# Create a custom source

> Add your own source to resolve selectors that the built-in sources don't handle.

Add your own source to resolve selectors that the built-in sources don't handle.

To create your own custom source, inherit from the [`Source`](/engine/6000.7/script-reference/unity/smartstrings/core/extensions/source.md) class, override the `TryEvaluateSelector` method, set `selectorInfo.Result` to the output, and return `true` when you handle the selector, or return `false` to let the next source try.

## Write the source

Override the `TryEvaluateSelector` method and set the `selectorInfo.Result` to the output. Return `true` when you handle the selector, or `false` otherwise to let the next source try.

Use `selectorInfo.CurrentValue` to read the value in scope, and `selectorInfo.SelectorText` to read the selector being evaluated.

If your custom source needs to be set up before it can be used properly (for example, if it needs to read the case-sensitivity setting), you can implement [`IInitializer`](/engine/6000.7/script-reference/unity/smartstrings/core/extensions/iinitializer.md) and its `Initialize(SmartFormatter)`. This method runs when the source is registered, and again after the settings asset deserializes.

The following source returns a random number when it sees the `random` selector. Combine it with the [Choose formatter](/engine/6000.7/manual/scripting/smart-strings/formatters/choose-formatter.md) to produce different responses.

```cs
public class RandomValueSource : Source
    {
        public override bool TryEvaluateSelector(ISelectorInfo selectorInfo)
        {
            if (selectorInfo.SelectorText != "random")
                return false;

            selectorInfo.Result = UnityEngine.Random.Range(1, 4);
            return true;
        }
    }
```

| Smart String                                          | Result   |
| ----------------------------------------------------- | -------- |
| `{random:choose(1\|2\|3):Hello\|Greetings\|Welcome}!` | Welcome! |

## Register the source

Add the source to the default formatter so it is available everywhere on the current thread:

```cs
Smart.Default.AddExtensions(new RandomValueSource());

            // "Welcome!"
            Smart.Format("{random:choose(1|2|3):Hello|Greetings|Welcome}!");
```

Or register it on a specific [`SmartFormatter`](/engine/6000.7/script-reference/unity/smartstrings/smartformatter.md) instance, which avoids changing global behavior:

```cs
var smart = Smart.CreateDefaultSmartFormat();
            smart.AddExtensions(new RandomValueSource());
```

You can also add a custom source to the project's default formatter without code: in the [Smart Strings settings](/engine/6000.7/manual/scripting/smart-strings/settings.md), select **Add** (**+**) on the **Sources** list and choose your source from the menu, which lists every available source type by name. Mark the class `[Serializable]` and give it a public parameterless constructor so it can be added and saved with the settings.

Sources are tried in order, so place your custom source where it should take priority over the built-in sources. Refer to [Smart Strings settings](/engine/6000.7/manual/scripting/smart-strings/settings.md) for information on how to configure the order in which sources are tried.

## Additional resources

* [Properties source](/engine/6000.7/manual/scripting/smart-strings/sources/properties-source.md)
* [Default source](/engine/6000.7/manual/scripting/smart-strings/sources/default-source.md)
* [Smart Strings](/engine/6000.7/manual/scripting/smart-strings.md)