasi: true);

var output = plugin.Call("count_vowels", "Hello World!");

Console.WriteLine(output);
// => Read 0 from key=count-vowels"
// => Writing value=3 from key=count-vowels"
// => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

output = plugin.Call("count_vowels", "Hello World!");

Console.WriteLine(output);
// => Read 3 from key=count-vowels"
// => Writing value=6 from key=count-vowels"
// => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}

F#:

let plugin = new Plugin(manifest, functions, withWasi = true)

let output = plugin.Call("count_vowels", "Hello World!")
printfn "%s" output
// => Read 0 from key=count-vowels
// => Writing value=3 from key=count-vowels
// => {"count": 3, "total": 3, "vowels": "aeiouAEIOU"}

let output2 = plugin.Call("count_vowels", "Hello World!")
printfn "%s" output2
// => Read 3 from key=count-vowels
// => Writing value=6 from key=count-vowels
// => {"count": 3, "total": 6, "vowels": "aeiouAEIOU"}