---
title: Custom parsing with IArgumentParser<T>
description: For types with no built-in support, implement IArgumentParser<T> and annotate the parameter with [ArgumentParser]. Annotate the parameter with [ArgumentParser(typeof(...))]:...
url: https://docs-v3-preview.elastic.dev/argh/parameters/custom-parsers
---

# Custom parsing with IArgumentParser<T>
For types with no built-in support, implement `IArgumentParser<T>` and annotate the parameter with `[ArgumentParser]`.

## Implementing a parser

```csharp
public class SemVerParser : IArgumentParser<SemVer>
{
    public static bool TryParse(string value, out SemVer result) =>
        SemVer.TryParse(value, out result);
}
```


## Using the parser

Annotate the parameter with `[ArgumentParser(typeof(...))]`:
```csharp
public static Task<int> Release([ArgumentParser(typeof(SemVerParser))] SemVer version) { … }
// myapp release 1.2.3
```


## Package reference

`IArgumentParser<T>` is defined in `Nullean.Argh.Interfaces`.
<tip>
  If you are building a shared library of custom parsers for reuse across multiple CLI apps, reference the interfaces package directly:
  ```xml
  <ItemGroup>
    <PackageReference Include="Nullean.Argh.Interfaces" />
  </ItemGroup>
  ```
  For normal apps that reference `Nullean.Argh` or `Nullean.Argh.Hosting`, the interfaces are already available transitively.
</tip>